Mammalian Genetics [Reprint 2013 ed.] 9780674732230, 9780674731141

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MAMMALIAN GENETICS

LONDON : HUMPHREY MILFORD OXFORD U N I V E R S I T Y P R E S S

MAMMALIAN GENETICS BY

WILLIAM E. CASTLE PROFESSOR E M E R I T U S OF GENETICS, HARVARD

UNIVERSITY

RESEARCH ASSOCIATE IN GENETICS, U N I V E R S I T Y OF CALIFORNIA

CAMBRIDGE,

MASSACHUSETTS

HARVARD U N I V E R S I T Y PRESS 1940

COPYRIGHT,

I94O

B Y T H E PRESIDENT AND FELLOWS OF HARVARD COLLEGE

PRINTED AT T H E HARVARD U N I V E R S I T Y PRESS CAMBRIDGE, MASSACHUSETTS,

U.S.A.

To an ever-faithful friend and inspiring teacher, Dr. C. B. Davenport, this book is dedicated by a grateful pupil.

PREFACE is intended as a summary of present knowledge of the genetics of mammals. T o be sure, this knowledge is not as complete as that concerning animals such as Drosophila or plants such as maize, but being ourselves mammals we are especially interested in such knowledge as concerns our animal group and that of our domestic animals. The heredity of man himself has not been included in this discussion, because it has to be investigated by methods other than those of experiment, which can be used in the case of the laboratory and domestic animals. Since experimental investigation of heredity forms the only sound basis for generalization, it is hoped that this summary will be useful to those engaged in the more difficult task of investigating heredity in man. Those who deal directly with farm animals and observe the evolution of breeds in progress will, it is hoped, find here material which will help them in correlating and interpreting their observations, and will point the way toward new investigations. T H I S BOOK

It is assumed that the reader will possess an elementary knowledge of the anatomy and physiology of mammals and of the development of an animal egg, as well as of the cytology of gametogenesis and fertilization. Such information is available in any general textbook of biology, if the reader wishes to refresh or increase his knowledge of these biological fundamentals. If this book should be used as a textbook in genetics, it would need to be supplemented with a laboratory study which the student himself could carry out, either of Drosophila, or, still better, if laboratory facilities are adequate, of a laboratory mammal such as the mouse or the rat. Several good laboratory guides are available to assist the student. One designed primarily for use with this text has been prepared by the author and is published by the Harvard University Press.

viii

PREFACE

The references given at the end of each chapter are not complete bibliographies, but suffice to put the reader in touch with the more important literature of the topic. In compiling the references, the exact titles of the papers cited have frequently not been given, but have been shortened or modified to show their bearing on the subject under consideration. The name of the author and place of publication will guide the reader who desires to consult original sources of information. W . E. C. University of California Berkeley January, 1940

CONTENTS I. II.

HEREDITY

IN G E N E R A L

MENDELIAN

3

INHERITANCE

11

III.

MODIFIED MENDELIAN RATIOS

IV.

L I N K A G E , AS I L L U S T R A T E D IN T H E S T U D Y OF R A B B I T G E N E S

V. VI. VII. VIII. IX. X. XI. XII.

20

K N O W N G E N E S OF R O D E N T S OTHER T H A N T H E R A B B I T

.

.

.

.

49

SELF-STERILITY;

55

BIPOLAR

SEXUALITY

SEX DIFFERENTIATION

59

SEX DETERMINATION

67

S E X - L I N K E D I N H E R I T A N C E IN D R O S O P H I L A AND IN M A M M A L S

79

I N H E R I T A N C E OF B L O O D G R O U P S IN M A N AND R A B B I T

88

.

.

.

DOMINANCE; MULTIPLE ALLELES LETHAL

GENES;

XIV.

GENES

HAVING

95

BALANCED LETHALS PATHOLOGICAL

EFFECTS —

102 "SUBLETHAL"

GENES

XVI. XVII.

XIX.

106

MATERNAL

INHERITANCE

110

T H E I N H E R I T A N C E OF B O D Y S I Z E VARIATION PURE

XVIII.

38

H Y B R I D I Z A T I O N AND H Y B R I D V I G O R

XIII.

XV.

25

AND

SELECTION;

117

QUANTITATIVE

CHARACTERS;

LINES

121

M A J O R C O N T R I B U T I O N S OF F I S H G E N E T I C S T H E MORE IMPORTANT MAMMALS INDEX

GENE

MUTATIONS

129 OF

DOMESTIC

141 167

MAMMALIAN GENETICS

CHAPTER I HEREDITY IN GENERAL HE DIFFERENCES which exist between individuals of the same species must be due either to differences in the germ plasm from which they develop (i.e., to heredity) or to differences in the forces to which they are subjected during development (i.e., to environment). W e know that agencies of both sorts are operative in producing the variations of individuals. A sample of seed of any variety of plant, if divided into two parts, one of which is planted in good soil, the other in poor soil, will produce plants very different in appearance and productiveness in the two locations, though all will be recognizable as being the same kind of plant. This similarity is due to the fact that they all had the same kind of germ plasm to start with. Such differences as the two lots of plants show are due to the different environments in which they have grown.

T

But if one sows side by side in the same soil seed of beans and seed of peas, bean plants will arise from one and pea plants from the other, regardless of how they are treated. This is because the germ plasm of beans is different from that of peas. It has different potentialities. Now heredity expresses itself in the resemblance of offspring to parents independent of environment, and is due to the fact that both parents and offspring develop from the same kind of germ plasm. In uniparental inheritance — as, for example, in parthenogenetically produced aphids — the offspring under uniform environmental conditions are commonly indistinguishable from the parent. T h e germ plasm from which the parent and its offspring develop is of one and the same sort. But in biparental inheritance the two parents differ from each other, sex being only one of the differences between the mother and the father. One contributes an egg and the other a sperm to

4

MAMMALIAN GENETICS

the production of a new individual. These so-called gametes differ from each other not merely morphologically but also in their potentialities. Hence, the germ plasm from which the new individual arises is not identical in character either with that of the mother or with that of the father, but is a combination of a part of the germ plasm of the mother with a part of the germ plasm of the father. In the more general features of its organization, in which the parents were alike, the new individual is like them. The progeny of two dogs, however dissimilar, are unmistakably dogs. One parent may have been a collie and the other an Irish terrier, but no one would hesitate to call the offspring dogs, though they differ in many respects from both parents. We must make a distinction, then, between the more general and the special characteristics of organisms. Heredity is concerned in both. Both have their basis in the germ plasm. But what we have learned about heredity in the present century — and this is much, being the greater part of our knowledge of the subject — concerns rather the special than the general features of organization. In crosses between different breeds of dogs, we can follow the inheritance of such special characters as long vs. short hair, straight vs. curly hair, erect vs. drooping ears, short crooked legs vs. ordinary straight legs, and many other differential characters. We have learned that the inheritance of each of these special characters is conditioned by a gene, a material body located within a particular chromosome of the nucleus of the germ cells and from them handed on directly to the body cells which arise from the germ cells in development. This is the theory known as the chromosome theory of heredity or the theory of the gene. It rests on ample experimental evidence, some of which we shall examine later. But in all dog crosses we can not trace the heredity of the general characters which constitute a dog rather than a cat; or the characters which constitute a carnivore rather than an ungulate; or those which characterize a mammal, or a verte-

H E R E D I T Y IN G E N E R A L

5

brate. All these are present, and it is commonly assumed that they, like the special characters, depend upon the existence of chromosomal genes. But we have no direct proof of this assumption, and it is difficult to see how such proof can be obtained. For the evidence as to the existence of the special unit characters of organisms is obtained by the method of crossing first employed by Mendel, in which an individual possessing a particular unit character is crossed with one not possessing it. The recurrence of the character in question in the two following generations of offspring forms a basis for explaining the method of transmission, which conforms with Mendel's law. In crosses of different varieties of dogs, both parents contribute the complex of genes indispensable to a vertebrate, a mammal, a carnivore, and a dog, so there exist in such an experiment no gene differences to indicate what these essential gene complexes are. It is assumed in the chromosome theory that if we could cross a dog with a cat we should find that the differences between them were of the same sort as those between different breeds of dogs, namely, dependent upon chromosomal genes; only the differential genes in such a case would be much more numerous. E g g and sperm, which unite to form a new individual, are equivalent to each other in chromosome content, except as regards the sex chromosome, to the discussion of which we shall return later. But the egg is not a mere container of chromosomes. Its influence on development is not restricted to its chromosome content. The cytoplasm of the egg is already organized at the time of fertilization. Its parts differ qualitatively, so that as cleavage progresses blastomeres with different potentialities are produced, although the nuclei which they contain are all alike in chromosome content. It is impossible to explain the primary organization of the egg as due to immediate chromosome influence, for it arises in the presence of exactly the same assortment of chromosomes as is found in each cell of the body. Yet those body cells are highly differentiated parts of a complete organism. The egg cell is potentially also a

6

MAMMALIAN GENETICS

complete organism, though it has the same chromosome content as the highly differentiated parts of the mother. Hence that differentiation is not a function of immediate chromosomal activity, but inheres in the unitary organization of the species, whether that organization finds expression in a single cell (the egg) or in a mass of differentiated cells (the body) derived in development from the egg. It is not necessary to assume that the egg has an organization as complete and elaborate as that of the adult. Students of experimental embryology are very emphatic on this point. The amphibian egg develops first a "gray crescent" in the blastula stage, which acts as an "organizer" to determine the fate of adjacent parts and ultimately of the entire egg. Those adjacent parts are undetermined when the gray crescent makes its appearance. They can be made to produce almost any part of an amphibian body by bringing them into proper relation with an organizer — either the primary organizer of the egg itself, or another organizer transplanted from another egg, even from the egg of a different species or genus. Thus the development of the egg is largely epigenetic, determined step by step, it being necessary to assume only a few primary features in the organization of the egg at the outset. In a similar way, the genetic determination of the inherited general characters (constituting dog, carnivore, mammal, and vertebrate) is probably relatively simple, inherent in the organization of the cytoplasmic portion of the egg itself, not of its chromosomes, except in so far as they control cytoplasmic organization. In support of this view may be cited investigations of the inheritance of body size in rabbits. Racial body size is a strongly inherited character in rabbits. Flemish Giant rabbits breed true to an adult body weight of 5000-6000 grams; Polish rabbits breed true to an adult body weight of 1300-1600 grams. A cross between the two breeds produces animals of intermediate weight. A comparative study of the embryology of these races shows that in the large race cleavage of the egg occurs at a more rapid rate than in the small race, while cleavage

H E R E D I T Y IN G E N E R A L

7

in eggs of the hybrid progresses at an intermediate rate. The faster cleavage of the large-race egg results in the production of a larger blastocyst upon which a correspondingly larger embryo is blocked out. At birth, young of the large race are nearly twice as heavy as those of the small race, and they subsequently grow faster and for a longer period before reaching maturity, thus attaining an adult weight four or five times as great as that of the small race. On the basis of the gene theory, it was formerly assumed that chromosomal genes determined the size of each part of the body, and that the size of these different parts was, to some extent, independently determined, thus necessitating the assumption of an enormous number of genes influencing the size of the parts. We now see that such complicated assumptions are unnecessary. Given at the outset only a more rapid developmental rate of the egg of the large race, all the observed differences in adult body size and in the proportions of the parts in large as compared with small races follow as necessary consequences. More rapid development results in greater birth weight and larger adult size. As the size increases the proportions alter, growth being more rapid in length than in width of elongated organs. Thus with continued growth the skull, ears, and leg bones become proportionally longer. The gene theory as at one time formulated required a multiplicity of genes with local effects to account for this situation. But a single initial difference in rate of development of the egg (dependent in part on cytoplasmic organization and in part on the existence of genes in the chromosomes which act as accelerators or retarders of development) accounts satisfactorily for the whole sequence of differences in body size and form which arise epigenetically. Why the egg of one species should exhibit an organization different from that of another we are unable to explain on the basis of either the chromosome theory or any other theory. Neither the chromosomes nor the cytoplasm, separately considered, constitutes a working organic system, a living organism.

8

MAMMALIAN GENETICS

But the two together do constitute such a system. The egg of each species contains both, in such a state of organization that only a stimulus to development (normally furnished by fertilization, but capable of being produced by other means) is required to enable it to develop into a typical individual of the species. It is not necessary to suppose that all the successive steps of differentiation are predetermined in the egg and represented there by distinct genes. Only a very few of the more general features of organization can be discovered in the egg, such as regional differences in the composition of its cytoplasm. The enormously more complicated organization ultimately attained comes into existence as development progresses, epigenetically. Chromosomal genes come into action as modifiers of these developmental processes, but there is no reason to suppose that they are wholly and immediately responsible for the processes. All processes of the living organism are probably subject to modification by chromosomal genes. But we must not confuse modification of a process with its causation. Genes may modify any portion of the life cycle, though we are more familiar with those which affect the adult stage. Thus in the silkworm a gene may affect the organization of the egg cytoplasm so that its superficial layer is either thin or thick, affecting the ease with which development is started, so that one-brooded or two-brooded or many-brooded races per annum result. Again, in gastropod mollusks, right-handed or lefthanded asymmetry of the body and shell depend upon the asymmetrical localization of materials in the egg cytoplasm, which may be influenced by a gene transmitted either by sperm or by egg nucleus. But it does not follow that because the organization of the egg cytoplasm can be modified by chromosomal genes therefore it is wholly produced by such genes. Chromosomal genes are comparable in their action to the hormones of the body. They affect the chemical composition of the cell so as to modify certain of its processes, just as hormones affect physiological processes in the body. Both are highly selective in their action. A sex chromosome is concerned,

H E R E D I T Y IN G E N E R A L

9

in the higher animals and plants, in the determination of the sex of the developing individual as male or female; but a contrary sex hormone arising from the sex glands of another individual of the species, of the opposite sex, if introduced at the proper stage may inhibit or reverse the course of sex development, changing a genetically determined female into an imperfectly functioning male. Sex hormones affect almost exclusively the sex organs (primary or secondary), being thus very specific in their action. Similarly, genes modify specific processes in development at very definite stages. They are not, perhaps, the determiners of vital processes, but only modifiers of those processes. We conclude, then, ( 1 ) that heredity includes all the resemblances between parents and offspring which are not due to purely environmental influences; (2) that these resemblances may be separated into two groups, one of which includes the general features of organization, the other its details only. The latter correspond with the unit characters of organisms and are inherited in accordance with Mendel's law. Their determiners are genes located in the chromosomes. The general features of organization — such as characterize entire phyla, classes, orders, and genera — are not demonstrably determined by chromosomal genes, but in all probability are dependent upon the general features of organization of the living substance in each group, such features of organization being not restricted to the chromosomes but certainly involving the cytoplasm also. Whether cytoplasmic organization is, in the last analysis, an indirect consequence of gene action remains at present an open question. We can only speculate at present as to how this organization arose, whether genes or cytoplasm came into existence first and later acquired the counterpart at present indispensable to life. Under the influence of the current study of viruses, which seem to lack cytoplasm, the working hypothesis of many biologists may be stated thus: The earliest form of life was probably associated with virus-like protein molecules which

MAMMALIAN GENETICS

10

acquired an organic plasma in which they were able to grow and reproduce. Whether this plasma arose independently w e do not know. A t present, viruses live and multiply only in the plasma of other organisms. Whether at an earlier time they had a plasma of their own and were thus capable of an independent non-parasitic existence, we do not know. A t present, every organism with which we are acquainted represents a permanent association of nuclear genes with a non-nuclear plasma. T w o hypotheses concerning their origin are possible and are currently held. One of these is that genes arose first as viruslike protein molecules which later developed a plasma in which to interact with the environment and draw nutrient substances from it. Genes of diverse constitution became aggregated in permanent groups forming chromosomes and nuclei within a common plasma (cytoplasm). T h e other hypothesis supposes that cytoplasm had an independent origin and that in it genes found a suitable medium for existence and multiplication, like the algal constituents of a lichen, and that thus a permanent condition of symbiosis was established between genes and plasma (nucleus and cytoplasm), each of which has subsequently lost its capacity for independent existence. REFERENCES

Β., 1932. Experimental analysis of development. 288 pp. New York: W. W . Norton & Co.

DÜRKEN,

RIDDLE,

O.,

1939. Epic of Life. Sei. Monthly 48: 530-533.

CHAPTER II MENDELIAN

INHERITANCE

N E W INDIVIDUAL begins its existence as a fertilized egg resulting from a union between an egg and a spermatozoon, each of which had the half ("haploid") chromosome number in consequence of a reductional cell division, so that the resulting cells known as gametes contained only one of each different kind of chromosome. The fertilized egg thus received a diploid complement of chromosomes. The act of fertilization initiates development of the egg into a many-celled aggregate whose parts become differentiated into the several organs of a body characteristic of the species. Very early in this process, one or more cells, as yet undifferentiated, are set aside as germ cells to form the gonads of the adult animal from which eggs or sperm will be produced to perpetuate the species. The germ cells are distinct from the body which contains them and may be removed from the body without interfering with its continued existence. If they are thus removed, it is possible to put in their place germ cells from another individual of the same species. Under favorable conditions the transplanted germ cells will persist and produce gametes, which, however, will have the genetic properties of the donor individual, not of the host. (See Figs. 1-6.) This shows that the germ cells derive their genetic potentialities directly from the fertilized egg, not from the soma, since a different soma may be substituted for the natural one without altering the potentialities of the germ cells. The more conspicuous differences between individuals of the same species of animal or plant are inherited in conformity with the law discovered by Gregor Mendel in 1866, now known as Mendel's law. Its validity failed of recognition in Mendel's own time, and his work was lost sight of until the year

A

12

MAMMALIAN GENETICS

1900, when the law was rediscovered independently by Hugo De Vries in Holland, Carl Correns in Germany, and E. Tschermak in Austria. This law, in its simplest form, applies to cases in which one of the two parents has a character which is lacking in the other parent (the so-called presence-absence relationship). A n illustration of this law in a cross between a black guinea pig and a white one is presented in Figs. 7-10. The albinism of the father is unseen (becomes "recessive") in the first-generation offspring but reappears in one fourth of the second-generation offspring (grandchildren). Fig. 1 1 shows in a diagram the course of transmission of full color ( C ) and of albinism (c) in this cross. This law also applies to cases in which a character occurs in a different form in each of the parents, the two forms being alternative and mutually exclusive. In general, Mendelizing characters relate to the finer details of organization of the individual, because the parents, in order to be able to join in the production of offspring, must be alike in their general features of organization; and so the inheritance of these general features can not be studied by the method of Mendelian crosses, as already explained. In the rabbit 19 different unit characters have been clearly demonstrated. Nine of these affect the color of the hairy coat, six affect the structure of the hair, one has to do with the color of the fat stored in various parts of the body, one concerns agglutinogens carried by the red blood corpuscles, which form a basis for the recognition of blood groups, one affects the body size of the animal, which may be dwarfed through the action of a gene which probably reduces hormone production in some gland of internal secretion (possibly the pituitary), and one affects the degree to which toes are developed, particularly on the front feet. The physical basis of each of these 19 unit characters, as found in the germ cells (eggs and sperm), is a gene borne in a chromosome. It will be observed that 15 of the 19 known genes of the rabbit affect details of structure or pigmentation of the hair, hair be-

FIG.

Ι

FIG.

6

R e s u l t s of o v a r i a n t r a n s p l a n t a t i o n in g u i n e a pigs. guinea pig (Fig.

Ovaries f r o m a small black

i ) w e r e t r a n s p l a n t e d into an a l b i n o ( F i g . 2 ) w h i c h , m a t e d

w i t h a n o t h e r a l b i n o ( F i g . 3 ) , p r o d u c e d black y o u n g ( F i g s . 4—6).

MENDELIAN INHERITANCE

FIG. I I . Diagram to explain the inheritance of color ( C ) and albinism (c) in the cross shown in Figs. 7 - 1 0 .

ing restricted in occurrence to the class Mammalia of the Vertebrata. Fat color (yellow or white) is due to a slight difference in the metabolism of fat storage from foods containing chlorophyl. Presence or absence of a haemagglutinogen is a detail of composition of the red blood cells of immaterial consequence

i4

MAMMALIAN GENETICS

to the life of the rabbit, since rabbits of all blood groups occur both in the wild state and under domestication and are all equally vigorous. But unimportant in one point of view as the unit characters are, they form the basis, to a large extent, of the differential characters of various breeds of domestic animals and the varieties of cultivated plants. The general features of Mendelian inheritance are now familiar even to elementary students of biology. We may review them briefly as seen in the inheritance of coat characters in the rabbit. Gray and black are alternative coat colors in rabbits. A gray individual of a true-breeding race, such as Gray Flemish Giant, if crossed with a black individual, produces only gray offspring. These first generation (Fx) young, if mated together, will produce a second generation (F 2 ) containing both gray and black young, the grays — in the long run — being three times as numerous as the blacks. Blacks of the second generation breed true, but only one third of the grays will be true-breeding, the other two thirds being like the Fi young in their capacity to produce both colors. If one of this group of grays, or one of the F x gray parents, is mated with a black individual, half of the young, on the average, are gray and half black. These blacks, like those of all the generations, are true-breeding, but the grays will be like the gray parent in their capacity to produce both sorts. These various statements can be expressed in a diagram, as in Fig. 12. In the gray coat of a rabbit, as well as in the black coat, there is found black melanin pigment. This is formed throughout the entire period of growth of the individual hairs of the black rabbit, but is not formed continuously in the growth of the hairs of a gray rabbit. In the newborn young the tips of the larger hairs are just emerging from the skin. These hair tips are heavily pigmented with black in both gray and black rabbits, and this sort of pigment continues to be produced in the hairs of the black rabbit until the hairs are full grown. But in

MEND ELIAN INHERITANCE

15

those of the gray rabbit, on the greater part of the body, after the formation of the black hair tip, there is produced a yellow pigmented zone of perhaps a quarter-inch in length; then the production of black pigment is resumed. But the ventral hairs of the gray rabbit — on belly, throat, and tail — have no black Black

Gray (pure)

Parents

F 1 generation

Gray (pure) Groy

Gray (impure Gray

3 Gray Μ Black

Gray (impure) 1 Gray: 1 Black

Black (pure) Black

F? 5generation F 3 generation

FIG. 12. Diagram of the results observed when a gray rabbit is crossed with a black one.

tip and are white except at the base. Also dorsally — on the neck of a gray rabbit — a black hair tip is replaced with reddishyellow pigment, forming a reddish neck patch. A l l these features of the gray coat are inherited together as a unit in heredity and may be referred to the action of a single gene, which we call the "agouti" gene, because a similar gray pattern is particularly well developed in the South American agouti. Gray rabbits may accordingly be called agouti animals in distinction from black ones, which may be called non-agouti. T h e agouti or wild type coat of gray rabbits may be referred to the action of a gene, A , agouti, which is dominant in crosses over its recessive allele, a, or non-agouti, these two being alternative conditions of one and the same gene. Or, if one prefers, he may regard one as the presence and the other as the absence of a gene determining the cyclical (agouti) activity of the hair follicles in producing melanin pigment, resulting in a gray coat. A being the contribution of the gray parent, and a that of the black parent in the cross discussed, the F x gray animal has the genetic constitution A a and is known as a heterozygote

MAMMALIAN GENETICS

16

or union of unlike alleles. (See Fig. 13.) The pure gray parent, on the other hand, is A A in constitution, a homozygote, or union of two like alleles. Likewise the black parent is homozygous, as are all black individuals; they are aa in constitution. The heterozygous gray animals form gametes of two sorts,

AA

ÄÄ-ÄS FIG. 13. Diagram showing the genetic constitution of the various rabbits of Fig. 12.

A and a, in equal numbers. Thus an Fx female produces eggs capable of fertilization, A -+- a, and an F x male produces sperm, A + a. Random unions of the two in fertilization result in zygotes A A + 2Aa aa, or three showing A (agouti or gray) to one lacking it. The two homozygotes A A and aa will each breed true, all the gametes of each being alike; but the heterozygous grays will form gametes of two sorts, A and a, and will thus have the same breeding capacity as the Fi animals. The F 2 ratio of three dominants to one recessive is the single character or unifactorial Mendelian ratio. Its mode of production may be shown graphically by the checkerboard method devised by Punnett, as in Fig. 14. Another unit character cross in rabbits may be made between a rex (very short-haired) and an ordinary rabbit. The F x young are all normal-haired, rex being recessive. The F 2 generation consists of three normal to one rex individual. If, now, we cross a gray normal with a black rex rabbit, we

MENDELIAN INHERITANCE

17

make simultaneously both the crosses already described, or a two-factor cross. Normal dominates rex, and gray dominates black, as when the crosses are made separately. So the F x animals are gray normals. In F 2 we obtain four classes of young, normal gray, normal black, rex gray, and rex black, approximating the ratio 9:3:3:1, this being the so-called bifactorial

A Sperm

A

AA

Aa

a

Aa

aa

Zygotes

FIG. 14. "Checkerboard" diagram of the gene combinations (zygotes) formed when rabbits are mated both of which are heterozygous (Aa).

Mendelian ratio. How it is produced may be explained algebraically as the combination or product of two independent 3 : 1 ratios, thus: 3 normal + 1 rex 3 gray + 1 black 9 gray normal

3 gray rex + 3 black normal -f-

1

black rex

The checkerboard explanation of the genesis of the F 2 dihybrid ratio requires a 16-place board, since there are four different possible kinds of eggs and as many kinds of sperm, all equal in probability of occurrence. Let R stand for normal and r for rex, A for agouti and a for non-agouti. Then we obtain the assortment of combinations shown in Fig. 15. A third unit character of rabbits occurs in the two alternative states intense or dilute melanin pigment. In the dilute state the pigment granules are not uniformly distributed throughout the cells of the hair but are clumped in certain parts of the cells. The consequence is that a dilute black rabbit has the coat color of a (Maltese) blue cat and is called a "blue." A dilute gray coat may be called a blue-gray coat. As dilution is recessive to intensity, a cross of blue with black

i8

MAMMALIAN GENETICS

produces black in F x and three blacks to one blue in F 2 , a regular unifactorial Mendelian result. If, now, we make this cross simultaneously with the two already discussed, as by mating a dilute gray non-rex rabbit

ΐ-ίΡ

AR

Ar

AR

Ar

aR

ar

AR

Ar

aR

ar

AR

AR

AR

AR

AR

Ar

aR

ar

Ar

Ar

Ar

Ar Z y ^ o t es

Sperm

aß ar

AR

Ar

aR

ar

aR

aR

aR

aR

AR

Ar

aR

ar

ar

ar

ar

ar

FIG. 15. Checkerboard diagram to show the gene combinations expected in F 2 when the F j parent rabbits are heterozygous for two independent pairs of genes, A vs. a, and R vs. r.

with an intense blac\ rex, we have a trihybrid or three-factor cross to deal with. Its expected result may be calculated algebraically by taking the combined products of all three single crosses, i.e., (3 normal + 1 rex) X (3 gray + 1 black) χ (3 intense + ι dilute). W e have already obtained the product of the first two of these factors in discussing the dihybrid F 2 ratio.

MENDELIAN INHERITANCE

19

(See page 17.) W e may now multiply that product by the third factor to obtain the trihybrid ratio, thus: 9 gray n o r m a l + 3 gray rex + 3 black n o r m a l 3 intense + 1 dilute

1 black rex

27 intense gray n o r m a l + 9 intense gray rex 9 intense black n o r m a l -j- 3 intense black rex -f- 9 dilute gray normal -)- 3 dilute gray rex -f3 dilute black normal -(- 1 dilute black rex

T h e checkerboard demonstration would require, in this case, a 64-square setup, there being eight different kinds of gametes to be supplied by either parent. If we let D stand for intense and d for dilute, the eight possible kinds of gametes are A R D , A R d , A r D , A r d , aRD, aRd, arD, and ard. This follows from the consideration that each of the four kinds of gametes shown in Fig. 15 might, with equal probability, occur in association with D or with d. If we were to add a fourth unit character to the cross, we should need, in the algebraic calculation of the expected result, merely to introduce a fourth (3 + 1) factor. T h e checkerboard exposition would now be too cumbersome for use, as the different kinds of gametes would amount to 16, and the checkerboard would contain 16 X 16 squares. Accordingly, the algebraic method of calculating Mendelian expectations, the method followed by Mendel himself, is altogether the simplest and most practicable. It is sufficient to bear in mind that 3 + 1 is the fundamental Mendelian ratio (three dominants to one recessive in the F 2 generation), and that this enters as a factor into the expected F 2 result as many times as there are independent unit character differences between the parents. REFERENCES CASTLE, W . E., 1930. T h e Genetics of Domestic Rabbits.

31 pp., 39 figs.

Cambridge, Mass.: Harvard University Press. MENDEL, GREGOR, 1866. Experiments in Plant-Hybridisation (Royal Hort. Soc. trans.). 41 pp. Cambridge, Mass.: Harvard University Press, 1938. NACHTSHEIM, Η.,

1936.

Vom

Wildtier z u m

Hausüer.

100 pp., 50

figs.

Berlin: A . Wegner. PICKARD, J. N., and F. A. E. CREW, 1931. T h e Scientific Aspects of Rabbit Breeding. 122 pp., 12 plates. London: Watmoughs.

CHAPTER III MODIFIED MENDELIAN RATIOS that a homozygous dominant combination can be distinguished by its appearance from the heterozygote. This is because two doses of a gene may be more effective than a single dose in giving expression to a character. A n example in point is the English pattern of spotting in the rabbit. This is due to a dominant gene which inhibits pigment development in certain parts of the coat. The action of a homozygote is more strongly inhibitory than that of a heterozygote. The latter possesses, from a fancier's viewpoint, about the right amount of inhibition; the homozygote has too much and so does not show enough pigmented spots or those which are extensive enough to present a pleasing harlequin aspect. The breeder selects the desired type for propagation, and in consequence secures regularly litters containing 50 per cent of wasters. For the progeny of two heterozygotes approximate a 1 : 2 : 1 ratio of homozygous English (too white to suit the fancier) to heterozygous English (the type desired), to selfs (unspotted). The homozygotes and the selfs are discarded by the breeder, and the heterozygotes are kept to produce the next generation, when the same three classes in the 1 : 2 : 1 ratio reappear. (Compare Figs. 34 and 35.) T FREQUENTLY HAPPENS

I

Among breeders of shorthorn cattle, self red and self white are standard colors, as is also roan, a heterozygote between the other two. Reds by themselves breed true, as do also whites, but roans produce all three color varieties in the ratio 1 red : 2 roans : 1 white. Fashion changes from time to time in favor of one or another of the three types. Either red or white truebreeding strains can readily be established, but the roan type is unfixable. (See Figs. 89-91.) The blue Andalusian fowl is another classical example of a

MODIFIED M E N D E L I A N RATIOS heterozygous type. It may be produced as a cross between black and splashed white, but the blues, when bred together, produce only 50 per cent of blue offspring, the remainder being equally divided between blacks and splashed whites, both homozygotes. Another example of an unfixable (heterozygous) combination of alleles is found in the standard Dutch rabbit of fanciers. This is a heterozygote between two types of Dutch (white and dark), the desired intermediate type being obtained by a combination of the two extremes, both of which are considered undesirable when homozygous and are accordingly rejected. Breeding together animals of the desired type (heterozygotes), the fancier thus obtains a 1 : 2 : 1 ratio, the middle term alone approximating the standard type. (See Figs. 37-39.) A recognizable difference in appearance between homozygous and heterozygous dominant individuals thus permits modification of the fundamental 3 : 1 Mendelian ratio into a 1 : 2 : 1 ratio. There are also certain modifications of the dihybrid ratio, 9 : 3 : 3 : 1 , which should be mentioned. Sometimes one gene is ineffective except in the presence of another. For example, the albino rabbit with pink eyes and white fur is a recessive mutation from the original colored type. W e designate its gene c, its dominant allele necessary for the development of color, C. If we cross a gray rabbit with an albino whose parents were black, we obtain gray offspring in F x but in F 2 there occur three types, gray, black, and albino, in the ratio 9:3:4. The cross is a dihybrid one in which the constitution of the parents is as follows: Gray (AC)

X Albino (ac) Gray (AaCc) 9 Gray (AC) : 3 Black (aC) : 3 White (Ac) : 1 White (ac)

Ρ Fi F2

T h e last two classes are both white, and so indistinguishable, their sum being 4. Another modification of the dihybrid ratio, 9 : 3 : 3 : 1 , is found in cases where the last three classes are indistinguishable. The ratio then becomes 9:7. There exist three races of short-haired (rex) rabbits, all similar in appearance but due to different

22

MAMMALIAN GENETICS

genes. Each behaves as a recessive and gives a regular 3 : 1 ratio in crosses with ordinary rabbits. But if one crosses two of these races with each other he obtains normal offspring (not shorthaired) in F a . In the F 2 generation there is produced a considerable number of short-haired individuals, but one can not, without a breeding test, determine to which parental race they belong, since all look alike. In a certain experiment I crossed two of the races, which may be called r x and r 3 . The normal F x offspring would accordingly be heterozygous for both genes and so of the constitution Ri r x R 3 r 3 . The expected F 2 combinations are as follows: Normal ι Ri Ri R3 R 3 2 Ri Ri R 3 r 3 2 Ri ri R 3 R 3 4 Ri r i R 3 Γ3

Short (r3) i Ri Ri r3 r3 2 Ri ri r, r 3

Short (ri) ι Γι Γι R 3 R 3 2 ri ri R 3 r3

Short (ri and r3) Γ! Γι r3 r3

The last three classes will all be short. Numerically, the normals should be to the shorts as 9 :y. Let the reader verify the calculation in a 16-place checkerboard. In an F 2 population of 88 individuals, 55 were normal and 33 short. The expected numbers on a 9:7 basis are 49.5:38.5. The deviation from expectation is 5.5. The probable error (result of chance alone, i.e., of "random sampling" from a population of infinite size) for this number of young is 3.14. The deviation is only 1.7 times the P.E., and so not significant. In other words, this really is a 9:7 ratio, as expected. Another modified dihybrid ratio is occasionally obtained when only one of the single dominant classes is distinguishable, all the other classes being similar in appearance. This results in a ratio of 13:3. A good example of this 13:3 ratio is obtained in a cross between two all white and true-breeding varieties of fowls, one of which is dominant, the other recessive, in crosses with colored breeds. The white of White Leghorn fowls is due to a dominant inhibiting gene which interferes with the production of melanin pigment in the plumage, though occasionally a part or all of a feather may escape from the inhibition

MODIFIED MENDELIAN RATIOS

23

and become pigmented. As a rule, however, the entire plumage is white. In crosses with colored breeds the F x birds are white, and the F 2 generation consists of 3 white birds to one colored. The cross may be diagrammed thus: White ( I C )

X Colored ( i C ) White ( I i C C ) White ( I I C C ) + 2 White ( I i C C ) + Colored (iiCC)

Ρ Fi F2

The white of White Wyandotte fowls and of certain other white breeds is recessive and may be regarded as due to a mutation in the color factor ( C ) . A cross of such whites with a colored breed may be diagrammed thus: White (cc)

X Colored ( C C ) Colored (Cc) Colored ( C C ) + 2 Colored (Cc) + White (cc)

Ρ Fi F2

If, now, a cross is made between the two white breeds, we get in F i only white birds and in F 2 , 13 white to three colored, as follows: White Leghorn ( I C )

X White Wyandotte (ic) White (IiCc) 9 White ( I C ) + 3 White (Ic) + 3 Colored ( i C ) + 1 White (ic)

Ρ Fj F2

The nine white IC individuals are substantially White Leghorns as regards color, though only one of the group is expected to breed entirely true for that character; the three Ic individuals have two reasons for being white; they combine both types of whiteness; the one ic individual is Wyandotte white in character; the three colored individuals (iC) inherit color from the White Leghorn parent without its inhibitor, deriving absence of inhibitor (i) from the Wyandotte parent. A modification of the trihybrid ratio is obtained when three dominant characters combine to produce a single visible effect, but if any one of them is absent no discernible effect is produced. Then a ratio of 27 (triple dominants) to 37 indistinguishable recessives is obtained. Emerson obtained this ratio in certain crosses of maize. Another modification of the trihybrid ratio (27:9:28) was

24

MAMMALIAN GENETICS

reported by Bateson and Punnett in sweet peas. Colored flowers must contain the color factor ( C ) ; those which contain c are white. But a second factor ( R ) must be present with C in order to produce colored flowers, which are then red. If a third factor ( B ) is present, the color becomes purple, but Β is ineffective except in the presence of both C and R. In a cross involving all three genes, the F i plant will be C c R r B b in formula, and its color purple bicolor (the color of the wild sweet pea). F 2 is expected to consist of the following classes: 27 CRB + 9 CRb + 9 CrB + 9 cRB + 3 Crb + 3 cRb + 3 crB + 1 crb Purple Red White White White White White White T h e several white-flowered classes will, of course, behave very differently, one from another, in crosses with a colored variety, as they differ in genetic constitution. Some of them would produce progeny with colored flowers if crossed with each other — as, for example, Crb with cRb, which would produce redflowered plants.

CHAPTER IV L I N K A G E , AS I L L U S T R A T E D I N T H E STUDY OF RABBIT GENES TWO GENES lie in the same chromosome they naturally have a tendency to stay together in transmission. But if they lie in different members of the same chromosome pair they have a tendency to stay apart in transmission, since each will go into a different gamete. This is the explanation given for genetic linkage on the basis of the chromosome theory. But the fact that genes do show tendencies to association and repulsion in transmission was known before the chromosome theory was formulated. As early as 1906 Bateson and Punnett had discovered examples of what they called coupling and repulsion in crosses of sweet peas. They suggested as an explanation that certain gametic combinations in the cell products of the reduction divisions might multiply more rapidly than others and thus be produced in excess. Thus, in cases of coupling, combinations A B and ab (present in the respective parents) would be more numerous in the gametes produced than the recombinations aB and Ab. But Bateson and Punnett were unable to suggest any reason why such differences should exist in rate of multiplication of cells, and, in fact, there is very rarely any multiplication of cells subsequent to the reduction division, in the genesis of gametes. HEN

W

As soon as sex-linked inheritance was clearly explained on the basis of the chromosome theory it became obvious that this same theory also afforded a complete explanation of both coupling and repulsion. Let us examine some examples of linkage encountered in rabbit breeding. The number of chromosome pairs is here so large (22) and the number of known genes so small (19) that it is surprising that as many cases have been encountered as are

26

MAMMALIAN GENETICS

now known, namely, three cases each involving three genes, and one other involving two genes. For 15 of the known genes of the rabbit only two alleles each are known, dominant and recessive; but for the others, three, four, or even six alleles are Μ

W FIG. 16 (at left). Β and C illustrate Morgan's conception of the linear arrangement of genes (like beads) in the chromosomes. A and D show how the constitution of a chromosome may change as a result of a single crossover. FIG. 1 7 (at right). A pair of homologous chromosomes, a, before; b, during; and c, after a double crossover. (After Morgan.

Μ

c

known, as shown in Table 1, where they are listed in the order of their dominance. These are cases of multiple allelism in which a gene assumes more than two alternative forms. A gamete can transmit only one allele, a zygote can contain only two, because it arises from the union of two gametes, but it can contain any two of the same series if the proper kinds of gametes enter into its production. Certain of the genes listed in Table 1 are indicated by Roman numerals as belonging in the same linkage group. Group I contains the genes Β and C. They were shown to be linked (borne in the same chromosome) by the following experiment: A cross was made between black chinchilla (Bc ch ) and brown Himalayan (bcH) individuals, which can be expressed in a diagram thus: B cCh the alleles placed above the line having been supplied by one

FIG. Ι Ν. Natural gray Flemish Giant.

FIG. 19.

Steel g r a y Flemish Giant ( E n F,).

FIG. 20. Steel gray Checkered Giant ( E u F. E n e n ) . T h e s e three are different color varieties of the largest breed of rabbits.

FIG. 22. Blue (aa dd).

FIG. 23. Polish (cc), the smallest breed of rabbits, having white f u r and pink eyes.

FIG. 2 4 . Klack-and-tan

(a'A 1 )·

FIG. 26. French silver (argente de champagne), a multiple-factor mutation.

FIG. 27.

Chinchilla (c c h c c h ) .

FIG. 28. Chinchilla r e x (c c h c c h r x r a ) .

FIG. 29.

Sable (aa c c h c c h ) .

L I N K A G E IN

RABBITS

TABLE 1 K N O W N ( M U T A T E D ) G E N E S OF THE RABBIT Linkage group

IV

Alleles w

Phenotypes

A , a', a

gray, black-and-tan, black

I

B, b Br, br

black, brown normal toes, brachdactyl

I

C , CCH3, CCH2, CCHL, c H , c

D, d

fully colored, chinchilla 3 , chinchilla2, chinchillaj, Himalayan, complete albino intense, dilute

D w , dw

normal size, dwarf

II

Du, du d , du w E D , E , e>, e

unspotted, dark Dutch, white Dutch steel (with A w ) , gray (with A w ) , Japanese, yellow

II

En, en F, f Hi, H2, Ο

English (spotted), unspotted normal, furless haemagglutinogen 1 in blood, haemag. 2 in blood, no haemag. in blood

II

L, 1

hair length normal, long-haired (Angora)

III

R i , rx

hair length normal, short-haired (rexi)

III

R2> Γ2 R3) Γ3 Sa, sa V, ν

hair length normal, short-haired (rex 2 ) hair length normal, short-haired (rexe) normal coat, satin coat self-colored, self-white (eyes blue) Vienna White

Y,y

white fat, yellow fat

W, w

normal agouti (with A ) , wide-banded agouti (with A )

IV

I IV

parent, those below the line by the other parent. The F i animals thus produced were all black chinchillas and, of course, double heterozygotes, and their gametes should be of four classes, equally numerous, if no linkage exists between Β and cch or between b and cH. A backcross was made between F i and the double recessive, brown Himalayan parent race. This

MAMMALIAN GENETICS

28

produced four phenotypes corresponding to the four kinds of gametes, and of the frequencies indicated: Bc c h Black chin. 244

bc ch Brown chin. 134

BcH Black H i m . 109

bc H Brown H i m . 233

Total 720

The classes which represent new combinations or crossovers are the brown chinchilla and black Himalayan, which together number 243. The non-crossovers, black chinchilla and brown Himalayan, total 477. The crossover (or recombination) percentage (243 720) is 33.75 ± 3.97, which indicates linkage, since free assortment should give 50 per cent recombination. The P.E.* for a population of 720 individuals, where equality of two classes (as crossover and non-crossover) is expected, is 3.97. The observed departure from equality, 50 per cent, is 16.25, which is more than four times the P.E. and hence statistically significant. Another cross was made between black full color ( B C ) and brown chinchilla (bc ch ), which we may diagram thus: Β

C_

b

c ch

The young were black full color and, of course, double heterozygotes (BbCc c h ), this cross differing from the one previously described only in that it contains a different color allele. A backcross was again made to the double recessive brown Himalayan. The four classes of gametes which the F i animals are expected to produce are: BC Black full color 52

bC Brown full color 31

Bc c h Black chin.

bc ch Brown chin,

29

50

* The probable error of a crossover percentage is found by the formula P(i-P) P.E. = .6745 , in which Ρ is the observed crossover percentage and η

η the number of individuals. In the above case Ρ = .3375 and η = 720. Let the reader calculate the P.E.

L I N K A G E IN RABBITS

29

The young of these four phenotypes which were produced were numerically as indicated, the parental or non-crossover classes being again in excess. T h e sum of the crossover classes is 60; the sum of the non-crossover classes is 102. T h e percentage of crossovers is 37.0 ± 2.6. The crossover percentage, in this case, between full color and black is similar to that previously found between another color allele, chinchilla, and bl,ack. In the cross just described C and Β entered the cross in the coupling relationship. Another cross was made, into which they entered in the repulsion relationship, brown full color ( b C ) X black chinchilla (Bc c h ), which may be diagrammed Β

c'h

A n F i individual which was black full color (BbCc c h ) in appearance was backcrossed with the double recessive brown Himalayan. T h e four classes of young were again: Black 11

Brown 22

Black chin. 21

Brown chin. 15

but in very different proportions. T h e crossover classes were now black full color and brown chinchilla, the ones which were non-crossovers in the previous case. T h e crossovers ( 1 1 -f- 15 = 26) are now 37.6 ± 4-0 per cent of the total, 69, in good agreement with the coupling experiment. If we combine the results of these experiments with others designed to test the strength of linkage between brown and chinchilla (or one of its several alleles), we have a total of 1000 young of which 346 are crossovers, this being a percentage of 34.6 ± 1.07. In accordance with the chromosome theory, we conclude that the genes Β and C, and their alleles, lie in the same chromosome, constituting linkage group I of the rabbit. A third gene, Y , y, for white vs. yellow fat, shows linkage with both Β and C and so must be referred to the same linkage group. This is indicated by the experiment now to be de-

3o

MAMMALIAN GENETICS

scribed. A brown chinchilla rabbit having yellow fat was crossed with black Himalayans having white fat, a cross which may be diagrammed thus: b Β

y Ϋ

cch ^

The F x animals showed the three dominant characters, black, chinchilla, and white fat, two of which dominants were derived from one parent, one from the other parent. If no linkage existed between the three pairs of genes, we should expect eight classes of gametes to be produced by F x animals, all equal numerically; but if linkage exists, we should expect the eight classes to be very unequal. Largest should be the original parental combinations, or non-crossover classes, next the single crossover classes, and smallest should be the double crossover classes. A backcross between the F i and triple recessive individuals (brown Himalayan having yellow fat) showed what the relative frequencies of the eight classes of gametes actually were. Let Ν indicate a non-crossover class, X a single crossover class, and X X a double crossover class, in which last case breaks in the parental chromosomes occur both between b and y and between y and cch, forming recombinations bYc ch and Byc H respectively. Ν XX Χ X X Χ XX Ν BYcH + BycH + bYcH + bycH + BYcch + Bycdl + bYcch + bycch 42 ι 15 ι 13 13 6 37 In a population of 128 individuals, 79 were non-crossovers; 14 were single crossovers between C and Y ; 28 were single crossovers between Y and Β ; and 7 were double crossovers, occurring simultaneously in both situations. The total crossovers between y and cch are 14 + 7 = 21, or 16.4 per cent; the total crossovers between b and y are 28 + 7 = 35, or 27.3 per cent; the apparent crossovers between b and cch are 14 + 28 = 42, or 32.8 per cent; the known crossovers (counting a double crossover twice) are 14 + 28 + (2 χ 7) = 56, or 43.7 per cent.

FIG. 30.

FIG. 31.

FIG. 32.

H i m a l a y a n A n g o r a ( a n c H c H 11).

H i m a l a y a n r e x ( a a c " c H Γ] r , ) .

Chinchilla rex A n g o r a

( c c h c c h i r , r , 11).

FIG. 33. G r a y rabbit, produced by a m a t i n g of the two white rabbits s h o w n in Figs. 34 and 35 ( C c c h E e V v ) , heterozygous f o r three recessive characters.

FIG. 34. Y e l l o w chinchilla (c c h c c h ee).

FIG. 35.

Vienna White ( v v ) .

FIG.

E n g l i s h , the type favored by fanciers, a heterozygote ( E n e n ) .

FIG. 37. E n g l i s h , the undesired type, homozygous ( E n E n ) .

FIG. 38. English-Dutch, a combination resulting f r o m a crossover between E n g l i s h and W h i t e Dutch ( E n d u w d u w ) .

FIG. 39. Standard

exhibition Dutch, heterozygous Dutch alleles (du w du' 1 ).

for

FIG. 41. Homozygous Dark Dutch (du d du 1 1 ).

two

L I N K A G E IN R A B B I T S

31

These data show that, of the three genes, Β and C are farthest apart. Accordingly Y must lie between them, if the order of the genes is linear. A map to show these relationships may now be constructed, laying off distances B Y and Y C proportional to 27.3 and 16.4 respectively. But the sum of these two, 43.7, is greater than the observed crossover percentage between Β and C, which is 32.8. The discrepancy is due to double crossing-over which is not recorded when one notes only the relation of Β to C. A double crossover would merely remove Y from its position between Β and C, leaving them undisturbed in relation to each other. Hence, in practice, maps are constructed on the basis of observed crossover percentages from gene to gene, and the map grows in length as more and more loci come under observation. The total map distance for a chromosome may exceed 50 or even 100, but the crossover percentage between any two genes in that chromosome can never exceed 50, since that would amount to complete independence, which two material bodies lying in the same viscid thread could not be expected to attain. In this case our chromosome map will be: C

16.4

Y

27.3

Β

3^8 or as more commonly expressed C

Υ

Β

ο

I6.4

43-7

A second linkage group of the rabbit includes the genes English, Dutch, and Angora. English was shown to be linked with Angora by crosses between the two, both in the repulsion relationship and in that of coupling. Backcrosses of the double dominant F x with double recessive individuals produced the following classes: English Short

Repulsion Coupling

602 14

English Angora

90 64

Self Short

Self Angora

98 65

617 10

32

MAMMALIAN GENETICS

Combining the crossover classes in both experiments, we get 9 0 + 9 8 + 1 4 + 1 0 = 212 in a total population of 1560, or 13.07+ 0.85 per cent crossovers. Dutch was also shown to be linked with Angora by backcrosses of F i double heterozygotes with double recessives (Dutch Angoras), which indicated 173 crossovers in a total of 1213 young, this being 14.26 ± 0.96 per cent crossovers. T h e order of the genes and their distances apart would therefore appear to be as follows: Du En

ο

L

1.19

14.26

In a direct cross between Dutch and English, Dutch is recessive, but it has the effect of reducing the size and number of the English spots. In a backcross with Dutch, approximately equal numbers of English and Dutch individuals are obtained, there being in one experiment 274 Dutch to 263 English. In a total of 730 backcross young a single crossover between Dutch and English was recorded, this being a frequency of 0.1 per cent. This individual was homozygous for Dutch and heterozygous for English, as shown by its eye color, coat markings, and subsequent breeding capacity. W e conclude accordingly, that the genes for Dutch and English lie very close together in the same chromosome with Angora, from which they are distant some 13 or 14 units, English being nearer than Dutch to A n gora. This chromosome we will call chromosome II of the rabbit. A third chromosome contains two recessive genes for rex (short coat), r x and r 2 , which are complementary in character, since a cross between them produces normals. If the two genes were not linked, but recombined freely, we should expect to get in F 2 , nine normals to seven short-haired, for the four phenotypes and their expected frequencies would be: Normal

9

Short (ri)

3

Short (r 2 )

3

Short (both ri and r 2 ) 1

(See Chapter III.) But if they were completely linked (never

L I N K A G E IN RABBITS

33

recombining) we should expect to get equal numbers of normal and short-haired individuals, namely, Short ( n )

1

Normal

2

Short (rs)

1

The observed numbers are nearer to the latter than to the former ratio, being 134 normal to 143 short, a deviation from equality of only 4.5, the P.E. being 5.6. The conclusion was drawn that the r x and r 2 mutant genes were probably borne in the same chromosome. Individual tests of F 2 rex individuals led to the identification of animals which were homozygous for ri and heterozygous for r 2 (ΓιΓ^ 2 Γ 2 ). From them, doubly homozygous individuals were later obtained, r 1 r 1 r 2 r 2 in formula. These produced only short-haired (rex) young when mated with either the pure r x or the pure r 2 race. When the double recessive was mated with an Fx doubly heterozygous individual, the double recessive parent would produce gametes all r 2 r 2 . But the F x individual would be expected to form gametes of four sorts, the non-crossover classes being r^Ra and R x r 2 respectively, and the crossover classes being R i R 2 and ΓχΓ2 respectively. Both of the non-crossover classes and one of the crossover classes would produce short-haired young in combination with the double recessive, but the R i R 2 sort of crossover gamete would produce normals. These would equal half the total crossovers. In such a backcross population of 384 young, 33 were normals, the rest being short-haired. Accordingly, twice 33, or 66, would be the indicated number of crossovers, which is 17.2 per cent of the entire population. The map for this chromosome would accordingly be: Ri ο

R2 17.2

A fourth linkage group in the rabbit is indicated by studies of Dr. P. B. Sawin, who found that a gene causing wide banding of the agouti hairs is borne in the same chromosome as the agouti gene itself, with about 7 per cent of crossing over between them. More recently Sawin has found that a third mutant

34

MAMMALIAN GENETICS

gene, dwarf, a lethal, is also borne in the agouti chromosome, with about 20 per cent of crossovers between agouti and dwarf. What the order of the three genes is can not be stated until the crossover percentage between dwarf and wide band has been ascertained. Besides the genes which enter into the four linkage systems, there are eight other genes which, so far as present information goes, are not linked with any other known gene of the rabbit and so probably lie each in a different chromosome. They are the genes which have been designated Br, D, E, F , H x , R : i , Sa, and V (Table 1 ) . If these are all independent, we are acquainted with genes of the rabbit borne in 12 different chromosomes, the total chromosome number for this species being 22. There remains much to be done before we shall have all the chromosomes of the rabbit tagged with mutated genes which will aid the further study of rabbit genetics. In the foregoing discussion, no mention has been made of certain genes identified by Nachtsheim which are responsible for inherited paralysis and other nervous disorders. The linkage relations of these has not yet been investigated, for which reason they have not been considered in this connection. The question is sometimes asked why, in linkage studies, a backcross to the double recessive is preferable to an ordinary F 2 population. The answer is that a backcross shows the exact character of each gamete contributed to the backcross by the F i parent. For the F x parent is a double heterozygote and so should form four kinds of gametes. If the genes come into the cross in the coupling relationship A B -f- ab, then F x is AaBb, and its gametes are A B -j- A b aB + ab, the first and last being non-crossovers. The double recessive parent will contribute only ab to each zygote. Thus all backcross phenotypes A b or aB will have arisen from crossovers. But if an F 2 is produced, the same four phenotypes will occur in unequal proportions, and all except one of them will consist only in part of either crossover or

FIG. 42.

G r a y r e x , or " c a s t o r e x "

FIG. 44.

Black furless

(aa

(Ι*! L'I).

ff).

L I N K A G E IN RABBITS

35 non-crossover gametes. Thus the phenotypes in an unmodified 9:3:3:1 ratio will be 9 A B : 3AI) : 3aB : iab. But they will arise thus: 3aB 3 Ab ι Ab + Ab iaB + aB 2Ab + ab 2aB + ab

9 AB ι AB + AB 2AB + Ab 2AB + aB 2Ab + aB 2AB + ab This phenotypc will derive f r o m gametes

less

than half of which are crossovers (8 out of 1 8 ) .

These phenotypes will derive from gametes two thirds of which (4 out of 6) are crossovers.

ι ab ab + ab

This phenotypc alone will derive f r o m noncrossovcr gametes.

If the genes enter the cross in the repulsion relationship, then gametes A B and ab will be crossovers, and the F 2 A B phenotype will arise from gametes more than half of which are crossovers (10 out of 18); the Ab and aB phenotypes will arise from gametes one third of which are crossovers (2 out of 6); and the ab phenotype will arise exclusively from crossover gametes. It will readily be seen, therefore, that the dihybrid ratio will be very differently modified in a case of linkage, according as it involves the coupling or the repulsion relationship of the genes. The strength of linkage can nevertheless be measured with considerable accuracy from an F 2 population, though with less accuracy than by the backcross method. But frequently one does not have the double recessive form to work with, as in the early stages of my rex experiments; or artificial crossing may be difficult and laborious, as in plants normally self-fertilized such as peas, beans, and wheat. Then an F 2 population may be studied and the crossover percentage estimated with fair accuracy, using tables such as those of Fisher and Balmukand (1928) or Immer (1930), based on the relation of the F 2 phenotypes, Ab X aB: A B X ab, i.e., the ratio of the products of crossover to non-crossover phenotypes. Interference. Studies of linkage in the rabbit, as well as those made on Drosophila, show that crossing-over may occur simul-

36

MAMMALIAN GENETICS

taneously in two different regions of the same chromosome, a thing which we call double crossing-over. In Drosophila, where the chromosomes are more elongated and more genes have been identified lying in a single chromosome, triple or even quadruple crossing-over can be detected. It was long since observed that in Drosophila crossing-over in one part of a chromosome interferes with crossing-over in adjacent regions. It probably serves to relieve the tension caused by twisting of the chromosomes about each other, and so to prevent additional breaks to some extent. In the rabbit chromosome, BYC, the crossover percentages observed in the first 477 cases studied were between Β and Y , 26.8 per cent; and between Y and C 14.4 per cent. If crossingover in each of these regions were independent of crossing-over in the other, i.e., were not influenced by it, we should expect simultaneous crossing-over in both regions to occur as the product of these two percentages, which is 26.8 X 14.4 = 3.8 per cent. This is the expected coincidence. The observed coincidence is 13 cases in a population of 477, or 2.7 per cent, which is less than three fourths of the coincidence expected through chance alone. Grüneberg has recently shown that interference occurs also in the albino chromosome of the rat. Interference was first discovered and has been repeatedly observed in Drosophila, and apparently is of general occurrence in other animals also. REFERENCES W. E., 1924. Linkage of Dutch, English, and Angora in rabbits. Proc. Nat. Acad. Sei. 10: 107.

CASTLE,

1924a. Linkage between albinism and brown pigmentation in rabbits. Proc. Nat. Acad. Sei. 10: 486. 1926. Color inheritance and linkage in rabbits. Carnegie Inst. Wash. Publ. No. 337. 1933· Linkage relations of yellow fat in rabbits. Proc. Nat. Acad. Sei. 19: 947. 1936. Further data on linkage in rabbits. Proc. Nat. Acad. Sei. 22: 222. and H . N A C H T S H E I M , 1933. Linkage interrelations of three genes for rex (short) coat in the rabbit. Proc. Nat. Acad. Sei. 19: 1006.

LINKAGE IN RABBITS

37

IMMER, F. R., 1930. Formulae and tables for calculating linkage intensities. Genetics 19: 1 1 9 . MATHER, K . , 1938. T h e measurement of linkage in heredity. London: Methuen & Co. SAWIN, P. B., 1934. Linkage o£ "wide-band" and agouti genes. Jour. Hered. 25: 477.

CHAPTER V K N O W N GENES OF RODENTS O T H E R T H A N T H E RABBIT HE RODENTS have been more readily available for genetic study than any other group of mammals because of their small size, short life cycle, and fecundity, which make it possible for the experimenter to produce a succession of generations including large numbers of individuals in a comparatively short time and at small expense. The rabbit has been propagated in captivity for something like a thousand years, largely for its value as a meat producer, the original domestication having been made in southwestern Europe. The guinea pig was domesticated in South America in pre-Columbian times, also as a meat producer. The house mouse has been bred in captivity in the albino variety for many centuries, and later in other mutant color varieties, one of the oldest probably being the piebald black-and-white waltzing mouse of China and Japan. The propagation of mice in captivity was originally pursued purely as a pastime or fancy, with no economic end in view until the time of Pasteur, when white mice began to be used in the study of pathology. The same is true of the tame Norway rat, a species unknown in Europe until the seventeenth century. Like the house mouse it was first bred in captivity in the albino variety. More recently a piebald (hooded) variety and mutations to self black and self yellow have made their appearance and been taken into captivity. Other and more numerous mutations have been observed to occur in captive wild stocks or in laboratory stocks deliberately inbred in order to secure greater genetic uniformity. Students of pathology and nutrition have made rat, mouse, guinea pig, and rabbit much-used laboratory animals, and this fact made them available in numbers for the study of genetics.

T

TABLE

2

G E N E S OF T H E H O U S E Linkage group

Alleles w

MOUSE

Phenotypes 1

Ac, ac B, b

yellow (lethal), agouti white belly, agouti uniform, black-and-tan, non-agouti normal brain, absent corpus callosum black, brown

C, c ch , c H , c

colored, chinchilla, "extreme dilute," albino

Ca, ca

caracul, normal coat

D, d D w , dw Er, er F,f Gl, gl Hd, hd

intense, dilute normal size, dwarf normal ear, hound ear normal tail, flexed tail normal, gray lethal normal, head dot

Hr, hr Hy, hy Hy2, hy2 J, j Ln, ln

normal coat, hairless normal, hydrocephalus normal, hydrocephalus normal, jittery intense, leaden

VI

Ν, η

naked, normal coat

I

Ρ, p

normal, pink eye and pale coat

V

P2, p2 p f , pf Pr, pr

normal, pink eye and pale coat (Roberts) parted frontals, normal normal, posterior reduplication

IV

R, r Re, re

normal, rodless retina rex coat, normal coat

III

S, s

self, piebald

Se, sc

normal, short ear

Sh, sh Sh2, sh2 Shs, shs

normal shaker normal, Zawadskaia shaker normal, shaker short

SI, si T , T 1 , t°, t 1

normal, silver normal tail, brachyuric ( T T 1 ) , tailless ( Λ 1 or T V ) normal, waltzer black-eyed white (lethal), unspotted normal coat, waved coat normal coat, waved coat

V

I VI II

III

II I

IV

Ay, A , A , a , a

V, ν W, w Wa, wa Wa2, wa2

4o

MAMMALIAN GENETICS

Lists of the k n o w n genes in the laboratory rodents (including Peromyscus) are given in Tables ι-6. A comparison of these shows that the same mutation (or one which produces a similar phenotype) has, in numerous cases, occurred in several of these species. In Table 7 is given a list of such gene mutaTABLE

3

G E N E S OF THE N O R W A Y R A T ( R a t t u s Linkage group

Alleles

norvegicus)

Phenotypes

Aw, a

agouti (white belly), non-agouti

II

A n , an

normal, anemic

II

B, b

black, brown

C , cr, c

colored, ruby-eyed, albino

C u , cu Cu2, CU2 D, d D w , dw H , h1, h H r , hr

curly coat, normal coat curly coat, normal coat intense, dilute normal, dwarf self, Irish, hooded normal, hairless normal, jaundiced

I II

J, j I

L,1

normal, lethal hypertrophied cartilage

I

Ρ, Ρ

normal, pink-eyed yellow

I

R, r

normal, red-eyed yellow

I

W, w Wo, wo

normal, waltzing normal, wobbly

tions as have occurred in two or more of the species. It includes 13 different types of mutation in genes possibly homologous in many cases but certainly not in all. Thus in the case of the house mouse we have two independent mutations to dilution (blue and leaden), these being neither allelic nor linked, but complementary, so that a cross between them produces normals. There are known also in the house mouse t w o independent mutations to pink eye, and two to a hairless condition ("hairless" and "naked"). These indicate that the same anatomical

RODENT

GENES

41

part or physiological process may be affected by different single gene agencies, resulting in apparently similar, but actually different, end products. The agouti gene apparently occurs in a common form ( A w , white-bellied wild type) in all six species compared in Table 7. TABLE

4

GENES OF THE BLACK RAT (Rattus Alleles

rattus)

Phenotypes

A w , A, a

agouti ( w h i t e belly), agouti uniform, non-agouti

C, c

colored, albino

D, d

intense, dilute

D

E , E, e

dominant black, recessive black, yellow

A mutation to non-agouti (a, uniform black) has been observed in all species except Peromyscus. A mutation to uniform gray ( A ) has occurred in three of the six species; black-and-tan (a') in two species. A mutation from black ( B ) to brown (b) pigment has occurred in all species except possibly the Black rat, though Patterson has described the occurrence of a cinnamon (brown agouti) mutation in a wild rat population of R. Alexandrinus in Texas. The color gene ( C ) has mutated to complete albinism (c) in all species except the guinea pig, where the Himalayan albino is the least heavily pigmented allele. T w o or three chinchilla alleles of the color gene are known in mouse, rabbit, and guinea pig, and one in the Norway rat. The mutation from ordinary intense pigmentation ( D ) to blue dilution (d) is known in four of the six species. The extension gene ( E ) , which cooperates with A w or A to produce a wild type of coat, has mutated to e (yellow coat or "restricted black") in rabbit, Black rat, Peromyscus, and guinea pig. It has mutated to E D , an intense black obscuring the agouti marking when A w is present, in Black rat and rabbit; and has produced curious inherited mosaics of black and yellow in the

42

MAMMALIAN GENETICS

Japanese rabbit (e 1 , an apparent mosaic of E D and e) and the tortoise shell guinea pig (e', an apparent mosaic of Ε and e). A mutation in which the tail is flexed through fusion of adjacent tail vertebrae occurs in mouse and Peromyscus. Mutations to hairless or furless have occurred in rabbit, mouse, Norway rat, and Peromyscus; in the mouse a second (dominant) mutation to hairless, called "naked," has occurred. TABLE

5

G E N E S OF PEROMYSCUS Linkage Group

Alleles

B, b I

I

ch

Phcnotypes

gray, cinnamon

C, c , c

fully colored, chinchilla, albino

E, e

black, yellow

E p , ep

normal, epileptic

F,f

normal tail, flexed tail

H r , hr

normal coat, hairless

N u , nu

normal coat, postjuvenile nude

ρ, ρ S, s

fully colored, pallid (red eyes and pale coat)

V, ν

normal gait, waltzing

self, spotted (white on tail, feet, and belly)

In Peromyscus has occurred a second (temporary) hairless mutation called "post-juvenile nude." A mutation to long silky hair (Angora) has occurred in rabbit and guinea pig. A mutation to pink (or red) eye and pale coat has occurred in mouse, Norway rat, Peromyscus, and guinea pig. A second phenotypically similar mutation in a different gene has occurred in mouse, Norway rat, and guinea pig (p 2 , r, and sa, respectively). In the rabbit three independent mutations have occurred to short hair and curly vibrissae. Mutations which affect the vibrissae in a similar way and modify the coat in lesser degree are known in mouse and Norway rat, four distinct mutations in the mouse (two dominant, two recessive), three in the rat (two dominant, one recessive).

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RODENT GENES

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A gene or genes which in their mutant form produce white spots in the coat are known for all species except the Black rat. In the rabbit there are certainly three distinct genes for white spotting, two recessive and one dominant in their mutant forms; in the mouse there are at least four, three recessive and one dominant. A silvered coat (with scattered white hairs) occurs as a domiT A B L E GENES Alleles

A w , A, a B, b C, c k , c d , c r , c H E , e1, e F,f L,1 M,

m

Ρ, p Px, px R,r S, s Sa, sa

6

OF T H E G U I N E A

PIG

Phenotypes

agouti (white belly), agouti uniform, non-agouti black, brown colored, two pale types, chinchilla, albino black, brindle, yellow normal red pigment, dilute red pigment short hair, long hair rough hair inhibitor, normal dark eye, pink eye polydactyl (semi-dominant lethal), normal rosetted (rough) coat, smooth coat self, white spotted dark eye, salmon eye

nant mutation in rabbit and Peromyscus, probably due to one principal gene with modifiers increasing or diminishing its somatic expression. In the mouse, silvering is reported to be recessive but likewise dependent upon independent modifiers for its full expression. Dwarf mutations have occurred in rabbit, mouse, rat, and guinea pig. In addition to parallel mutations occurring in two or more species of tame rodent, certain mutations are as yet known to have occurred in one species only. Such in mice are ( 1 ) tailless, and (2) rodless retina resulting in complete congenital blindness. Mutations peculiar to the rabbit are ( 1 ) yellow fat

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MAMMALIAN GENETICS

(known, however, also in sheep), (2) blood-group mutations (allied, however, to related mutations in man and several other mammals), and (3) a modifier of agouti (wide banding). Peculiar to the guinea pig are ( 1 ) the dominant mutation rosetted (rough) coat, and (2) its likewise dominant and partially inhibiting modifier, and (3) a dominant lethal mutation for polydactylism. On the whole the cases of parallel mutation are much more numerous than those of unique occurrence. Also recurrence of the same mutation in the same species is being reported with increasing frequency. In a colony, of captive wild rats inbred by Dr. King at the Wistar Institute, not only have several new mutations been observed, but a majority of the gene mutations previously known have reappeared undoubtedly as de novo gene changes. REFERENCES i.

RABBIT

(See also Chapters II and III)

W. E., 1905-1919. Coat characters. Carnegie Inst. Publ. 23: 114, 288.

CASTLE,

1907. Color varieties. Sei. 26. 1 9 1 1 . Heredity in evolution and animal breeding. Appleton. and

H . D . FISH,

and

P. B. HADLEY,

1915. Black-and-tan.

AM.

New York:

D.

Nat. 49.

1915. English. Proc. Nat. Acad. Sei.

1.

C., 1 9 1 2 - 1 5 . Color. Jour. Genet. 2: 221; 5: 37.

PUNNETT, R . SCHULTZ, W . ,

1919. Color factors concealed in albinos made visible. Zeit. ind.

Abst. 19: 27. Pap, E., 1921. Color. Zeit. ind. Abst. 26: 185. ONSLOW, H . , CASTLE, W .

1922. Steel. Jour. Genet. 12: 91. E.,

1924. Japanese. Jour. Genet. 14: 225.

1925. Dutch. Jour. Genet. 16: 189. P U N N E T T , R. C., and M. S. P E A S E , 1925. Dutch pattern. Jour. Genet. 15: 375; 16: 197. KOSSWIG, C . ,

1927. Albino alleles. Zeit. ind. Abst. 45: 368.

NACHTSHEIM, Η . , CASTLE,

1929. Rex. Zeit. ind. Abst. 52: 1.

W. E., 1929. Mosaic coat patterns. J. Exp. Zool. 52: 471.

PICKARD,

J.

W.,

1929. Brown-black-angora mosaic. Jour. Hered. 20: 483.

1936. Black-blue mosaic. Jour. Genet. 38: 337. CASTLE,

W. E., 1933. Furless. Jour. Hered. 24: 81.

Ρ

F,

Pi

FIG. 62.

A

d i h y b r i d cross b e t w e e n

k n o w n as b l a c k h o o d e d .

a wild N o r w a y

rat a n d a t a m e

varietv

P, parents; w i l d g r a y at left, b l a c k h o o d e d at r i g h t .

F j a h e t e r o z y g o t e , g r a y like the w i l d parent, b u t w i t h w h i t e feet, an e v i d e n c e that it carries the recessive h o o d e d pattern. F^ the f o u r s e c o n d - g e n e r a t i o n classes of o f f s p r i n g . F r o m l e f t to r i g h t , g r a y self, g r a y h o o d e d , b l a c k self, a n d b l a c k h o o d e d , n u m e r i c a l l y as 9 : 3 : 3 : 1 . genes involved.

L e t the reader i d e n t i f y in T a b l e 3 the m u t a n t

FIG. 63. A trihybrid cross between a black hooded rat (top left) and a self yellow (top right). F 1 } one of the first-generation progeny, gray self. F 2 , the eight classes of second-generation progeny, from left to right, black hooded, black self, gray hooded, gray self, yellow self, yellow hooded, cream self, cream hooded. Expected proportions of the classes, 3:9:9:27:9:3:3:1. Formulae of the parents, aa hh R R and A A H H rr. Let the reader determine the formulae of the F, and the F 2 classes. Which will be true breeding, and for what characters only? (Consult Table 3.)

Ρ

R

F2

FIG. 64. A cross was m a d e between the two rats shown at the top,, a pink-eyed cream hooded rat and an albino (aa C C hh pp χ aa cc hh P P ) . T h e F t progeny w e r e black hooded as shown. T h e F 2 population included three phenotypes, cream hooded, black hooded, and albino, their proportions being close to the 3 : 9 : 4 ratio, slightly distorted by linkage between genes c and p, w h i c h are borne in chromosome I. L e t the reader express the genetic f o r m u l a e of the FI a n d the F 2 classes.

RODENT GENES

47

KEELER, C. E., a n d W . E . CASTLE, 1934. Blood groups. Jour. Hered. 2 5 : 4 3 3 ; Proc. N a t . Acad. Sei. 19: 92, 98, 403; 20: 273. SA WIN, P. W., 1934. W i d e band linked with agouti. Jour. Hered. 25: 477. KOPEC, S., 1 9 2 4 - 1 9 2 6 .

Significance of birth weight.

Jour. Genet.

14:

241;

1 7 : 187. HAMMOND, J., and A . WALTON, 1928. Genet. 20: 401.

Attempted hare-rabbit cross.

Jour.

II. MOUSE

KEELER, C. E., 1 9 3 1 .

T h e laboratory mouse.

Literature.

C a m b r i d g e , Mass.,

H a r v a r d University Press. LOEFFLER, L., 1932. Open-eye, recessive with overlaps. Zeit. ind. Abst. 6 1 : 85, 409. DUNN, L . C., 1933. Multiple alleles, agouti and albino. Jour. Genet. 3 3 : 443. 1934· Mosaicism in color. Jour. Genet. 29: 3 1 7 . PINCUS, G., 1929. Black-brown mosaic. J. E x p . Zool. 52: 439. FISHER, R. Α., 1930. Tricolor mosaic. Jour. Genet. 2 3 : 77. CREW, F. A . E., and L . MIRSKAIA, 1 9 3 1 . Hairless. Jour. Genet. 2 5 : 17. CREW, F . A . E., 1933. W a v e d recessive. Jour. Genet. 2 7 : 95. KEELER, C. E., 1935. W a v e d 2 , recessive. Jour. Hered. 26: 189. CARNOCHAN, F. G., and L. C. DUNN, 1937. Caracul, dominant. Jour. Hered. 28: 333. CREW, F. A . E., 1939. R e x , dominant. Jour. Genet. 38: 3 4 1 . DANFORTH, C. H., 1927. Hereditary adiposity of yellow. Jour. Hered. 18: 1 5 3 . III. NORWAY RAT

ROBERTS, E., 1924-26. Hairless. Anat. Record, 29: 1 4 1 ; 34: 172. WILDER, W . , et al., 1932. Hairless. Jour. Hered. 23: 481. FELDMAN, Η. W., 1935. T w o independent hairless mutations identical.

Jour.

Hered. 26: 162. 1935a· K i n k y , recessive. Jour. Hered. 26: 252. KING, H . D., 1932. Curly, dominant. Proc. 6th intern. 2: 250.

Congress Genetics

BLUN, C. T., and P. W . GREGORY, 1937. Curly2, dominant. Jour. Hered. 28: 43. GUNN, C. H . , 1938. Jaundice, recessive. Jour. Hered. 29: 137. LAMBERT, W . V., and A . M. SCIUCHETTI, 1935. D w a r f , recessive. Jour. Hered. 26: 9 1 . CREW, F. A . E., and S. Κ . KON, 1933. A lethal a n e m i a ( P ) . Jour. Genet. 28: 25. SMITH, S. E., and R . BOGART, 1939. A n e m i a , recessive lethal. Genetics 24: 474. MOORE, L . Α., and P. J. SCHAIBLE, 1936. Hernia, recessive with overlaps. Jour. Hered. 27: 273.

48

MAMMALIAN GENETICS

HESTON, W. E., 1938. Bent-nose. Interaction of genes and diet causal explanation. Jour. Hered. 29: 437. SADONIKOWA-KOLTZOWA, M. P., 1928. Differences in temperament involve plural genes. Zeit. ind. Abst. 49: 131. KOLLER, P. C., and C. D. DARLINGTON, 1934. Chromosomes. Jour. Genet. 29: 159. IV. BLACK RAT

FELDMAN, Η. W., 1923. Dominant and recessive black. Sei. 58: 163. 1926. A w and A, yellow and blue mutations. Genetics 1 1 : 456. CREW, F. A. E., 1923. Yellow. Jour. Hered. 14: 221. V. PEROMYSCUS

CASTLE, W . E . , 1 9 1 2 . Albino. Sei. 35.

SUMMER, F. B., 1924. Hairless. Jour. Hered. 15: 475. 1928. Grizzled multifactor mutation. Am. Nat. 62. 1930. Genetics of 3 subspecies. Jour. Genet. 23: 275. 1932. Literature. Bib. Genetica 9: 1. BARTO, E., and R. R. HEUSTIS, 1933. White star. Jour. Hered. 24: 245. FELDMAN, Η. W. 1936. Piebald genes, 2 recessive, 1 dominant, which interact. Jour. Hered. 27: 301. HEUSTIS, R . R . , a n d E . BARTO, 1 9 3 2 .

Yellow.

Sei. 76.

1934· Brown and silver, independent recessives. Jour. Hered. 25: 219. 1936. Flexed tail, recessive, 2 factor? Jour. Hered. 27: 73. 1936. Trembling, recessive lethal. Jour. Hered. 27: 436. HEUSTIS, R. R., 1938. Ivory, recessive. Jour. Hered. 29: 235. CLARK, F. H., 1936. Pink-eye and albinism linked. Jour. Hered. 27: 259. 1938. Pectoral BufI Spotting. Jour. Hered. 29: 79. v i . GUINEA PIG

CASTLE, W. E., 1905-1914. Carnegie Institution Pub. Nos. 23, 49, 179, 288. and J. C. PHILLIPS, 191 I. Germinal transplantation. Carnegie Inst. Publ. 144. and S. WRIGHT, 1916. Carnegie Inst. Publ. 241. WRIGHT, S., 1915. Albino alleles. Am. Nat. 49. 1920. Piebald pattern. Proc. Nat. Ac. Sei. 5. 1923. T w o new color factors. Am. Nat. 59. 1925. Albino alleles. Genetics 10. 1934· Polydactylous, dominant lethal. Jour. Hered. 25: 359. WRIGHT, S., and Ο. N . EATON, 1923. Otocephaly. Jour. A g r . Res. 26.

SOLLAS, I. B. J., 1913. Dwarf. Jour. Genet. 3: 201.

CHAPTER VI HYBRIDIZATION AND HYBRID VIGOR which are very similar to each other can be successfully crossed. Ordinarily only individuals belonging to the same Linnean species can produce offspring, but occasionally species hybrids can be produced, as between horse and ass, resulting in offspring known as mules, normally incapable of further propagation, but themselves vigorous and valuable. This cross has been made continuously since early Greek and Roman times, and its produce, the mule, is the bestknown example among animals of a sterile hybrid. Species hybrids, in general, are intermediate between the parents in all characters such as size, shape, and proportions, but if the parents differ in a mutant character, i.e., possess different alleles of the same gene, then the inheritance for that character is typically Mendelian. Thus when a bay-colored mare is mated with a dull-colored ass the mule colt is bay in color but with more black in the coat than a bay horse possesses. The modification of the bay color may be regarded as due to the influence of genes introduced by the ass parent, but the bay color itself is due to dominance of the allele carried by the mare.

O

N L Y ORGANISMS

In crossing a wild Brazilian cavy, Cavia rufescens, with the guinea pig, C. porcellus, it was found by Detlefsen that the wild parent introduced an allele of the agouti gene different from that possessed by the guinea pig, light belly of the guinea pig being dominant over uniform agouti of the Brazilian cavy. But the expression of the character in the hybrids varied much, and it was only when the residual heredity had become largely guinea pig through repeated backcrosses with the guinea pig that the uniform agouti character was clearly and uniformly developed. This case shows that Mendelizing genes can be transmitted in species crosses as they are in matings between

50

MAMMALIAN GENETICS

individuals of the same species, but that the genetic background affects the degree of their expression here as elsewhere. The hybrids produced by this cavy cross had one remarkable feature. They were larger than either parent species in weight and skeletal dimensions. Such an occurrence is very common in species hybrids. It is known as hybrid vigor. It is not confined, however, to species hybrids, but is usually found in crosses between different breeds or varieties of the same species and so may better be called crossbred vigor. It tends to disappear in later generations if the crossbred individuals are inbred, i.e., mated with each other. It is a result of the union of two gametes which are unlike in their genetic constitution. As regards Mendelian genes, it is associated with a maximum amount of heterozygosis. On the implied supposition that genes alone are responsible for the phenomenon, it has been termed heterosis. Whether the greater vigor of the organism is due to the unli\eness of two sets of genes brought together in fertilization, or to dissimilarity in constitution of egg and sperm in other respects, is at present uncertain. The question merits investigation by means of merogony or some similar method. The cross of Cavia rufescens with the guinea pig, like the cross of mare with jackass, produces offspring which are invariably sterile in the male sex, but the female hybrids are regularly fertile with males of the parent species in the case of cavies, and very exceptionally in the case of mules. The F i hybrid males produce no sperm in either of these crosses, or in the similar crosses of domestic cattle with the bison, yet the F i females in both the cavy and the cattle crosses are fully fertile. It is possible, therefore, to backcross the female hybrid with the domesticated species and thus obtain %-blood guinea pigs or cattle, respectively. Again, the cavy hybrids are sterile in the male sex, though the females are fertile, thus making possible another backcross with the guinea pig. But the %-blood guinea pig hybrids were also sterile in the male sex, though a few of them produced motile sperm sparingly. A third backcross produced 15/16-blood guinea pig hybrids resembling guinea

HYBRIDIZATION AND HYBRID VIGOR

51

pigs closely in appearance, size, and psychology, and a few of such hybrids produced motile sperm and viable young. Enough of the characteristics of one species (the guinea pig) were now associated together to make a fully functioning organism. We know that in the case of maize and certain other plants, mutation in a single gene may produce male sterile offspring, a condition transmitted in the egg cell as a Mendelian recessive character. But such an explanation is not possible in this case. It is the hybrid character of the males that makes successful spermatogenesis impossible, not a sex-limited gene. The current explanation of the failure of species hybrids to reproduce sexually is that their chromosomes are incompatible, i.e., are so diverse in constitution that they refuse to conjugate at synapsis, and so their gametes, if produced at all, contain unbalanced assortments of genes and are inviable. The strongest support for this explanation is found in cases of polyploidy following hybridization, several of which are now on record.* These are the exceptions which support, if they do not prove, the rule of sterility following species hybridization. In such cases each chromosome derived from either parent becomes duplicated without the occurrence of cell division. Then, when the time comes for gametogenesis, each chromosome has a mate of like constitution (its duplicate) with which to conjugate, and thus the hybrid condition is transmitted in its entirety in each of the gametes formed. A constant hybrid thus results. It is characteristic of such fertile species hybrids that they grow with unusual rapidity and are large and vigorous. They manifest the same hybrid vigor as the mule, but without its sterility. A cavy species cross showing a maximum amount of hybrid vigor in the F! generation, but without the usually attendant sterility, was made by the writer some years ago. The male parent was a species of small wild cavy, C. Cutleri, obtained in Arequipa, Peru, which was crossed with the domestic guinea pig. The males of the guinea pig race employed in the cross weighed, when adult, about 800 grams, twice the adult weight * Exclusively among plants.

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of the Cutleri males. The Fi hybrid males weighed about 900 grams, and all their bone measurements were greater than those of the guinea pig. They were in every respect larger animals than either parent species. The Fi females showed a like superiority in size and vigor over their parents. But in the F 2 generation there was a notable decline in size. Males in this generation averaged about 700 grams in adult weight, as much less than guinea pigs as the Fx generation had been greater. This result conforms with the general rule regarding crossbred vigor. It is at a maximum in the F j generation and is rapidly dissipated in later generations if inbreeding succeeds the outcross. The gene theory explains hybrid vigor as due to heterosis, the bringing together in one zygote of a maximum number of unlike pairs of genes. For some reason, it is thought, pairs of unlike genes produce a more vigorous metabolism than pairs of like genes. Why this should be so we do not know. The number of unlike pairs of genes would theoretically be reduced by 50 per cent in each successive generation subsequent to F^ If, then, hybrid vigor is due to heterosis, we can see why it should rapidly decline in generations subsequent to Fi. But if hybrid vigor is due to unlikeness of constituents of the gametes other than chromosomes, a different formulation of the explanation must be sought. D. F. Jones has offered a gene explanation of hybrid vigor alternative to that already stated. He supposes that specific dominant genes are responsible for growth energy; that such genes are numerous and located in different chromosomes. A cross which brings together different sets of such growthpromoting genes makes all of them operative in F 1 } whereas in F 2 many combinations will arise containing only part of the dominant genes; hence the average growth energy of F 2 will be less than that of F^ To this hypothesis it may be objected that some, at least, of the F 2 segregates should contain the maximum number of growth-promoting genes and should breed true for that condition, whereas such F 2 and F 3 strains with growth energy equal to that of Fi do not occur. Jones

HYBRIDIZATION A N D HYBRID VIGOR

53

maintains that they do occur in rare instances, and he cites the King inbred rats as an example, which, however, is unconvincing, because, as Livesay and Castle and Pincus have shown, increased vigor of growth follows an outcross of the King rats, which, accordingly, can not be regarded as a race in which maximum vigor has been fixed as a genetic trait. Jones explains the rarity of occurrence of F 2 and F 3 families with vigor equal to that of F i by suggesting that certain of the dominant genes are linked, so that following a cross they tend to repel each other and emerge in different gametes. Thus the condition of maximum vigor is unfixable, or attainable only with great difficulty. The persistence of crossbred vigor undiminished in heteroploid species hybrids, generation after generation, and in asexually propagated hybrids, indicates strongly that such vigor is dependent upon gene constitution rather than plasmatic character, since there is no elimination of genes in the case of such hybrids alone, whereas the plasmatic relations are not different in this from any other category of hybrids. If hybrid vigor is dependent upon gene activity we are confronted with two possibilities. Either the gene activity of dissimilar pairs of genes (heterozygotes) is more energetic than that of homozygous pairs, or genes promoting activity occur in linked groups so that a maximum result is attainable only in heterozygotes. The former is the view originally advanced by Shull, and by East and Hayes; the latter view was advanced as a substitute for the former by D. F. Jones. In favor of the view that crossbred vigor is due to unlikeness of the uniting gametes may be cited the situation in some of the lower plants, where only gametes which are in some way unlike will unite to form a zygote. The difference between the uniting gametes, in the algae and fungi, is usually morphological as well as physiological, but in some cases only the latter can be recognized. Hartman finds that in certain marine algae union occurs only between zoospores which are respectively + and —, or between one which is more strongly plus and one which is less strongly plus in relation to the same minus strain. (See

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Fig. 65, Chapter VIII.) In other words, there must always be a sexual difference between the gametes which unite. Now, the consequence of a sexual union is to produce an organism more vigorous than either uniting organism would be by itself. The change is not one of mere increased bulk or cell size, as is shown by artificially produced polyploids in mosses, according to Wettstein, which, notwithstanding their increased cell size, may be less viable under natural conditions than the ordinary haploid forms. Crossbred vigor in the higher plants and animals marks a similar advance in vigor over unions in which the gametes are similar in genetic makeup. But gametes produced by the heterozygous F x individuals will, in consequence of recombination, be in general more alike than were the gametes which produced the F x individual itself. Hence a less vigorous stimulation to growth may be expected to result among the F 2 individuals, as is actually found to occur. REFERENCES W. E., 1916. Studies of inheritance in guinea pigs. Cavia Cutleri crosses. Car. Inst. Wash. Publ. no. 241.

CASTLE,

1926. The explanation of hybrid vigor. Proc. Nat. Acad. Sei. 12: 16. 1914. Studies of a cavy species cross. Car. Inst. Wash. Publ. no. 205. E A S T , Ε. M., and D. F. J O N E S , 1919. Inbreeding and outbreeding. Philadelphia: J. B. Lippincott Co. D E T L E F S E N , J. Α . ,

D. F., 1917. Dominance of linked factors as a means of accounting for heterosis. Proc. Nat. Acad. Sei. 3: 310.

JONES,

CHAPTER VII SELF-STERILITY; BIPOLAR SEXUALITY of self-sterility in plants and animals has certain aspects which remind us of the primary sex differentiation of green flagellates and of bread moulds, and other aspects which suggest hybrid vigor or heterosis. It has long been known that in certain species of flowering plants, even though the flowers are "perfect" — that is, produce both egg cells and pollen — nevertheless the individual plant is infertile to its own pollen, though it is fully fertile to pollen produced by other individuals of the same species. T o one who thinks teleologically, this is a device to secure cross-fertilization. But why, it may be asked, is cross-fertilization important? Selfsterility is less common among animals, even when they are hermaphrodite and produce both eggs and sperm simultaneously. Nevertheless self-sterility does occur in the group Tunicata, as I discovered many years ago (1896) when studying the embryology of Ciona. Plough (1932) has since shown that it occurs also in other tunicates, such as Styela. The occurrence of self-sterility depends upon a particular genetic constitution of the parent. In species of plants in which both self-fertility and self-sterility occur, self-sterility is commonly inherited as a recessive Mendelian character. This shows that in such species a single mutated gene is immediately responsible for the occurrence of self-sterility. Prell suggested the need of an "opposition factor" in the case of self-sterile plants, if successful fertilization is to occur. He suggested that in such cases, the pollen tube bearing a particular self-sterility gene must always meet, as its grows through the pistil of the mother plant, a different allele of that gene, never one identical with itself. Otherwise the pollen tube grows too slowly to accomplish fertilization. The slowness of its growth may preHE PHENOMENON

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vent its reaching the micropyle before the flower withers, or if there is foreign pollen also present, this will out-distance the growth of the plant's own pollen and reach the micropyle first. In either case self-fertilization is prevented. In line with this suggestion it was found by East and Mangelsdorf that in a self-sterile plant, if self-fertilized, the growth of the pollen tube occurs at a uniform rate, whereas if cross-fertilization occurs, bringing in a different allele, growth is accelerated. The regular reaction of pollen in plants not selfsterile is one of accelerated growth, hence this may be regarded as the normal relation. It has been lost by a gene mutation in self-sterile species, and only heterosis can restore it. In the case of a self-sterile individual of the tunicate Styela, Plough (1932) has shown that freshly discharged eggs and sperm will not unite with each other in fertilization, so that if foreign sperm is present, cross-fertilization occurs. But if foreign sperm is not present, after a time some change occurs in the discharged eggs — perhaps some inhibiting substance escapes from them — and if fresh sperm of the same individual is then added, fertilization will occur. Previous to the work of Prell and of East, Correns had shown the existence of group sterility as well as of individual self-sterility in certain Cruciferae, i.e., that a plant may be infertile not only to its own pollen but also to that of any plant of similar genetic constitution. East found the same to be true in Nicotiana. A l l plants bearing the same allele of the self-sterility gene are mutually sterile. East showed that there are multiple alleles (as many as 17 in one case) of the self-sterility gene. But an individual plant being diploid may carry two (and only two) self-sterility alleles in the tissue of its pistil, and it will be sterile toward pollen carrying either of these. For the pollen is haploid, although the maternal tissue through which it must grow to accomplish fertilization is diploid. Thus the maternal tissue may be s ^ in constitution (s being the self-sterility gene). The plant will then form pollen either Si or s2 in character, but neither sort can normally effect fertilization, since it will meet with its li\e in the maternal

SELF-STERILITY; BIPOLAR SEXUALITY

57

tissue. But foreign pollen, s3 in character, can effect fertilization in an s ^ plant. If the foreign pollen comes from a plant S]S3 in formula, only the s3 pollen will be effective, and the progeny will be either s ^ or s2s3 in character. If these two sorts are intercrossed, only the Si pollen of the first sort of individual and the s2 pollen of the second sort of individual will be effective, i.e., will not meet its like in the tissue of the plant pollinated. T h e SiS3 parent will produce s ^ and s2s3 progeny, the s2s3 parent will produce s ^ and SxS3 progeny, none of the progeny being identical in constitution with its mother plant. A self-sterility gene functions as a gene lethal to fertilization by failing to impart acceleration of growth to the pollen tube, such acceleration occurring normally in self-fertile species, but not in self-sterile species unless the allele borne in the pollen tube is different from those carried in the pistil of the plant pollinated. W h a t has been called bipolar sexuality occurs in certain fungi (basidiomycetes) in which two different genes have to occur in heterozygous combination in order that fertilization may occur. This differs from the ordinary condition of self-sterility in flowering plants in that a second and independent mutation has occurred which involves a physiological weakness, which can nevertheless be overcome by heterosis. There occur in such cases four classes of gametes which may be called ab, A b , aB, and A B respectively. Fertile unions can occur only between A B and ab, or between aB and A b , since these alone result in double heterozygotes. A t gametogenesis the zygote will give rise again to the four types of gametes, which in turn will recombine only in such ways as will reconstitute the double heterozygote. Finally sex determination in Habrobracon, according to W h i t i n g and to Snell, is a case among animals in which a heterozygous union of gametes shows its superiority over homozygous unions, in that it alone can call into action the chain of events leading to somatic femaleness (egg-production). W e shall refer to this case again later.

MAMMALIAN GENETICS

5«

REFERENCES et al., 1917-29. Studies on self-sterility. Genetics 2: 505; 3: 353; 4: 341; 1 1 : 466; 14: 455. Proc. Nat. Acad. Sei. 9: 166.

EAST, Ε . M . ,

Kniep, H., 1922. Ueber Geschlechtsbestimmung und Reduktionsteilung [in Basidiomytes] Verh. Phys.- Med. Gesell. Würzburg 47: 1. M O R G A N , Τ . H . , 1910. Cross and self-fertilization in Ciona. Arch. f. Entwick. 30. P L O U G H , Η . H . , 1932. Self-sterility in the Styela egg. Proc. Nat. Acad. Sei. 18: 131· P R E L L , Η . , 1921. Anisogametie etc. Arch. f. Entwick. 49: 463.

CHAPTER VIII SEX DIFFERENTIATION EX is ONE of the most general and fundamental characteristics of organisms both plant and animal. Sex differentiation is found in all except the lowest types of organisms such as the bacteria. In the unicellular algae it has perhaps its simplest expression, where swarm spores consisting of free-swimming, one-celled, green individuals, furnished each with a pair of flagella, unite in pairs to form zygotes. The uniting gametes are not morphologically different, but there is reason to think that physiologically they are different. Hartmann finds that among certain marine algae (Ectocarpus) the gametes fall into two classes, male and female, or minus and plus. Union occurs ordinarily only between a minus and a plus individual, but there are different degrees or grades of minusness and plusness, so that Hartmann distinguishes strong and weak male, and strong and weak female gametes. They form a continuous graded series, as shown in Fig. 65. Any female may unite with any male gamete. Hartmann finds that, exceptionally, union may occur between gametes of the same sex, provided they are sufficiently different in their sexual strength. Thus a strong female may unite with a weak female, or a strong male with a weak male — a phenomenon which Hartmann calls relative sexuality.

S

The occasion for the occurrence of sexual union between individuals among the algae is usually the onset of conditions less favorable for further asexual multiplication. Upon such a cell union there frequently follows a quiescent or resting stage which may continue until conditions are again more favorable for rapid asexual multiplication. The zygote has twice the bulk of the gametes which unite to produce it and frequently encysts or surrounds itself with a protective coat to enable it to survive

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an unfavorable period of heat, cold, or dryness. Thereafter, its twofold store of reserve materials, and a possibly heightened metabolic energy due to the dissimilarity between the gametes which united to produce it, enable it to assimilate and grow more energetically than would a single haploid individual. Sexuality is thus advantageous to the species in its struggle for existence. Otherwise we can not suppose that it would have arisen or persisted. There is reason to think that it has arisen

many times independently in the course of the evolution of plants and animals, for it has assumed a great variety of forms. Sometimes it occurs in regular alternation with asexual methods of reproduction; sometimes it becomes the exclusive method of reproduction; or it may fall into disuse again and be replaced once more by asexual or parthenogenetic methods of reproduction (as in the aphids, where sexual reproduction occurs only as an emergency measure). Or it may be lost altogether in organisms which become exactly adapted to a very special environment. Thus males no longer occur in many species of Entomostraca (among the Crustacea) and in many species of insects, all individuals being parthenogenetic females. Obviously,

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in such species sexuality at one time occurred, but has for the present been discontinued. The mothers still produce eggs, but the offspring are fatherless, there being no union of gametes in their formation, which is the essence of sexuality. What causes cells to unite we do not know, but one of the conditions essential to such unions is apparently a physiological dissimilarity between the uniting gametes. Hartmann has shown this to be true for algae, even when no morphological differences are discernible between them; and Blakeslee and others have shown that, in bread moulds and related fungi, mycelial growths originating from the same spore will not react sexually toward each other to produce zygospores, nor will they react thus with mycelia derived from any other spore of the same species unless it has a different sex reaction. A plus mycelium will form zygospores only when brought into contact with a minus mycelium, and vice versa. Since the plus mycelia are usually more vigorous vegetatively than the minus mycelia, the former may appropriately be designated female and the latter male. A plus mycelium of one species may give the characteristic sex reaction (attempted zygospore formation) with a minus mycelium of another species or genus, with which hybridization is impossible, but it will not give such reaction with plus mycelia of the other species. This shows that the same qualitatively plus and minus sex differentiation occurs in different species. Successful cell unions, however, are in general limited to those individuals which belong to the same species. Indeed, ability to unite sexually is one of the criteria of species identity. The uniting individuals, however, in addition to being of the same species, and thus in most respects very much alike, must be of different sex — one plus, the other minus. A zygote, or product of the cell union of gametes differing in sex, is a diploid organism. When it reproduces sexually, i.e., forms gametes, these will be haploid and of two sorts, plus and minus, or female and male. C. E. Allen (1919) has shown that the liverwort, Sphaerocarpos, in its sporophyte (diploid) generation produces four haploid spores from each spore mother

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cell. In the production of these spores, reduction to the haploid state occurs. When the four sister spores are germinated separately, it is found that two of them produce female gametophytes (egg-forming plants) and two produce male gametophytes (sperm-producing plants). There is a chromosomal difference between the two classes of spores. One sort (presumably the female) contains a large X-chromosome, the other sort (presumably the male) contains a small Y-chromosome. Sex determination here accordingly occurs very much as in Drosophila, by an X-Y chromosome apparatus, the difference between the two cases being that in Sphaerocarpos the haploid generation is dimorphic as to sex, but the diploid is monomorphic. In Drosophila both generations are sexually dimorphic, the haploids being distinguished as egg (female) and sperm (male) gametes; and the diploid generation as egg-producing and sperm-producing individuals, respectively. In most other plants, as in the liverwort, Sphaerocarpos, only the haploid generation is sexually differentiated into egg-producing (female) individuals and sperm-producing (male) individuals. The diploid generation or zygote arises by the union of an egg with a sperm. It thus contains the potentialities of both sexes united in one common type of individual. These two potentialities segregate at spore formation in Sphaerocarpos, as we have seen, and also in other plants with a heterothallic gametophyte generation, i.e., with separate individuals for the production of eggs and sperm. But there exist also plants with a homothallic gametophyte generation, i.e., which are not distinguishable as female or male, but which are capable of producing gametes of either sort. Thus the prothallus of the fern Aspidium, though haploid throughout, produces on its under surface, but on regionally separate areas, both antheridia (containing male gametes) and archegonia (containing female gametes). The only sexual differentiation recognizable in such cases is the primary one of gametes, either male or female. The same haploid tissue presumably exists throughout the soma of the prothallus, but in one region

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63

(the indentation of the heart-shaped prothallus) it produces archegonia containing eggs (female gametes), while in another region it produces antheridia containing sperms (male gametes). Since both sorts of gametes arise without chromosome reduction from the same haploid tissue, it must be that that tissue contains both potentialities, and the determination of which sort shall be produced must depend upon the internal environment, i.e., local conditions within the soma. A similar situation exists within the flower bud of diploid flowering plants. The more centrally located tissue produces egg cells contained within one or more pistils; a zone of tissue peripheral to the pistil produces pollen grains contained in anthers. The definitive gametes arise by reduction division in both cases, but the determination of which sort of gamete (male or female) is to be produced in each gonad antedates the occurrence of reduction. It obviously depends upon the internal environment. Before we proceed further to the discussion of sex determination, it may be well to sum up our ideas about sex differentiation. ( 1 ) This begins as a differentiation of one-celled flagellates, potential gametes all morphologically alike, into two groups physiologically different, plus and minus or female and male respectively. Ordinarily, cell union will occur only between two gametes of opposite sex, or at least between those which differ in the strength of their sexual reaction. (2) A second stage is reached when male and female gametes become morphologically — as well as physiologically — distinct. They are now recognized as eggs and sperm respectively. Unions occur only between egg and sperm. The same soma may give rise to both under different conditions of the environment internal or external. (3) A third stage is reached with somatic differentiation of haploid individuals into exclusive producers of eggs or of sperm. They are now called female and male individuals respectively. Such are the heterothallic liverworts, mosses, and fungi. A somatic chromosome difference may here exist between the sexes (Allen), or the character of the sex reaction (as plus or minus) may depend upon cytoplasmic differ-

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ences (Correns). In the cases thus far considered we are dealing exclusively with gametic (haploid) structures, either actual gametes or gametophytes. Diploid structures thus far show no sex differentiation. They are neutrals produced by the union of a plus with a minus gamete, and are transitory stages in the life history. But in the ferns and flowering plants, and in metazoan animals quite generally, the diploid stage acquires greater importance, becomes the predominating one, and finally acquires sexual differentation. The haploid generation now practically disappears as a distinct somatic existence and may be, as in Metazoa, represented solely by gametes formed directly at the reduction division. (4) The diploid soma may or may not be differentiated as male or female. In most flowering plants and in many Metazoa, such as snails, flatworms, and tunicates, there is a single type of soma which bears gonads of two sorts, male and female. The sex differentiation is here regional, not individual. In certain "proterandrous" animals and plants male gonads are first produced by the individual and later female gonads. Less often in certain plants the succession is reversed. External conditions, such as a deficient or a superabundant food supply, may prolong one stage indefinitely, and the individual thus becomes unisexual. A n internal change, either chromosomal or plasmatic, may act in a similar way and thus become sex determinative. Thus an internal X - Y chromosome apparatus converts us mammals, potential hermaphrodites, into male or female individuals, respectively, i.e., producers of eggs or sperm. Occasionally this or some other agency, internal or external, gets out of order, and the usual sex balance is upset, and a true hermaphrodite, or an intersex, is produced. Sex, then, may be said to be primarily a differentiation of potential gametes, one-celled organisms, along a physiological gradient, with a tendency for those sufficiently different from each other to unite in pairs. Many-celled organisms show sexuality (a tendency to cell unions) only when in the one-celled stage. Cells differentiated sexually so as to be capable of union are called gametes. Following such a union the tendency toward

SEX D I F F E R E N T I A T I O N

65

union is lost. Plus and minus differences have cancelled each other, and there will ordinarily be no further recurrence of the tendency to cell fusion until the diploid organism produced by fusion has returned to the haploid state by a reduction division. With the resumption of that state, sexuality reappears as plus and minus differentiation in the haploid segregants, which have a tendency to unite and so to cancel out their differences once more. As an exception to the generalizations just stated, diploid, and even polyploid, gametes have been artificially produced in mosses by inducing gametophyte production through regeneration from sporophyte tissue. This discovery we owe to the Marchals. Accordingly, sex differentiation in this case does not depend upon the existence of any particular chromosome complement, but is primarily a physiological state. Riddle (1932) maintains that the sex gradient (suggested by the observations of Hartmann) is one of metabolic activity, males in general having a higher oxidation rate than females. This is in harmony with an earlier and more general theory of Geddes and Thompson (1890), who held that females are more anabolic, males katabolic, in metabolism, as shown, for example, in storage of reserve food in the egg in contrast to the absence of storage and the high motility of the sperm. Observational evidence on metabolism in relation to sex is necessarily confined chiefly to diploid individuals differentiated as to sex. Riddle cites observations on basal metabolism (heat production) in human beings, albino rats, fowls, and two species of doves, showing a higher oxidation rate in relation to body weight in males than in females. He shows also that the oxygen carriers (erythrocytes) are either more numerous or contain a higher concentration of haemoglobin in males than in females, thus making a higher rate of oxygen consumption possible. Accepting the cytological evidence that a chromosome apparatus is active in sex determination, he regards it as probable that the chromosome differences become effective through their influence on oxidation rate.

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CASTLE, W. E., 1930. The quantitative theory of sex and the genetic character of haploid males. Proc. Nat. Acad. Sei. 16: 783. HARTMANN, Μ., 1930. Sexualität der Protisten und Thallophyten. Zeit. ind. Abst. Vererb. 54: 76. ALLEN, EDGAR (editor), 1932. Sex and internal secretions. 951 pp. Baltimore: Williams & Wilkins Co.

CHAPTER IX SEX D E T E R M I N A T I O N that sex is primarily a function of gametes so differentiated that they have a tendency to unite in pairs, plus with minus, sexuality, as we commonly think of it, relates not to the primary differentation of gametes as eggs or sperm respectively, but to the somatic differentiation of individuals as egg producers or sperm producers. By sex determination we refer ordinarily to such determination of the individual as a producer of eggs (female) or of sperm (male). This terminology is applicable whether the individual is a haploid organism, as in mosses and liverworts, or diploid, as in the higher plants and animals. Three different agencies have been recognized as operative in sex determination. They are ( i ) chromosomal, (2) environmental, and (3) endocrine — the last being largely restricted to vertebrates. Chromosomal determination of sex was first definitely established in the case of an insect, the squash bug (Anasa tristis), by Ε. B. Wilson, who showed that the male produces two types of sperm, containing respectively 1 1 chromosomes and 10 chromosomes. The matured egg, capable of fertilization, contains invariably 1 1 chromosomes. If it is fertilized by the 10chromosome type of sperm, a male zygote is produced having 21 chromosomes in each of its cells. If it is fertilized by an 11-chromosome sperm, a female zygote results containing 22 chromosomes. The odd chromosome is thus seen to be a female sex determiner. The probable significance of an odd chromosome (found in many insects) as a determiner of sex had been previously suggested by another American zoologist, C. E. McClung. It remained for Wilson to produce demonstrative evidence of its correctness. When, a few years later, Morgan discovered what he called OTWITHSTANDING THE FACT

N

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sex-linked inheritance of white eyes in Drosophila, he recognized that the odd chromosome of male insects would furnish a suitable basis for such inheritance, since it would be represented singly in males, doubly in females. He assumed, therefore, that the female Drosophila contains two X-chromosomes, which are sex-determiners and bearers of sex-linked genes, but that the male contains only one X-chromosome. This is indeed correct; but cytological study showed later that in the male there is a synaptic mate of the X-chromosome called the Y-chromosome, though this is distinguishable from the X morphologically, since it commonly has a hook on the end. Accordingly there is, strictly speaking, no odd chromosome in Drosophila, as there is in Anasa and some other insects. In Anasa the sexes are X X = female, X O = male. In Drosophila they are X X = female, X Y = male. But the X-chromosome in both cases functions as the bearer of sex-linked characters. The Drosophila type of sex determination and sex-linked inheritance occurs in most groups of insects and in mammals, where the male is the heterozygous sex and the female homozygous. See Fig. 66. The contrary type of sex determination and sex-linked inheritance, in which females are the heterozygous sex, occurs in moths among insects and in birds among vertebrates. Here the differential chromosome apparatus is usually called W Z = female, ZZ = male, but to render comparison easier, I shall in Fig. 67 retain the X-Y terminology, understanding by X, in both systems of sex determination, a chromosome which has a female determinative influence, and by Y one which has a male determinative influence. In moths and birds, the Y-chromosome carries sex-linked genes, whereas in Drosophila and man the X has this function. The X-chromosome of birds is probably not homologous with the X of mammals. For there is no reason to think that chromosomes retain an individuality in the process of evolution, since chromosome numbers vary so widely from group to group of animals and plants. The fact that both among insects and among vertebrates we have contrary types of sex

SEX DETERMINATION

69

determination makes it seem probable that the sexually separate state (dioeciousness) has evolved independently in both groups from an antecedent hermaphrodite or monoecious state. This is certainly true in the flowering plants, which are commonly monoecious and only in exceptional cases have be-

FIG. 66. Sex determination in Drosophila and mammals (weak X and males heterozygous).

come dioecious. When this has occurred, an X - Y chromosome apparatus can usually be demonstrated as the probable cause. It was the belief of Correns, who gave much study to the subject of sex determination in plants, that each individual of a species contains the potentialities of both sexes, whether or not it is functionally unisexual. In female flowers the rudiments of anthers usually occur, and in male flowers rudimentary pistils. Suppression of one sex function may call the other into action,

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as in the tropical papaya, in which decapitation of a male shoot may cause female shoots to sprout out laterally from the base of the wound, or in Melandrium, in which parasitic infection of the anthers by a smut fungus will cause functional development of pistils. The best experimentally supported theory of chromosomal

9

cr

FIG. 67. Sex determination in moths and birds (strong X and females heterozygous).

sex-determination is at present that of Bridges, known as the theory of genie balance. From the progeny of triploid Drosophilas, which transmit the various chromosomes in various combinations, some in duplicate and some single, it was possible to infer the sex tendency of each chromosome. The Xchromosome was found to have a strong female tendency which surpassed the net male tendency of the autosomes (the II, III,

SEX DETERMINATION

71

and I V pairs of chromosomes). Bridges states that if we rate the sex influence of the X-chromosome as -f- 100 (female), then that of the three autosomes jointly would be about — 80 (male). The following combinations have been identified by Bridges and his associates. Net tendency

X

A

+

3*

2

300

160

+

140

4

4

400

320

+

80

3 2 I

3 2 I

300

240

+

60

200 100

160

+ 4° -f- 2 0

Intersex

2

240

-

40

Ο.83

I

3 2

200

Male

100

160

-

60

Ο.63

3

100

240

— 140

Ο.42

Superfemale Female

Supermale

. ..

I

-

80

Ratio 1.88 I.25

* Obtained where 2 X ' s cling together in one gamete.

The combination 2 X 2 A is the normal constitution of a female, but X 2 A is a male (though sterile), and X Y 2 A is the constitution of normal males. Triploid individuals, 3 X 3 A , are also female, and so too are tetraploids, 4X4A. So long as an X is present for each set of autosomes, a female results. Even haploid tissue, X A , on mosaic individuals, is female, the anterior pair of legs lacking the sex comb. This bears out the generalization of Bridges that X has a female-determining tendency superior to the male determining tendency of a single set of autosomes. Yet two sets of autosomes, as already stated, have a male-determining influence superior to the female-determining influence of a single X . In moths and birds the X has an overwhelming influence in sex determination, its female tendency overbalancing the male tendency of a single Y . What the sex tendency of the autosomes is in these groups has not been ascertained as yet, but if their influence is held constant, X is more influential than Y , a female resulting. Yet in the absence of X , two Y's combined with the regular set of autosomes produce a male. A table like that of Bridges but omitting the autosomes might

72

MAMMALIAN GENETICS

be constructed thus, assuming the influence of X to be three times as strong as that of Y : Female Male

Χ

Y

+

ι ο

ι ι

100 ο

—

Net tendency

33 67

+ 67 — 67

Accepting a suggestion made by Winge, we might suppose that the two contrasted types of chromosomal sex determination may have arisen independently from a previous hermaphrodite state in which all chromsomes were paired, both members of each pair being alike. If, now by mutation in a chromosome pair with net female tendency, one member of the pair should lose that character or drop out altogether, a sex difference such as is found in Anasa and Drosophila would result between X X (hermaphrodite) and X O or X Y (male) individuals. But if the mutation occurred in a chromosome with net male influence, then a dimorphism like that of moths and birds might arise between Y Y (hermaphrodite) and X Y (female) individuals. With progressive differentiation, these might become exclusively male and female, respectively, in function. There is reason to think that in moths and birds the Y-chromosome (which carries more known genes than the X ) represents the original state of an equal chromosome pair, and that the X is a derived structure, which in some way has acquired a gene or genes with a sex-determining influence contrary to that of Y . It is k n o w n that in fishes, where both systems of sex determination occur, genes cross over from Y to X and from X to Y , so there can be little doubt that the X - Y pair represent the modified derivatives of an original equal pair. This interpretation is borne out also by the results of castration experiments in birds. T h e male has usually the more conspicuous and decorative plumage, the color of the female being protective or concealing in character. T h e common duck furnishes a good example. If a female duck is castrated, it loses its modest plumage and acquires that of a male. But no such change follows castration of the male. This shows that the characteristic species color is that of the male, and that this is

SEX D E T E R M I N A T I O N

73

merely inhibited or suppressed by hormones produced in the female, the inhibition being removed with the removal of the ovary. The X-chromosome complex would accordingly seem to be the more recent acquisition, the Y the older one possessed by both sexes. The haploid males of hymenoptera present a problem in sex determination difficult for the theory of genie balance to explain. For so far as chromosomal balance is concerned, they have exactly the same constitution as the diploid, but each chromosome is represented singly rather than doubly. It seems that genetically these haploids are female, for they produce only a single type of sperm, and this is female-determining, since the unfertilized egg develops into a male, but the fertilized egg develops into a female. There is only one true cell division instead of two in the spermatogenesis of hymenoptera. The first cell division is abortive and buds off a small cell devoid of chromosomes. A true cell division follows in which the chromosomes separate into two haploid groups, and two spermatozoa (not four) result. These are both female-determinative, since the egg, if fertilized at all, develops into a female; but if it remains unfertilized, develops into a male. (See Fig. 68.) It is possible to assume that in this group the cytoplasm has a net male-determining influence which a single set of chromosomes is unable to overcome, but which a double set of chromosomes does overcome, throwing the balance toward femaleness. This interpretation could be applied also to sex determination in certain scale insects (Schräder), where haploid males also exist and fertilized eggs develop invariably into females. But in another scale insect, Lecanium, according to Thomsen (1927), both male and female diploid individuals occur. Sex determination is here, as in Drosophila, by an X-Y chromosome apparatus, X sperm and Y sperm being produced. If, however, the egg develops parthenogenetically, it does not produce a haploid, but is restored to the diploid state by fusion of the haploid egg nucleus with a haploid second polar-cell nucleus, and thus becomes a female

74

MAMMALIAN GENETICS

( X X ) . This case gives a clue as to how haploid males have arisen, namely, by loss from a species, originally with sex determination as in Drosophila, of the capacity to produce Y sperm. The remaining sperm are exclusively X in constitution (femaledetermining) . The haploid individual is somatically male, per-

9

cf

FIG. 68. Sex determination in species having haploid males (weak X and neither sex heterozygous; cytoplasm with male tendency greater than female tendency of one X but less than that of two X ' s ) .

haps because the cytoplasm has an inherent male tendency, which a single set of chromosomes will not overbalance, but which two sets of chromosomes will overbalance. A further complication is met with in the parasitic wasp, Habrobracon, according to Whiting (1933). This species ordinarily produces females from fertilized eggs, males from unfertilized eggs. The latter are haploids and produce each a single type of sperm. But these haploid males and the sperm

SEX D E T E R M I N A T I O N

75

which they produce are of two types, either X + A or Y + A . Females, which are invariably diploid, are always heterozygous (like females of moths and birds). Their chromosome constitution is X -+- Y + 2A. If, at reduction, the egg becomes X + A , such an egg can ordinarily be fertilized only by Y -f- A sperm; but if, at reduction, it becomes Y + A , then it can be fertilized only by X + A sperm, a female resulting in either case. The homozygous diploid is ordinarily not produced, but occasionally one arises either from the fertilization of an X egg with an X sperm, or of a Y egg with a Y sperm. These are comparable with the homozygotes produced under exceptional circumstances by self-sterile plants (East, 1933). Ordinarily such zygotes can not arise, since only heterozygous combinations will enable the pollen tube to grow fast enough to accomplish fertilization, outdistancing homozygous combinations. We may assume that in Habrobracon also the cytoplasm has a male sex tendency so strong that neither X -f- A nor Y + A chromosome combinations (either haploid or diploid) can overcome it. Heterosis, however, resulting from the X + Y combination of chromosomes, either directly or indirectly through an altered metabolism, evokes the female developmental response. Whiting reports that the diploid males are produced only when the parents are closely related; i.e., when a minimal amount of heterosis is to be expected. Apparently like will unite with like gamete only when there is no competition with unlike combinations. This is similar to the case in self-sterile plants and in the tunicate, Styela. In support of the idea that in some species a definite sex tendency may inhere in the cytoplasm may be cited the observation of Correns that in reciprocal crosses of female and hermaphrodite individuals in certain species of flowering plants, the progeny are regularly like the mother in sex. Environmental determination of sex may occur in an organism potentially hermaphrodite but in which special conditions are necessary for one sex function to express itself. Thus hemp is a plant ordinarily dioecious and probably with an X - Y

76

MAMMALIAN GENETICS

chromosome apparatus for sex determination. But individuals which under ordinary conditions would function as males will function as females if an environmental condition — richness of the soil or length of day — is properly controlled. Thus Schaffner ( 1 9 3 1 ) found that plantings of hemp in the greenhouse from May 1 to July 15 gave normal sex ratios, but later plantings gave an increasing percentage of sex reversals of male to female, amounting, in November plantings, to 100 per cent such reversals. Similarly in maize, Schaffner showed that by control of light periodicity the tassel, normally staminate ( $ ) only, may become partly carpellate ( 2 ), or even neuter (vestigial). It is known that gene mutations may lead to similar effects (Jones, 1934), the responsible gene mutations being known as seed-tassel, male sterile, etc. Μ. M. Rhoades found that one race of "male sterile" inherited its peculiarity through the cytoplasm, not the chromosomes (fide Emerson). Endocrine control of sex is found particularly in vertebrates, but to some extent also in other animal phyla. Thus in the mollusk Crepidula individuals normally function first as male, then as female. But small individuals, if isolated, remain neuter. Yet in proximity to an older (female) individual, they develop sperm and become functional males. Some substance given off by the functional female stimulates spermatogenesis in them. A still more striking case of endocrine sex control in an invertebrate is that of the worm Bonellia, according to Baltzer. A n isolated individual develops into a female. But if larvae settle upon her elongated proboscis, they become attached to her body and function as parasitic males which impregnate the eggs of their host. Vertebrates in general are potential hermaphrodites having the essential organs of both sexes but with those of one sex inhibited, that is, only partially developed and so not functional, or else completely vestigial. In male frogs a Müllerian duct (oviduct) is present in addition to the vas deferens, but it is in a rudimentary state. A n X - Y chromosome apparatus is involved in sex determination in frogs, females being homozygous

SEX D E T E R M I N A T I O N

77

( X X ) as in Drosophila, males heterozygous ( X Y ) in constitution. But in most species both sexes pass through a female stage at metamorphosis, one in which ova are present in the gonad. Later these primitive ova disappear in the X Y individuals, and sperm is produced in the gonads. The X X individuals continue to function as females, producing mature ova. If, however, two tadpoles potentially of opposite sex (one X X , the other X Y in constitution) are united parabiotically, the male gonads of the one may so influence the development of the potential female that she too becomes a functional male. Such a change is called sex-reversal. It occurs, ordinarily, from the female to the male condition and under the influence of substances produced in the gonads of the male — an endocrine influence. The most complete demonstration of hormone influence in sex reversal has been found in cattle. Though long known to cattle breeders as the phenomenon of freemartin production, its full explanation was first made known by Lillie. A female calf born as a co-twin to a male calf is usually more or less completely changed as to sex, having male characteristics, but sterile. It is called a freemartin. Its primary (chromosomal) sexdetermined character is female ( X X ) , while that of its co-twin is male ( X Y ) . During development the two foetuses become closely crowded together, and their placental circulation becomes united, so that blood from one embryo passes directly into the other. Because male gonads develop earlier, male sex hormone from the male embryo inhibits development of the gonads in the other embryo into ovaries and modifies them into imperfect testes. The extent of the modification depends upon how early the intercommunication between the blood systems of the foetuses becomes effective. If, as in rare cases, it fails to occur, then the development of the female is entirely unmodified, as in other mammals in which males and females are produced in the same gestation. Endocrine glands other than gonads may also influence unequally the development of male and female sex characters, and may thus in a sense become sex-determinative. This is true,

78

MAMMALIAN GENETICS

in particular, of the thyroid and the pituitary glands. Anterior pituitary extract is able greatly to accelerate sexual maturity and egg production in mice. A mouse at the age of a month or six weeks can thus be made to produce ova several weeks earlier than normally and at three or four times the normal rate. Pituitary extracts act in a similar way on ovaries and on testes, as sex stimulators without which the gonads can not function. REFERENCES F., 1925. Untersuchungen an Bonellia. Pub. Sta. Zool. Nap. 6: 223. C. B., 1930. Haploid Drosophila and the theory of genie balance. Sei. 72: 405.

BALTZER, BRIDGES,

GOLDSCHMIDT, R . , JONES,

1927. Physiologische Theorie der Vererbung. Berlin.

D. F., 1934. Unisexual maize plants. Genetics 19: 552.

F. R . , 1917. The freemartin; a study of the action of the sex hormones in the foetal life of cattle. J. Exp. Zool. 23: 371. S C H A F F N E R , J. H., 1931. Sex reversal in hemp. Am. Jour. Bot. 18:424. W H I T I N G , P . W , , 1935. Selective fertilization. Jour. Hered. 26: 17. W I N G E , O., 1930. On the occurrence of X X males in Lebistes. Jour. Genet. 23: 69. LILLIE,

CHAPTER Χ SEX-LINKED INHERITANCE IN DROSOPHILA AND IN MAMMALS Drosophila has contributed more to the development of Mendelian theory than any other genetic study made since the rediscovery of Mendel's law. T o it must be credited ( i ) the demonstration that the genes responsible for Mendelian inheritance are located in the chromosomes, in linear order; and (2) that the coupling or repulsion which, under certain conditions, exists between different genes is due to the fact that such genes lie in the same chromosome pair; and (3) that the curious form of inheritance now called sexlinked is due to the fact that the genes of sex-linked characters lie in the sex chromosomes. These discoveries were made in the zoological laboratory of Columbia University by Τ . H . Morgan and his pupils, Bridges, Sturtevant, and Muller, in the decade of the World War, and have received merited recognition in the award to Professor Morgan of a Nobel prize. HE STUDY OF

T

The first use made of Drosophila in biological experimentation was in the Harvard Zoological Laboratory in the years 1900-1906. My pupils and I carried out a series of experiments directed chiefly toward a study of the effects of inbreeding. Flies were inbred, brother with sister, for 59 successive generations. At first the flies were raised on fermenting grapes, but later bananas were used with better success. Morgan, on taking up Drosophila breeding five years later, also used fermenting banana, then added agar to give it greater consistency, and later — when it had been shown by Baumberger that what the larvae live on is the yeast of fermenting fruit, not the fruit its e l f — Morgan devised the corn meal, molasses, agar mixture which is now employed in most laboratories. This small fly is popularly known as the vinegar fly, pomace

8o

MAMMALIAN GENETICS

fly, or wine fly, indicating its frequent occurrence where natural fermentation is in progress. In reality it feeds on the yeast which causes the fermentation. Its life cycle is very short, a generation being completed, under favorable conditions, in a fortnight or less. It is known to entomologists as Drosophila melanogaster Meig. D. ampelophila Low., now a synonym, was the name given to the species when I began raising it. Males are slightly smaller than females and have a conspicuous blacktipped abdomen, which gives the name to the species and serves readily to differentiate males from females. Other sex differentials easily demonstrable with a lens are the sex combs on the first pair of legs of the male. These serve as grasping organs when the males seize the females at mating. The males also have a genital pore ventrally located toward the posterior end of the abdomen, whereas on the female the genital opening is terminal. In order to examine the adults carefully, it is best to etherize them lightly. But if the etherization is too prolonged, they will not recover. When the wings are seen to stand out at right angles to the body, resuscitation has become difficult or impossible. Flies are among the most highly evolved insects. They have a single pair of functional wings and pass through a complete metamorphosis. From the Drosophila egg hatches a maggot or larva which, after feeding for two or three days, pupates within a straw-colored chitinous shell (pupa cover) and then, three or four days later, hatches out as an adult winged fly, with brilliant red compound eyes and a grayish body. Females are incapable of mating within 12 hours after emergence, but usually mate within the next 12 hours and are then ready to lay eggs if a suitable substratum is provided, which must be moist and smell of yeast. A female is capable of laying several hundred eggs. In our inbreeding experiments, fertility was measured by the number of pupae produced by a mother. The pupae were removed and counted before they had an opportunity to hatch. Hence few adults came under observation. When Morgan be-

SEX-LINKED INHERITANCE

81

gan breeding Drosophila, he directed his attention to the adults, and in 1911 reported on the discovery of a white-eyed male. This was a momentous discovery, for it revealed the method of sex-linked inheritance. Later Morgan and his pupils observed the occurrence of other variations, some of which, like white eye, are sex-linked, others not sex-linked, in heredity. In all, several hundred mutations have been reported in this species, which seems to be extremely subject to genetic variation and in which FEMALE

X

X

MALE

X

Y

FIG. 69. Diagram of the four pairs of chromosomes found in the female and in the male of Drosophila melanogaster. Pair I is designated X X in the female, X Y in the male. Pairs II and III are the long chromosomes above, and pair IV the small round ones at the center. (After Morgan.)

mutations can be induced by treatment with X-rays or other external agencies, as first shown by Muller. Drosophila melanogaster has four pairs of chromosomes, three of which consist of elongated rod-like structures, while the fourth pair consists of small oval elements. One of the large chromosome pairs consists of the sex chromosomes, X X in the female, X Y in the male. The form of Y is slightly different from that of X , having a hook on one end. The sex chromosomes constitute pair I, as it is called. The autosomes (alike in both sexes) are II and III, both long chromosomes, and IV, which is the small, rounded chromosome pair.

MAMMALIAN GENETICS

82

The white-eyed male fly discovered by Morgan was mated with normal females, but since white eye is a recessive mutation there were produced only normal red-eyed offspring. These were then bred with each other, and the expected 25 per cent of white-eyed descendants was obtained in F 2 , but all white-

Flies

Chromosomes

Gametes

FIG. 70. Sex-linked inheritance of white vs. red eyes in Drosophila. Parents, white-eyed male and red-eyed female; Flt red-eyed males and females; F 2 , redeyed females, red-eyed males, and white-eyed males, as 2:1:1. A black X indicates an X-chromosome bearing the gene for red eye; a white X indicates a bearer of the gene for white eyes; Ο indicates that an X is wanting; in later publications Morgan replaced it with a Y. (From Conklin, after Morgan.)

eyed individuals were males. (See Fig. 70.) When, however, these males were mated with F x females (which had a whiteeyed father), then white-eyed individuals were produced in both sexes in number equal to the red-eyed individuals. Morgan explained these facts as follows. The gene for white eye is borne in an X-chromosome. Its normal allele is a gene necessary for the production of ordinary red eye, and this is dominant

SEX-LINKED INHERITANCE

83

over white eye. The original male carried the white-eye gene in his single X-chromosome. This he transmitted to all his daughters but to none of his sons. T o them he transmitted Y , not X . But the daughters would receive both a red X from their mother and a white X from their father; and the former, being dominant, would give them red eyes. Nevertheless, they would transmit the white X in half of their eggs. Those eggs which were fertilized with X sperm would become females and red-eyed, half of them homozygous, half of them heterozygous. Those which were fertilized with Y sperm would become males. Such as got the white X would be white-eyed, the others red-eyed. But if one of these Fx females was mated with a white-eyed male, the expected result would be production of red-eyed and white-eyed individuals in both sexes, as was actually observed by Morgan. A further substantiation of Morgan's explanation may be obtained by crossing white-eyed females with ordinary redeyed males. Here the female carries only white X's; the male carries a red X . (See Fig. 71.) Accordingly, the white-eyed mother produces white-eyed sons and red-eyed daughters. This Morgan called crisscross inheritance. The F 2 generation consists of equal numbers of red-eyed and white-eyed individuals in both sexes, for the mating to produce it is of exactly the same nature as the backcross of white-eyed male with F j heterozygous red-eyed female. All the breeding tests which could be applied to the case thus supported Morgan's explanation, but no cytological evidence for it was yet forthcoming. This evidence was supplied by one of Morgan's pupils, Calvin B. Bridges, in his doctor's thesis (1916). He had observed, as early as 1913, exceptions to Morgan's "crisscross" inheritance in the transmission of white eye — cases in which white eye descended directly from mother to daughter or red eye descended from father to son. A further study of these exceptions led to the discovery of what Bridges called non-disjunction. If the two X-chromosomes of a whiteeyed mother fail, in certain of her eggs, to disjoin, and instead

84

MAMMALIAN GENETICS

remain clinging together and pass, at the reduction division, to the same pole of the spindle, then the egg ready to be fertilized will contain either two X's or no X. If the two-X egg is fertilized with Y sperm, a white-eyed female results, but it will be of unusual constitution. It will be a female because it con-

Flies

Chromosomes

6

fx? Χ a

χ

Parents

9

d

Gametes

9

: 2 X 9X

Μ π 9'

Fi Gnmeles

Χ XI

Fz

$

FIG. 71. Reciprocal cross to that shown in Fig. 70. Parents, red-eyed male and white-eyed female. F 1 ; white-eyed males and red-eyed females ("criss-cross inheritance" — Morgan). F 2 , equal numbers of red-eyed and white-eyed individuals in both sexes. (From Conklin, after Morgan.)

tains two X's, but it will also contain a Y . It will be white-eyed because both X's are white X's supplied by the mother. Bridges showed, by cytological demonstration, that these exceptional daughters actually do contain three sex chromosomes instead of the usual two. T w o X-chromosomes are present and also a Y-chromosome, in addition to the regular complement of six other chromosomes (pairs II, III, and I V ) . T h e other type (no-X type) of egg resulting from non-dis-

IN

I

1 0.0 1.5

YELLOW WHITE

5.5 7.5

ECHINUS RUBY

13.7 - - CROSSV'NL'SS

STAR

0.0

9 0

0.0

17

R0UCH0ID 0.0,; ;nBENT 05/ \ S H A V E N 0.9 EYELESS

TRUNCATE

14.0

STREAK

20.0 -- C U T

215

-

-

TAN

330

VERMILION

36.1

MINIATURE

25.3 SEPIA \ 25.8 : = HAIRY 29.0 - - DACHS

43.0 - - S A B L E

/

44.4

\

GARNET

46.5 - - BLACK 52.4--

56.5.

„FORKED

57.0'

"BAR

65.0

68.0

CLEFT BOBBED

FIG. 72. Linkage maps of Drosophila melanogaster showing only the more important mutant genes. Numerals show the calculated map-distance of each gene from the upper (zero) end of the chromosome. (After Morgan, Sturtevant, Müller, and Bridges.)

PURPLE

65.0+VESTIGIAL

38.5

DICHAETE

42.0

SCARLET

45.5

PINK

54.0 - - S P I N E L E S S 59.0 --

CLASS

63.5

DELTA

6 5 5

67.5 70.0

LOBE

73.5

CURVED

HAIRLESS EBONY

72.0 - - W H I T E - O C E L L I

8 6 5 - - ROUCH

97.5 • - A R C

\ /

98.5 -103.0 105.0

95.4

CLARET

95.7

MINUTE

PLEXUS BROWN SPECK

101.0

MINUTE-C

86

MAMMALIAN GENETICS

junction in a white-eyed female, if fertilized with X sperm of the red-eyed father, will produce a red-eyed male, like the father in appearance but lacking the usual Y-chromosome. Such males are sterile, showing that the Y-chromosome, though not essential to sex determination, is essential to certain physiological processes in the male. By evidence of this sort, both genetic and cytological, Bridges showed that the X-chromosome actually carries something which differentiates red-eyed from white-eyed individuals of Drosophila. This something we call a gene. After other sexlinked characters had been discovered, and attempts were made to combine these in a single race by means of crosses, evidence was obtained that each such gene is located in a particular part of the X-chromosome, and that they are arranged in a definite linear order. Chromosome maps of Drosophila indicating the loci of some of the more important gene mutations are shown in Fig. 72. Sex-linked inheritance is of rare occurrence among mammals, which indicates that in this group the X - Y chromosome pair, although of prime importance in genetic sex determination, carries few other genes subject to mutation. In rodents no cases of sex-linked inheritance have been observed, notwithstanding the intensive laboratory study which has been made of several species. In the cat, one sex-linked gene has been identified, yellow; and in cattle a dark mahogany red color has been described as sex-linked, but possibly is only sex-limited, i.e., owes its peculiarity to the agency of the male sex hormone rather than to a gene borne in an X-chromosome. In man, however, several cases of sex-linked inheritance are known, which indicates that the X-chromosome of the human species bears numerous genes other than those concerned in the determination of sex. Color blindness, night blindness, haemophilia, and Gower's muscular atrophy are well-known human mutations sex-linked in inheritance.

SEX-LINKED INHERITANCE

87

REFERENCES CASTLE, W . E . , a n d F . W . CARPENTER, A . H . C L A R K , S . O . MAST, a n d W .

M.

BARROWS, 1906. Effects of inbreeding, cross-breeding, and selection upon the fertility and variability of Drosophila. Proc. Am. Acad. Arts and Sei. 4 i : 731·

Τ. H., and C. B. BRIDGES and A. H. STURTEVANT, 1925. The genetics of Drosophila. Bib. Genetica II, 262 pp. Literature.

MORGAN,

MORGAN, Τ . H . , a n d A . H . STURTEVANT, H . J . MULLER, a n d C . B . BRIDGES, 1 9 2 3 .

The mechanism of Mendelian heredity. New York: Henry Holt & Co. MÜLLER, H. J., 1927. The problem of gene modification. Zeit. ind. Abst., Supb. i.

CHAPTER

XI

T H E I N H E R I T A N C E OF BLOOD GROUPS IN M A N AND RABBIT HEN A PERSON has lost much blood from a wound or a hemorrhage, his life may be endangered. It has long been the aim of physicians to replace extreme loss of blood by transfusion, i.e., by introducing into the depleted circulation blood from another individual or from an animal. But the blood from another species will not answer, because in its presence precipitins are formed which increase in amount and seriousness with increasing unrelatedness of the donor. Thus the amount of precipitin formation which occurs when the blood of one species is mixed with that of another has been found to be a criterion useful in classification. The closer the relationship between two species, the less the precipitin formation when their bloods are mixed. All human beings belong to the same species. There is no precipitin reaction when their bloods are mixed. But human beings differ in certain other properties of their blood, falling in this respect into four principal groups, known from their genetic differences as Ο, A , B, and A B . If blood from an individual of one group is transfused into an individual of an incompatible group, its corpuscles will clump together or "agglutinate," and the consequences may be fatal to the recipient. Consequently it is a rule in present-day hospital practice always to determine the blood group of any patient for whom transfusion is thought desirable and to introduce into his circulation only blood from an individual of the same blood group or of a compatible group.

W

Blood agglutination results from a two-fold agency: ( i ) an "agglutinogen" carried in or on the red blood corpuscles of the donor, and (2) an "agglutinin" carried in the blood plasma of the recipient. Reaction between these results in clumping of

I N H E R I T A N C E OF BLOOD GROUPS

89

the blood cells which bear the agglutinogen in the presence of blood plasma which bears the corresponding agglutinin. The agglutinogens of human blood which form the basis of the commonly recognized four blood groups are known as A and B, the corresponding agglutinins are known as a and b, a terminology not to be confused with that of Mendelian alleles. Agglutination will occur only when agglutinogen A meets agglutinin a, or Β meets b; not when A meets b, or Β meets a. Thus each agglutinogen has its specific agglutinin and is unaffected by any other. The occurrence of agglutinogens and agglutinins in human blood is as follows: Group

Agglutinogen

Agglutinin

Ο A Β AB

None A Β A and Β

a and b b a None

The relationship is a reciprocal one; if an agglutinogen is present, the corresponding agglutinin is absent, and vice versa. The two can not coexist or the blood would clump. For practical purposes we may disregard the agglutinins in the donor plasma and consider only the agglutinogens in the blood corpuscles. Blood containing an agglutinogen must not be transfused to an individual which lacks it. If this rule is followed, no incompatible transfusions will be made. Reaction between transfused plasma and red blood cells of the recipient is found not to occur, probably because the introduced agglutinin (if any) is neutralized by tissues other than the blood corpuscles. Thus A B blood may be transfused only into A B individuals; A blood may be transfused into A or A B individuals; Β blood, into Β or A B individuals; and Ο blood into any sort of individual. The Ο group is sometimes called the universal donor group; the A B group, the universal recipient group. As regards inheritance, the agglutinogens are inherited as dominant characters, A and Β being alleles. The agglutinins are not to be regarded as independently inherited characters but

MAMMALIAN GENETICS

purely as characters correlated with the agglutinogens — their reciprocals, so to speak. As regards their genetic properties: AB individuals form gametes A or Β (never AB)

A

Β Ο On this basis the expectation for any mating between individuals of the same or of different blood groups may readily be calculated. The mating A B χ Ο is a critical one, as, on the hypothesis stated, the children will all be different from either parent. They will be A or B. Blood tests on many thousands of individuals present only rare exceptions to the foregoing generalizations. These exceptions are found chiefly in the earlier data and are more likely due to faulty technique in classification or poor test sera than to any other single cause. Questionable paternity may also account for a few exceptions. The generalizations are now regarded as so well established that they are being used in legal medicine as evidence in cases of doubtful parentage. Blood groups being inherited in a simple way, it is to be expected that populations will differ in the relative frequencies of the groups. Among Amerindians the Ο group is all but universal; among Australian natives A and Ο are the commonest groups. Among Europeans A is commoner than B. Among Asiatics in general Β is commoner than A. The suggestion has been made that the Ο condition is primitive in man; that A is a mutation which occurred among the ancestors of Europeans; that Β is a mutation which occurred among Asiatics; and that AB's have resulted from racial crosses. But against any such explanation it may be said that similar blood groups occur among anthropoid apes. More probably, therefore, blood groups occurred among the common ancestors of man and anthropoids and have been handed down by inheritance to both.

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The study of blood groups in man was greatly stimulated by the World War, when blood transfusions were frequently required and blood group determinations were made on large numbers of soldiers. The suggestion that the agglutinogens were properties of blood cells inherited as simple Mendelian dominants led to a search for similar phenomena in other mammals. Several of my pupils looked vainly for blood groups in rabbits, rats, and mice. A step forward was taken by Landsteiner and Levine of the Rockefeller Institute, in 1928, when they showed that there exists in man another pair of agglutinogens, Μ and N , capable of demonstration by the method of immune sera presently to be described. These also are inherited as Mendelian alleles, each dominant over its absence, the two being able to coexist in the same individual, like the A and Β agglutinogens. Thus there are found M N , Μ, N , and Ο individuals, as regards these agglutinogens, and their occurrence is apparently unrelated to the occurrence of the A and Β agglutinogens. They have arisen through mutation in a different gene probably borne in a different chromosome. The employment of immune sera for the demonstration of agglutinogens led to a renewed attack upon blood grouping in animals other than man. Fischer and Klinkhardt in Germany, and Landsteiner and Levine in America, working independently, showed that two agglutinogens occur in the red blood cells of rabbits. A study of their inheritance was undertaken by Keeler and Castle at the Bussey Institution. The Rockefeller Institute generously supplied for this purpose a stock of tested rabbits and a small amount of test serum, with instructions on methods of preparing such sera. The procedure is as follows. A rabbit whose blood contains an agglutinogen serves as donor. A few cc of his blood are injected subcutaneously or into the body cavity of each of several other rabbits. If the recipient is of the same blood group as the donor, nothing happens. But if the recipient lacks the agglutinogen of the donor, then there arises, as an antibody, an

92

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agglutinin in the blood plasma of the recipient. If repeated injections are made (at least six to ten in number) at intervals of two or three days, the agglutinin attains sufficient strength to cause clumping of the corpuscles in any blood which bears the agglutinogen. Blood plasma of the recipient may now be drawn off and stored in glass tubes in a refrigerator and kept for months or years without losing its agglutinative property. By its use, a few drops of blood from any rabbit may be tested for presence or absence of the corresponding agglutinogen from its blood cells. The genes for the agglutinogens of rabbits' blood have been designated by us H i and H 2 . They are alleles of the same gene, as are respectively both the A and Β and the Μ and Ν of human blood. It seems a remarkable thing that these blood-group genes occur in pairs yet neither member of a pair interferes with the action of the other. There is no dominance of one over the other, as in ordinary alleles. Each has its specific action and develops its specific antibody, regardless of the presence of the other. They might be compared to stereo-isomeres of the same organic molecule. No third agglutinogen has in any case been demonstrated, though a modified A, called A 1 , has been found in man. Linkage studies of rabbit agglutinogen inheritance are thus far negative. They indicate that the Η gene is not located in any chromosome bearing another known gene. The agglutinogens are natural constituents of rabbit red blood cells. They are present and functional in unborn embryos even before the blood cells have lost their nuclei, since addition of the proper serum will cause such blood cells to clump. They can be demonstrated at all subsequent ages as constituents of the red blood corpuscles (or attached to them), but are demonstrable in no other tissue, unless disintegration of foreign blood containing them has occurred. The agglutinins are, in the rabbit, induced antibody components of the blood plasma, or if not produced entirely de novo, they have been raised in titer by injection of foreign blood to a level where they are able to

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93

produce agglutination of such blood. On the other hand, the a and b agglutinins of human blood are normal constituents of it sufficiently high in titer to cause agglutination of blood carrying either A or Β agglutinogen. Origin of such an agglutinogen by mutation must have been attended by suppression of the corresponding agglutinin; otherwise the mutation would have been lethal. Some light is thrown on this question by experimental results obtained from the rabbit studies. Agglutinins borne in the blood plasma pass freely through the placenta from mother to embryos in utero. This is shown by the following experiment. A rabbit of the Ο group is mated to a Ο male. The embryos will all be likewise O, since absence of an agglutinogen is recessive to its presence. The mother is given repeated injections of H j H , blood and develops hi and h 2 agglutinins, so that her plasma will agglutinate either H 2 or H 2 blood cells. These agglutinins are found to occur in similar titer in the embryos before birth or immediately thereafter. Hence they must pass from the mother's blood to that of her embryos through the placenta. If, in the foregoing experiment, the double recessive (O) mother is mated with a double dominant (H X H 2 ) buck, embryos are produced half of which are Hi and half H 2 in character. Now, when the mother receives injections of H i H 2 blood, she develops hi and h 2 agglutinins, as usual, and these presumably pass through the placenta as before. But if an examination is made of the newborn young, it will be found that the Hi individuals contain h 2 agglutinin but no hi agglutinin. The agglutinin which would cause clumping of an embryo's blood is missing. Presumably it has been disposed of, neutralized, or in some way counteracted in the embryo's body as fast as it was received, although the other (harmless) agglutinin is allowed to remain. If, in such a way, embryos are able to protect themselves against an agglutinin which, if of sufficient strength, would agglutinate their blood and cause them to perish, it seems probable that the absence of harmful agglutinins from human blood

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may be similarly accounted for. In Ο individuals both a and b agglutinins occur naturally in strengths sufficient to agglutinate foreign blood containing A or Β agglutinogens. But in A blood, the a agglutinin is missing, taken care of in some way, before it attains harmful titer. Similarly, in Β blood, b agglutinin is missing, and in A B blood both a and b agglutinins are missing. The rabbit experiment serves to explain also why, in human beings, a child of a blood group incompatible with that of the mother does not perish before birth. If the mother is Ο and the child A , why does not the a agglutinin of the mother's blood pass into the embryo and clump its blood ? The answer is that doubtless such agglutinin does pass into the embryo but is neutralized there before it attains harmful concentration, just as an A individual also keeps down any a agglutinin which its own blood would generate if A agglutinogen were not present. REFERENCES SNYDER, L. H., 1929. Blood grouping. 153 pp. Baltimore: Williams and Wilkins Co. KEELER, C. E., and W. E. CASTLE, 1932-1934. Blood group inheritance in the rabbit. Proc. Nat. Acad. Sei. 19: 92, 98; 20: 273, 465; Jour. Hered. 25: 433·

CHAPTER XII DOMINANCE; MULTIPLE ALLELES

I

N ORDINARY MENDELIAN INHERITANCE

One

of

tWO

alleles

IS

dominant over the other. The question suggests itself: Why is it dominant? Why does it alone find expression in the zygote? We can see why a difference should exist between a homozygous dominant and a heterozygous dominant, since in one the dominant gene is twice represented, whereas in the other it is represented only once. But why does the recessive count for naught ? Bateson suggested that it really is naught, that the recessive gamete is simply one which lacks altogether the gene present in the dominant gamete. Thus an albino animal would be one lacking the color gene indispensable for the production of pigment. This presence-absence theory encountered difficulty when it was found that two or more different kinds of albino recessives might occur in the same species. The rabbit furnished the first critical case, as was pointed out by Sturtevant in 1913. Here there are two kinds of albinos represented in the breeds Polish and Himalayan. Polish rabbits develop no pigment of any kind in any part of the body. Himalayans are also pink-eyed and white-coated, for the most part, but have pigmented extremities (feet, ears, nose, and tail). They have a low capacity for developing pigment, whereas Polish have none at all. Both sorts are born unpigmented and commonly remain unpigmented in the first coat. But if Himalayan young are chilled while the hair is developing, the first coat may be slightly pigmented a sooty white all over. In its second coat, attained at the age of two or three months, a Himalayan is regularly pigmented at the extremities of the body only, those being naturally the coolest parts of the skin, through loss of body heat by radiation. But if a patch of fur is removed from the back of a Himalayan rabbit (normally white) by shaving or

φ

MAMMALIAN GENETICS

plucking the hair, and the animal is then kept in a cold place while new hair grows on the denuded area, the hair comes in pigmented. These facts show that temperature affects the production of pigment in a Himalayan rabbit. A temperature lower than the normal internal body heat of a rabbit is necessary for the production of pigment in the coat. Himalayan rabbits, as commonly bred, are homozygous for non-agouti intense black, because such a genetic constitution develops the greatest amount of pigment in the coat and makes the characteristic markings most conspicuous. If a cross is made between Himalayan and Polish breeds, the F i young have Himalayan markings, and F 2 consists of three Himalayan to one Polish. This shows the Himalayan gene to be dominant over the gene for Polish albinism. Himalayan crossed with fully colored rabbits gives colored F x animals, and three colored to one Himalayan in F 2 . Polish crossed with colored rabbits gives colored in F l 3 and three colored to one Polish type in F 2 . The three genes form a genetic triangle.

Ö

FIG. 73. A genetic triangle involving triple alleles, full color ( C ) , and two different albino alleles, Himalayan ( c H ) and Polish (c).

Full color is dominant over both types of albinism. Each type retains its distinctive character and emerges unmodified from a cross with the ancestral colored type as well as from a cross between the two. Each is entirely stable and has, in all probability, originated independently of the other, not as a modification of it. The hypothesis is tempting that each has arisen as a

DOMINANCE; MULTIPLE ALLELES

97

different modification of the same complex organic molecule, which has in each case lost (or acquired) a different side-chain or other constituent. The stability of genes suggests that they have a very definite chemical constitution, subject to change only on rare occasions, and when so changed retaining the new organization as firmly as the old was retained previously. The presence-absence hypothesis of Bateson would account for the occurrence of a single type of albino, but not for two. It has accordingly been generally abandoned in favor of a hypothesis that alleles arise through change in a gene rather than by its loss. There are on record, however, a few cases in which a recessive seems to have resulted from a dominant gene actually lost from the genetic complex. One such case was discovered by W. H. Gates in the house mouse. The waltzing character of Japanese Waltzing mice is inherited as a recessive Mendelian character, but it is not due to a lost gene but to a changed gene, as Gates was able to show when an actual loss did occur. In a cross of a normal mouse with a waltzer, a waltzing individual was obtained in Fi, where only normals were expected. This surprising result would have led most of us to suppose that some mismating had occurred. But Gates had confidence in the accuracy of his records and pedigrees and investigated the case further. He found that this individual, a female, ( 1 ) when mated to a waltzing male (homozygous, of course), produced only waltzing offspring (21 in number), and (2) when mated to normal males, produced only normal offspring (13 in number). So far she behaved like an ordinary waltzer. But (3) when she was mated with one of her normal sons, she produced a mixed litter of five normals to two waltzers. Further investigation showed that her normal offspring were of two sorts, one of which transmitted waltzing while the other did not. This, then, is the explanation arrived at: In the original cross which produced the waltzing Fi female, the waltzing parent transmitted waltzing (v), but the normal parent transmitted, not the normal allele of waltzing (V), but nothing. In other words, that particular gamete (egg) lacked the V gene,

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possessing neither of its alleles (V or v). The F x female was accordingly of the constitution vo. Her gametes would be ν -{- o. Mated with a waltzer, the resulting combinations would be vv -f- vo. All would be waltzers, but half of them would be ordinary homozygous waltzers, carrying two doses of waltzing. The others would be like their mother, carrying only one dose of waltzing. Genetic tests proved this to be true. If the F i female were mated to a normal male, whose gametes were all V , the resulting combinations would be V v + Vo. All would be normal, but only half of these normals would transmit the waltzing gene. Genetic tests proved this to be true. One such test was a backcross with the mother. Both normal young ( V v ) and waltzing young (vo) were obtained, but no normals of constitution oo, which might be expected to occur, were discovered. Probably they are inviable. A cytological study was made by Painter of some of Gates's mice which lacked the waltzing gene. He concluded that the loss occurred by the breaking off of a part of the end of one chromosome which involved the complete loss of the V locus. Other genes may have been lost in the same chromosome fragment, without which a viable embryo could not be produced. Hence the failure of oo normals to be found. Unfortunately the chromosomes of the mouse are so numerous, so small, and so much alike that it is not possible to identify beyond question the chromosome which bears the V gene. The mouse has 20 pairs of chromosomes, as does also the Black rat. The Norway rat has 21 pairs, and the rabbit 22. When, in Drosophila, the bar mutation occurred, it apparently arose from the origin of a new genetic locus or a profound change in some previously existing locus. It behaves as a dominant in crosses with normals, which presumably lack the bar gene. But two doses of bar are more effective than one in modifying eye form, since homozygous bar females have narrower eyes than heterozygous females. Males, of course, ordinarily get only one dose of bar, since this character is borne in the X-chromosome. A true-breeding race of bar-eyed Droso-

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99

phila consists of females all homozygous for bar and males all bar-eyed, i.e., having the bar gene in their single X-chromosome. In such bar-eyed races of Drosophila, occasional reversion to normal has been observed to occur, and such reversions breed true, indicating that the bar locus has been lost again. This interpretation is supported by the observation that an extreme form of bar called ultra-bar or double bar arises occasionally in true-breeding bar stocks. This, again, reverts to normal or to ordinary bar with frequencies comparable with the reversion of bar to normal. Sturtevant and Morgan have shown that these various alterations in the bar character are commonly associated with crossing over in the region of the bar locus in the X-chromosome. Using forked and fused (adjacent genes) as markers, they have shown the probability that double bar and normal round eye result from the same process in a homozygous female, a crossing over in which the breaks in the X-chromosome occur on opposite side of the bar locus, so that one chromosome has two bar genes tandem, and the other has none. Thus double bar results in one type of derivative, no-bar (round) in the other. A further qualitative mutation in bar of a less extreme character, called infra-bar, has been observed. It occurred in a male individual and so was probably not due to loss or duplication. It is an allele of ordinary bar. Morgan regards its origin, as well as the original production of bar, as due to qualitative mutation in a previously existing gene, but the reversion to normal and the double bar production he regards as presenceabsence phenomena in Bateson's sense. The three alleles of the color gene of rabbits which have been described had long been known when a new color variety of rabbit (chinchilla) came to the notice of rabbit breeders soon after the World War. The chinchilla rabbit resembles the wild gray type in having black-pigmented agouti-banded hairs, but differs from it in containing little or no yellow pigment in the agouti bands. These, accordingly, are white rather than yellow, as are also the ventral surfaces and the nape of the

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MAMMALIAN GENETICS

neck. The chinchilla rabbit may be described as a squirrel-gray rabbit. When chinchillas are crossed with albino rabbits, either Polish or Himalayan, chinchilla offspring are obtained in F 1 } and 3 chinchillas to ι albino in F 2 . Chinchilla is dominant over albino types, exactly as Himalayan is over Polish. When chinchilla is crossed with gray, the latter is found to be dominant in F l 5 and a good 3 : 1 ratio of gray to chinchilla is obtained in F 2 . Accordingly, it seems that chinchilla is an albino allele intermediate in position, as in expression, between full color and Himalayan albinism. In my original studies of chinchilla ( 1 9 2 1 ) I was able to distinguish two different chinchilla alleles, one much darker than the other. In a subsequent study, my pupil, Dr. Sawin, was able to distinguish a third intermediate between the other two. Thus the albino series of alleles in rabbits, as at present known, consists of six members which, in order of dominance, are: ( 1 ) full color; (2) dark chinchilla; (3) intermediate chinchilla; (4) pale chinchilla; (5) Himalayan albinism; (6) complete (Polish) albinism. They form a graded series. Polish is unable to produce pigment. Himalayan produces only black pigment, and that sparingly and only at low body temperatures. Chinchilla produces black pigment in considerable abundance, but no yellow ordinarily. Full color produces both black and yellow pigments abundantly. Dominance is not in all cases complete. Thus pale chinchilla heterozygous for albinism (either Polish or Himalayan) produces a type much paler than homozygous chinchilla. Non-agouti pale chinchilla heterozygous for albinism is known in the rabbit fancy as sable (German, Marder). A series of multiple alleles similar to the albino alleles of mammals, but even more extensive, occurs in Drosophila as regards eye color. The first noticed difference was the widest one — that between normal red eye and a completely colorless white eye. A faintly tinged reddish eye, known as eosin, represents a stage close to white, very much as Himalayan is related to Polish albinism in rabbits. Eight other stages intermediate between eosin and full red eye have since been distinguished.

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101

They form a graded series, but with complete discontinuity between its eleven members. These, in the order of their intensity and dominance, are red, apricot, coral, ivory, ecru, bufi, tinged, blood, cherry, eosin, and white. Multiple alleles are supposed to be different forms of a single gene. This view is supported by the following evidence, ( i ) When one allele shows linkage with a second gene, any other allele also shows linkage with that same gene and in like degree. (See Chapter IV.) (2) When two alleles are united in the same zygote, there is no intensification of the character affected. The condition expressed is more often intermediate. It does not exceed that of the stronger allele. (3) Α heterozygote between two alleles of the same gene transmits each of them separately in half of its gametes. A zygote can not contain more than two alleles because it has only two chromosomes as carriers of that particular gene. Thus a gray rabbit heterozygous for albinism may produce only one type of albino offspring, either Himalayan or Polish, according to its ancestry, but not both. Yet a Himalayan rabbit heterozygous for Polish may produce both types of albinos, but no full-colored ones. The evidence which we have indicates that a recessive character results from the action of an allele of the same gene as produces the dominant character, but which acts less energetically than the dominant allele. REFERENCES A . H . , 1913. T h e Himalayan rabbit case, with some considerations on multiple allelomorphs. A m . Nat. 47.

STURTEVANT,

1925—1928. T h e effects of unequal crossing over at the bar locus in Drosophila. Genetics 10: 1 1 7 ; 1 3 : 401.

CHAPTER XIII LETHAL GENES; BALANCED LETHALS is one which causes the death of an individual in which it occurs in a homozygous state, though in a heterozygous state it may have no seriously harmful effect. The cause of death is probably inability of the organism to perform some of its vital functions in the absence of the normal allele of the mutant gene. T w o categories of lethals can be distinguished, dominant and recessive. A dominant lethal alters the phenotype when heterozygous, but kills it when homozygous. Such, for example, is the gene of cattle which when heterozygous produces the short-legged Dexter type, but when homozygous produces "bulldog" calves which die before or at birth. A recessive lethal has no observable effect when heterozygous, but kills when homozygous, just as a dominant lethal does. LETHAL GENE

The first discovered case of a lethal gene among animals was of the dominant type, its expression in the heterozygous animal (yellow mouse) being due to a dominant allele of the agouti gene. Yellow mice are invariably heterozygous, as was first shown by Cuenot, who observed that they do not breed true but regularly produce mice of a different color in addition to those which are yellow. When yellow is mated with yellow, a ratio of two yellow to one black or gray is obtained. This is a modification of the usual 3 : 1 Mendelian ratio in which the expected homozygous dominant (yellow) individual perishes at an early stage of development, and only the two yellow heterozygotes survive. Another dominant lethal of mice is known as black-eyed-white. Heterozygotes have extensive white areas of a characteristic pattern in their coat, homozygotes are completely white but with colored eyes. They survive for only a

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103

few days, at most two weeks, after birth, and then die of apparent malnutrition. A n interesting case of a dominant lethal is found in the dwarf mutation of rabbits. Heterozygotes are of reduced body size as compared with their normal litter mates, being about one third smaller at birth and retaining this disparity in size throughout life. They usually attain full sexual maturity, but if mated one with another produce one in four young which are homozygous for the dwarf gene and perish a few days after birth. These homozygotes are less than half as large as normals, and are unable to suck effectively, apparently because of a too short upper jaw. Although active in their movements, they die of starvation within a few days. This gene probably operates through reduced activity of the pituitary gland, since a similar but non-lethal dwarf mutation of the mouse is known to act in this way. Dwarf mice (homozygotes) which are undersized and sterile may be made to grow to full size and to reproduce by grafts of pituitary glands from normal mice, or by injections of the secretions of such glands. Most lethals are recessive. A recently discovered lethal of rats, anemia, will serve as an illustration. This gene when heterozygous has no observable harmful effects, as carriers of anemia are vigorous and produce large litters. But in a homozygous state, it causes death of the rat at about two weeks after birth, through increasing deficiency of haemoglobin in the blood. Soon after birth anemic individuals can be distinguished by their paler color. Though they may get an ample supply of milk from the mother, as shown by their distended stomachs seen through the body wall, yet their growth is retarded, so that at 10 days of age they are only one-half or one-third the size of their normal litter mates. Also, the paleness of the body has increased and they have taken on a jaundiced appearance, and a few days later they die. Several recessive lethals have been discovered among improved breeds of dairy cattle and are of much concern to cattle breeders. "Hairless," "amputated," and "short-spine" are some

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MAMMALIAN GENETICS

of those described by Möhr and Wriedt among cattle in Norway, names chosen for some of their more conspicuous phenotypic features. In heterozygotes these lethals, being completely recessive, are not harmful and remain undetected unless inbreeding is practiced with a view of intensifying the desirable characters of outstanding ancestors. Then homozygous lethals are produced and perish. An interesting situation arises when two different lethal mutations occur in the same chromosome pair at closely adjacent loci. This relation, which was first discovered in Drosophila, is known as one of balanced lethals. If lethals a and b are each fatal only when homozygous, then heterozygotes bearing a in one chromosome of a pair and b in the other will be viable and fully normal (if both lethals are recessive), or of a distinctive type (if one or both are dominant). Such heterozygotes will also be true-breeding, producing only living offspring which are heterozygous like their parents. The expected zygotic output of such heterozygotes would of course be aa -(- 2 ab + bb, but the homozygous combinations, aa and bb, are by hypothesis lethal, so that only heterozygotes will survive. These will constitute only 50 per cent of the expected population, and in mice, where among mammals this situation has been most exhaustively studied, a reduction of average litter size from eight to four has actually been observed, and the stage in development at which lethal embryos perish has been determined. The balanced lethals of mice have been studied chiefly at Columbia University by Dunn and his colleagues. The case involves mutant genes which produce a shortening of the tail, or in extreme cases complete taillessness. Three different mutant alleles of a gene for normal tail development are apparently involved. Any one of the three is lethal, if homozygous. Mutant T ' is a dominant lethal which in a heterozygote with normal tail (T) produces a short-tailed phenotype called "brachyuric" ( T T ' ) . The other two mutant alleles (t° and t 1 ) are recessive lethals and so have no visible effect in heterozygotes with normal tail (Tt° and Tt 1 ). But in hetero-

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105

zygotes with the dominant lethal they enhance the effect of the latter to complete taillessness (T't° or T ' t 1 ) . Embryological studies have shown that the two recessive lethals have unlike degrees of deadliness. The homozygote t°t° perishes at about the eleventh day of gestation, but t V perishes before implantation and so must be regarded as the more deadly of the two. REFERENCES CHESLEY, P., and L . C. DUNN, 1936. T h e inheritance of taillessness (anury) in the house mouse. Genetics 2 1 : 525. D U N N , L . C . , a n d S . GLUECKSOHN-SCHOENHEIMER, 1 9 3 9 .

T a i l l e s s n e s s i n a sec-

ond balanced lethal line. Genetics 24: 587. EATON, Ο. N., 1937. Lethal characters in animals and man. Jour. Hered. 28: 320. GREENE, H . S. N., C. K . HU, and W. H. BROWN, 1934. A lethal dwarf mutation of the rabbit. Sei. 79: 487. MOHR, Ο. L., 1929. Letalfaktoren bei haustieren. Zeit. Zuchtk. 4: 105. SNELL, G. D., 1 9 3 1 .

Inheritance in the house mouse (lethal dwarf mutation

described). Genetics 16: 42.

CHAPTER XIV GENES HAVING PATHOLOGICAL EFFECTS — "SUB-LETHAL" GENES there are others which have effects harmful to the organism but not necessarily fatal. These effects frequently take the form of what are known as hereditary diseases and so may be termed pathological effects. Such effects in mammals may be manifested in a variety of organs and ways, affecting either the nervous system and the sense organs, the muscular system, the blood system including the heart, the excretory system (kidneys), or the reproductive system. I. Genetically determined disorders of the nervous system have been studied in the rabbit especially by Nachtsheim (1938). He has described four different disorders. They are: 1. Shading palsy (symbol tr. = tremor) which is inherited as a simple recessive. It is first noticed in the young when about 10 days old as a continuous trembling, but passes into more extreme shaking movements and finally into convulsions which usually prove fatal at an age of about three months. In two male individuals reared by Nachtsheim the symptoms of the malady were less severe, and he was able to rear them to maturity and obtain from them numerous young by unaffected mothers, which were all unaffected. 2. Spastic paralysis. (Symbol spp.) This, like shaking palsy, manifests itself in young individuals before they leave the nest, and frequently can be detected at birth, as a stiff paralyzed condition of the hind legs. This condition becomes permanent in affected individuals without affecting other parts of the body or interfering seriously with growth. Mature individuals, however, are unable to reproduce for mechanical reasons. The mutation is inherited as a simple recessive. ESIDES GENES WHICH ARE LETHAL,

B

SUB-LETHAL GENES

107

3. Syringomyely. (Symbol syr.) An asymmetrical spastic paralysis usually affecting merely a hind leg, occasionally a front leg also. Affected individuals are normal until four to six months old, sometimes until a year or more of age. The character is a simple recessive, but irregular in its manifestation. Two affected individuals may have part of their young apparently normal. A particular genetic background may be required for the manifestation of the malady. 4. Epilepsy. It has long been known to rabbit breeders that in the Vienna White breed epilepsy is of occasional occurrence. Both Nachtsheim and I independently have observed it in our laboratory stocks. Affected individuals behave in every way as normals, unless they are suddenly startled by an unusual noise or a quick movement of the caretaker. Then the rabbit will dash wildly round the hutch until it falls in a swoon, and lies on its side seemingly dead. Presently, however, it begins to breathe again, and gradually resumes normal behavior. The character is sporadic in its occurrence, but is restricted to homozygous (vv) individuals. It has never been observed to occur in colored individuals derived from Vienna White crosses. For this reason Nachtsheim concludes that it is less probably an effect of a gene closely linked with the Vienna White gene than of the Vienna White gene itself. In this opinion I concur, but I have noticed that following outcrosses of a Vienna White race (in which epilepsy occurred) to a colored race, an extracted Vienna White race did not manifest epilepsy. Hence the epilepsy, if a by-product of the Vienna White gene, probably manifests itself only on a particular genetic background. II. Eye disorders genetically determined include the following, found in rabbits: i. Buphthalmus (symbol = bu) or hydrophthalmus (Nachtsheim). This is a condition in which the eye becomes distended with fluid and its contents opaque, and loss of vision follows. It is inherited as a simple recessive but with normal overlaps, indicating that the gene attains full expression only on a particular genetic background. It is first seen at an age of

io8

MAMMALIAN GENETICS

3 or 4 months, after which its severity increases, usually with one eye more severely affected than the other, or with one eye affected, the other normal, until blindness ensues. The general health of affected individuals is impaired and frequently results in death, but many affected individuals survive and breed successfully. This mutation was observed and studied by Castle in a race of New Zealand White rabbits. Nachtsheim observed its occurrence in a race of chinchilla rabbits. 2. Cataract. Opacity of the lens is observed to occur occasionally in various races of rabbits. The onset occurs in my experience in animals nearly or quite full grown, but Nachtsheim, who alone has made successful experiments with its inheritance, reports the incidence as occurring at an age of about nine weeks. He found the inheritance to be that of a recessive, Fx individuals being normal. In a backcross population of 19, he observed the occurrence of three affected individuals, the expectation being nine or 10. This indicates the probable occurrence of normal overlaps as in other eye and nerve defects. Other eye defects in rabbits have been recorded in which heredity seems to be involved, but no exact study of their inheritance has been made. They include coloboma, microophthalmy, and anophthalmy (fissure in the wall of the eye, small eyes, and eyelessness). Nervous disorders of mice due to mutated genes resulting in uncoordinated or erratic movements are waltzing, shaker, shaker 2, shaker short, and circling. All are recessive in inheritance. Similar mutations in the Norway rat are waltzing and wobbly. In mice, rodless retina is a simple recessive resulting in complete blindness (Keeler). In Peromyscus, epilepsy similar to that of rabbits has been described by Dice. Its manifestations are evoked particularly by auditory stimuli (as by rattling of a bunch of keys). It also is recessive. Leukemia- (blood cancer) in mice is inherited as a multifactorial recessive character, usually fatal (MacDowell). In dogs of the Dalmatian breed, a peculiar functioning of the kidneys occurs, resulting in the

S U B - L E T H A L GENES

109

production of urine with abnormally high uric-acid content and frequent formation of kidney stones. This is a recessive character. In rabbits, hypospadias (split penis) is a hereditary defect of the male sex organs, usually resulting in sterility. It is apparently a multifactorial recessive, and sex-limited. REFERENCES KEELER, C . E., 1927. Rodless retina. J. Exp. Zool. 46. NACHTSHEIM, Η., 1937. Er'opathologische Untersuchungen an K a n i n c h e n . Zeit. ind. Abst. 73: 463. See also Chapter V .

CHAPTER XV MATERNAL INHERITANCE are properly regarded as the chief agency for the transmission of characters from generation to generation. The only qualification which need be made is that it is not certain that heredity is exclusively a function of chromosomes. A new individual arises from the union of two gametes ordinarily distinguishable as egg and sperm, respectively. Each of these is haploid as regards its chromosome equipment, and is composed of equivalent chromosomes except in the case of the sex chromosomes. As regards cytoplasmic equipment, egg and sperm are very different. The egg has a very large amount of cytoplasm, relatively larger than the cytoplasmic equipment of any other cell of the body. But the sperm has so little cytoplasm that it is usually not even mentioned in genetic literature as a component of the male gamete. It is assumed that if the cytoplasm were concerned in heredity the egg must be enormously more influential than the sperm, but this assumption ignores the consideration that qualitative resemblances and differences may be quite as important as quantitative ones. The sperm nucleus (as well as the sperm plasma) is relatively small in relation to the huge egg nucleus (and egg cytoplasm), but after the minute sperm nucleus has entered the egg, it enlarges to become equal to the egg pronucleus in bulk as well as in genetic influence. For all we know to the contrary, the sperm cytoplasm may also be eventually influential, though its original mass is inconsequential. HROMOSOMES

C

There is one category of cases in which the cytoplasm is known to be the exclusive vehicle of transmission. The green hydra (Hydra viridis) has in the cells of its endoderm what are regarded as symbiotic algae, the so-called zoöchlorellae. In sunlight they synthesize organic products just as green plants

MATERNAL INHERITANCE

III

in general do, and also give off oxygen, both of which products are utilized by the hydra. In sexual reproduction of the hydra, zoöchlorellae are contained in the cytoplasm of the egg, and they go over bodily into the individual which develops from it, which accordingly becomes green like the parent hydra. But the sperm, though produced by green individuals, contains no green bodies. Thus the transmission of the zoöchlorellae is a purely maternal function performed by the cytoplasm of the egg alone. A similar situation occurs in many of the higher plants in which plastids occur in the cytoplasm. These may be carried over bodily in the cytoplasm of the egg cell, but not in the pollen grains; in which case, as in the hydra, the inheritance is purely maternal. In many white-spotted or striped plants, certain parts only of the plant may contain green plastids. Flowers which arise on the white portions of the plant produce only white seedlings, whether pollinated from flowers borne on the green or on the white branches, but flowers on the green parts of the plant produce only green seedlings, however pollinated. This shows that only egg cells carry the chloroplastids. For when they have green plastids, only green seedlings result, but when they lack green plastids, only white seedlings result. T h e source of the pollen grains makes no difference, for they are not carriers of chloroplastids. In another class of variegated plants a chromosomal gene conditions the partial or full development of the chloroplasts. In such cases the inheritance of variegation is typically Mendelian, and well illustrates the nature and function of genes. T h e gene is not the character (in this case the plastid), but it modifies the character, or may be indispensable to the production or possession of the character by the organism. Many years ago Halsted (1918) showed that in reciprocal crosses between species or varieties of tomatoes having fruits of unlike size there was a difference in the size of fruit borne by the reciprocal F x plants, fruit size being always closer to that of the maternal parent — a result which must be ascribed to plasmatic influence.

112

MAMMALIAN GENETICS

In mosses, Wettstein, by a series of ingenious experiments on unusually favorable material, has established the occurrence of inheritance by means of the plasma (plasmon) as well as through the genes of the chromosomes (genome). He first crossed varieties of the same species (Funaria hygrometrica), and observed in such cases the occurrence of regular Mendelian inheritance undoubtedly due to differences in genes alone, not in plasma. Reciprocal crosses in this case gave identical results, indicating that the same sort of plasma was possessed by both parents. Next he crossed this same species with another widely different species of the same genus (F. mediterrane a), but here observed the occurrence of strongly matroclinous inheritance expressed in such sporophyte characters as the position, form, and color of the capsule and of its peristome formation. By induced regeneration of sporophyte tissue, he was able to produce diploid reciprocal gametophytes which differed from each other particularly in form of leaf tip, leaf rib, and paraphyses. This was interpreted as showing that although the gene content (genome) was the same in the reciprocal hybrids, they must owe their differences to differences in the plasma, which had been derived from a different species in each case. Germination of several hundred spores from Fi sporophytes produced haploid gametophytes which were different in the two reciprocal hybrids. The genome in each group would presumably be the same, representing all possible segregations from the two sets of chromosomes. But the plasmon in one group would be derived from F. hygrometrica, in the other group from F. mediterranen. When species of different genera were crossed, as F. hygrometrica with Physcomitrium piriforme, still stronger evidence of plasmatic inheritance was obtained. Reciprocal F x sporophytes were obtained which produced abundant (haploid) spores. These would derive their plasma from the maternal species, but their genome from one or the other parent species or from a combination of the two. Those haploid gametophytes which were least like the maternal species were now

MATERNAL INHERITANCE

113

backcrossed with the pure paternal species to secure, if possible, a pure genome of one species in a plasmon of the other, but this attempt failed. A l l plants resembling the paternal species were completely sterile. Only plants having apparently the pure maternal genome could survive and reproduce in the maternal plasma. A n attempt was made, however, to change over the character of the plasma from that of one species to that of the other by repeated backcrossing with the paternal species. If, it was thought, the plasma owes its character to prior influence of the genes, then if genes of the other species are introduced in every generation in repeated backcrosses the plasma should at last become habituated to their presence and show their influence. Five such backcrosses were made, but still only forms purely maternal were segregated and fruited. Another way in which it was sought to increase the influence of the paternal genome was by means of polyploidy. By regeneration from sporophytic tissue, diploid and triploid Hygrometrica gametophytes were produced which were then crossed with haploid Piriforme plants. Thus sporophytic genomes were produced which were two-thirds and three-fourths Hygrometrica respectively, but no increase in the production or the fertility of types showing Hygrometrica influence resulted. Wettstein concludes that the plasmon of the maternal species retains its distinctive character even in the presence of a foreign paternal genome represented two or three times over and introduced afresh in a series of successive generations. T h e genetic independence of the plasmon, he believes, is conclusively demonstrated. Another case of maternal inheritance occurring in the flowering plants was studied by Correns. In Satureia hortensis and in Cirsium oleraceum occur strains some of which are purely female, others hermaphrodite. Each sort reproduces only its own kind as regards sex. In the case of the females this is surprising, since the female progeny in each generation are produced from pollen produced by the hermaphrodites. It

11 4

MAMMALIAN GENETICS

seems that the pollen in this case does not influence the sex of the offspring, but that the sex is determined by the mother alone, and presumably by the plasma. That such is the case is shown by an experiment in which two different species were crossed, from one of which (Cirsium oleraceum) a purely female strain was used, while from the other (C. canum) a hermaphrodite strain was used. The offspring were purely female. They were backcrossed a second and a third time with the hermaphrodite C. canum, resulting in progeny which showed increasing resemblance to the paternal species in many characters, but as to sex were still purely female. Correns concludes that as regards the genome these third backcross hybrids must have become largely C. canum, but as regards their sex they still showed unimpaired control by the plasma of the maternal species. Correns also reviews an investigation made by Harder (1927) on certain fungi indicating plasmatic inheritance. Strains of Pholiota were crossed which were morphologically very different. When crossing occurs, the mycelia first fuse and their plasmas unite, but the nuclei remain distinct in the common plasm until spore formation takes place. Harder was able to remove one of the nuclei with a micro-manipulator, leaving the nucleus of only one parent species in the fused plasmas. It was found that certain characters were determined by the nucleus alone, notably the sex behavior as plus or minus followed that of the strain which furnished the nucleus. But in other characters, sometimes one parent, sometimes the other, was more closely resembled. Harder explains this as depending upon the relative amounts of plasma which in each case were furnished by each parent. In mammals it has been found that reciprocal crosses between species or varieties frequently give unlike results. Yet the genome in such cases is of identical constitution in female offspring, and different in males only as regards sex-linked genes (those borne in an X or a Y chromosome). Accordingly the differences observed in reciprocal crosses must be assigned pri-

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marily to a difference in maternal influence. A difference in body size between the parent races regularly results in offspring resembling the mother more closely than the father. Thus in crosses between horse and ass, the mule (produced by a mare impregnated by a jackass) is a larger animal than the hinny (produced by a female ass mated with a stallion). This is the general opinion among breeders with opportunities for comparing the two hybrids, but has not been made a subject of critical study. In the case of rabbits and mice, however, several critical experimental studies have been made of reciprocal crosses between races of unlike body size, all with the same result, that the average body size of the offspring is nearer to that of the mother's race than to that of the father's race. That this is due to constitution of the egg, not to a gestational influence, is indicated by the observation of a like result in salamanders, in which development of the egg occurs wholly outside the maternal body. A superior influence of the mother in transmission of susceptibility to cancer has also been observed in mice by Murray and Little and ascribed by them to non-chromosomal inheritance. The view has been expressed by some that the cytoplasm forms merely a suitable medium in which the chromosomal genes may act, that the cytoplasm of different species being chemically different are not equally suitable media for action of the genes, hence the different behavior when the same genome is brought into different plasmas. But since life is a product not of nuclear activity alone nor of plasmatic activity alone, but of both acting together, it is illogical to ascribe the action exclusively to either. Hydrochloric acid and marble brought into contact produce a vigorous chemical transformation, but would any chemist maintain that the action was due wholly to the acid, and that the marble was nothing but a medium in which the acid might act?

MAMMALIAN GENETICS

ιι6

R E F E R E N C E S

and W . H . G A T E S , S. C . R E E D , and L . W . of a size cross in mice. Genetics 2 1 : 66, 310. CORRENS, C . , 1928. Ueber nicht-mendelde Vererbung. Supplb. i. CASTLE, W . E . ,

1936. Studies

Zeit. ind.

Abst.,

1918. Reciprocal crosses in tomatoes. Jour. Hered. 9: 169. and C . C . L I T T L E , 1935. Genetics of Mammary tumor incidence in mice. Genetics 20: 466. S I R K S , M. J., 1938. Plasmatic inheritance. Bot. Rev. 4: 113. W E T T S T E I N , F . V . , 1926-30. Plasmatische Vererbung. Nach. wiss. Gesell. HALSTED,

B.

LAW,

MURRAY, W .

D.,

S.,

Göttingen. Math.-Phys. 1926: 250; 1930: 105. •

1928. Ber. Deut. Bot. Gesell. 46: 32.

CHAPTER XVI T H E I N H E R I T A N C E O F B O D Y SIZE in mammals is a matter of inheritance will be questioned by no one. Each wild species has its characteristic body size, the maximal or minimal limits of which are rarely transgressed. When subspecies or geographic races differ in body size such differences commonly have a genetic basis, as numerous breeding experiments show. Among domestic mammals, racial differences in body size are frequently greater than in individual wild species in a state of nature. This is because under domestication artificial selection has been substituted for natural selection, and extremes of size have been perpetuated, even when their survival value in a state of nature would be small. In the European rabbit under domestication, in the course of less than a thousand years, body size has probably been increased by several hundred per cent. The wild rabbit, unmixed with domestic stock, scarcely exceeds 3 or 4 pounds in weight; some breeds of domestic rabbits regularly attain 9 - 1 2 pounds. Such a large size is of course attained only under favorable environment. What is inherited in breeds of large body size is a capacity to utilize a favorable environment and abundant food supply to produce rapid and prolonged growth, a capacity which the wild stock does not possess. For domestic breeds which resemble wild stocks in body size are unable to increase body size beyond a very definite upper limit, even under optimum conditions, and there is no reason to think that genuine wild stocks could do so. HAT BODY SIZE

When a cross is made between breeds of rabbits which differ in adult body size, as between Polish (2-3 pounds) and Flemish Giant ( 1 0 - 1 4 pounds), F i animals of intermediate body size are obtained, and populations produced by a backcross of F i

ii8

MAMMALIAN GENETICS

to one of the parent breeds are again intermediate between their respective parents. This result indicates that multiple gene differences are involved, some of which are perhaps dominant while others are recessive in inheritance. For if all mutant size genes accumulated in the large race were dominant in behavior, the F i animals should be as large as the large parent race, which they are not. Or if all such genes were recessive, then the F i animals should be of the same size as the small parent race, which again they are not. But if part of them were dominant and part recessive, then the resultant should be intermediate as observed. A n alternative assumption would be that size genes are devoid of dominance, which is contrary to the experimental evidence. Genes affecting body size are with certainty identifiable only by other morphological or physiological effects, and in all cases critically studied they behave either as dominants or recessives. In mice, one dominant gene (yellow) has been identified which increases body size in heterozygotes, though in homozygotes it is lethal. T w o others have been identified which are recessives, but harmless and apparently without effect in heterozygotes. In homozygotes the recessive brown gene increases body size by 2 to 5 per cent, and the dilution gene by a less amount, usually below ι per cent. Combined in action, brown and dilution have a greater effect than either one by itself. T h e largest race of mice producible by selection should include in its genetic constitution A y bb dd, since this genetic aggregation would tend to increase body size over what it would otherwise be. It would be of interest to k n o w whether Goodale (1938) in his selection experiments, designed to increase the body size of albino mice, unconsciously utilized this gene complex. Besides mutant genes which increase body size, there are others which have an opposite effect. Thus in mice the following mutant genes are known to act, when homozygous, as reducers of adult body size: p, p2, se, and dw. A l l are recessive in inheritance, as most mutant genes are. If in a large race recessive mutant genes had been accumulated which, when

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119

homozygous, increased body size, and in a small race recessive genes had been accumulated which tended to decrease body size, then in a cross between the two races an F i of intermediate size would result, since both sets of mutant genes would become ineffective in the heterozygotes. Possibly this is what happens in the crossing of a selected large race, such as the checkered giant rabbit, with a selected small race, such as the Polish rabbit. Jones, in his explanation of hybrid vigor, has assumed that genes which impair vigor and thus reduce growth and size are in general recessive, and that different sets of such genes have accumulated in different races. A cross between such races renders the differential recessive genes ineffective and thus increases growth energy. This is a reasonable assumption, but it is doubtful whether it is an adequate explanation of all the phenomena of heterosis, as has been elsewhere indicated. Castle and Gregory have shown that size genes of rabbits act by altering developmental rate. Eggs of a genetically largebodied race of rabbits undergo cleavage more rapidly than the eggs of a small-bodied race, so that a larger embryonic disk is formed and larger-bodied young are born, and these continue to grow at a more rapid rate until maturity is attained. The parental influence on size is exerted through sperm and egg alike, i.e., is shared by both parents, and presumably equally as far as chromosomal genes are concerned. But the total influence on the body size of offspring exerted by the mother is greater than that exerted by the father, which is thought to indicate an influence of the cytoplasm of the egg on developmental rate, as explained in the chapter on maternal inheritance. Cases similar to those studied in mice, in which particular mutant genes have an influence either as accelerators or as retarders of growth, undoubtedly exist among plants as well as among animals. A single instance may be cited from the observations of Halsted (1918) on tomato crosses. He found that size of fruits was increased by the recessive mutant genes, dwarf and coarse-leaved foliage, but reduced by yellow foliage

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(recessive allele of green) and yellow fruit (recessive allele of red fruit). In the particular cross which he studied, two of the favorable mutations and one of the unfavorable ones were introduced by one parent, so there can be no question of the uniformly superior influence of one parent but only of a differential action of alleles in which the parents differed. Of the four recessive alleles, two are favorable to increased size of fruit and two unfavorable. The extent to which favorable alleles increase the weight of fruit in the F 2 population was as follows: dwarf habit, 17.5 per cent; coarse foliage, 13.4 per cent; green foliage, 3.8 per cent; red fruit color, 11.6 per cent. Whether or not these genes had a cumulative favorable action was not investigated. REFERENCES CASTLE, W .

E., 1929-1934. Size inheritance in rabbits.

J.

Exp. Zool. 58, 60, 67.

1934· Body size of reciprocal hybrids in rabbit crosses. Proc. Nat. Acad. Sei. 20: 621. 1936. Superior influence of the mother on body size in reciprocal hybrids. Sei. 83: 627. A size cross in mice. See Chapter X V . GOODALE,

H. D., 1938. Body weight of albino mice increased by selection.

Jour. Hered. 29:101. HALSTED,

B. D., 1918. Reciprocal tomato crosses. See Chapter X V .

CHAPTER XVII VARIATION A N D SELECTION; QUANTITATIVE CHARACTERS; PURE LINES of an organism which are measurable vary about an average from which small deviations are more frequently found than large ones, those which are plus in direction and those which are minus being about equally common. The result, if expressed graphically, resembles a "normal" or frequency-of-error curve, such as would represent the results of coin tossing or dice throwing — in other words, the results of chance, when two or more independent contingencies are involved. Men vary in height and weight in this so-called "continuous" fashion, any two variates, however extreme, being connected by a complete series of intermediates. Such measurable characters of an organism are often called quantitative characters. Their accurate description is made possible by statistical methods which are the special field of biometry. The usual magnitude of a character is expressed by its average or mean magnitude, or if the variation is not equally frequent in plus and minus directions, then by the mode, or commonest class of variates. The amplitude of the variation is expressed by the standard deviation. The coefficient of variation allows one to compare the amount of variation in one character with that in another character, and the coefficient of correlation shows to what extent variation in one character is causally connected with variation in a second character. These are the more important biometric tools found useful in the study of quantitative characters.*

T

HE CHARACTERS

* For a discussion of biometric methods and their uses, the reader is referred to special treatises on biological statistics. Several recent textbooks of genetics contain adequate reviews or summaries of statistical methods useful in genetics.

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The inheritance of quantitative characters is one of the most difficult problems in the whole field of genetics, and a complete solution of it can not be said to have been reached as yet. Quantitative characters involve multiple causation; otherwise there would not be a continuous variation curve, but only discontinuous classes, as in ordinary Mendelian inheritance. Further, the multiple causes may be either environmental or genetic, or probably more often a combination of the two. When we institute a comparison between parents and offspring as regards a quantitative character, we must first eliminate environmental agencies before we shall be in a position to determine what genetic agencies are operative. Then we are confronted with the question, how many genetic agencies are at work and how are they interrelated. These are difficult or impossible of determination. For it is conceivable that the genetic agencies may be chromosomal, plasmatic, or both. If they are chromosomal alone and are severally of equal influence, it is possible to calculate how many are involved by a method devised by Castle and Wright ( 1 9 2 1 ) based on the comparative variability of the F t and the F 2 generations. But this method has little practical utility because of the uncertainty as to whether each of the multiple agencies involved has an influence equal to that of each of the others. It may well be that one genetic agency has much greater influence than another, in which case the result obtained by the Castle-Wright formula would be misleading. There is the further uncertainty as to whether the inheritance may not be plasmatic rather than chromosomal, or partly plasmatic, partly chromosomal. There is only one feature of blending (quantitative) inheritance about which we can feel reasonably certain. The increased variability of F 2 as compared with F x can be best explained as due to the recombination of chromosomal genes, for there is no known mechanism for reduction and recombination as regards the plasma, even supposing that both parents contribute plasma to the offspring, which again is uncertain.

FIG. 74.

I n h e r i t a n c e of a recessive pattern of w h i t e s p o t t i n g seen in " h o o d e d "

rats. T h e p a r e n t s (at the l e f t ) a r c a h o m o z y g o u s h o o d e d m o t h e r ( h h ) a n d a heterozygous " I r i s h " father ( H h , black with white belly). their y o u n g is s h o w n at the r i g h t .

m o t h e r , five a r e h e t e r o z y g o t e s l i k e the f a t h e r . N o t e Such

fluctuations

-2

FIG. 7 5 .

A

A n e n t i r e litter of

F o u r are h o m o z y g o u s

hooded

fluctuations

like

the

in b o t h classes.

a r e f o u n d to be in p a r t heritable.

- 1

0

+1

+ 2

+3

+4

series of g r a d e s f o r c l a s s i f y i n g the plus a n d m i n u s v a r i a t i o n s the h o o d e d p a t t e r n .

of

VARIATION A N D SELECTION

123

It is conceivable that a quantitative character may be determined basically by plasmatic organization of the egg alone, or of both egg and sperm, and that the chromosomal genes may act as modifiers of this basic character, which will then show such differential variability between F i and F 2 as is due to recombination of the chromosomal genes. Quantitative characters in the variation of which only environmental agencies are concerned are well illustrated in "pure lines" as defined by Johannsen. A pure line is a group of completely homozygous organisms, such as theoretically should be found to occur in any species which has long been self-fertilized. Under self-fertilization an organism should theoretically become homozygous for all chromosomal genes in a dozen generations or less. Beans, peas, and wheat are plants regularly self-fertilizing, and Johannsen was able to show that cultivated varieties of such plants are made up, for the most part, of mixtures of numerous pure lines. He found that, starting with a commercial sample of beans, selection of the larger seeds produced a larger-seeded race; but if he confined the selection to the seeds borne on a single plant, no change resulted. In the last case he was selecting within a pure line, each seed having the same genetic properties as any other seed so far as regards hereditary seed size. But the individual seed might be larger or smaller owing to its position in the pod, or the number of seeds in the pod, or some other environmental circumstance. This affected only its individual (somatic) character, not its genetic character. On the other hand, in a field crop there are many pure lines growing side by side. Some of these are largerseeded than others for genetic reasons. If all are harvested together, then selection of the larger beans from the mixture will produce more plants of the large-seeded lines, and so the average seed size of the crop will be increased. Johannsen thus established the important principle that selection within a pure line is ineffective. Selection is effective only when genetic differences are present in the material upon which selection is made, one genetic combination being given preference over

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MAMMALIAN GENETICS

another. Before Johannsen's experiments were made, it was generally supposed that genetic variability was always present in every species and under all circumstances, and consequently that selection, either artificial or natural, was always in a position to alter the race in desired or advantgeous directions. Johannsen showed that this was not so within a pure line, so long as it remained pure. It might become impure in consequence of an outcross with a plant of different genetic constitution, or by a spontaneous mutation within the pure line; but such occurrences in beans were rare. In plants and animals which are not self-fertilizing, pure lines do not commonly exist, and selection is effective up to the limit of the existing variability. It is possible, nevertheless to produce strains of animals which approximate pure lines in genetic constitution by long-continued inbreeding, as by brothersister matings for a series of 25 or more successive generations. A series of 25 brother-sister matings should be as effective as about nine generations of self-fertilization in producing completely homozygous individuals. Dr. Helen Dean King has inbred albino rats for an even greater number of generations, so that their homozygosity should be beyond question. The strain of rats thus purified genetically has been of service in the interpretation of a long-continued selection experiment. Some years ago Castle and Phillips selected a colony of hooded rats for modification of the hooded pattern in plus and minus directions, i.e., toward increased and decreased extent of the pigmented areas in the coat. The selection was effective throughout the experiment, which was continued for more than twenty generations and involved the production of some 35,000 individuals, but the modification secured in each generation as a result of selecting the darkest individuals in the plus series, and the lightest individuals in the minus series, decreased in amount as the experiment progressed. This indicated that the maximum amount of genetic variability was present at the beginning of the experiment, and that it had

VARIATION A N D SELECTION

125

been gradually diminished thereafter, though it was not completely exhausted even by twenty generations of intensive selection. N o w it is important to bear in mind that the hooded pattern of rats is the result of mutation in a single gene. Crosses between a hooded and a non-hooded (self) strain give no hooded individuals in F l 5 and a 3 : 1 ratio of self to hooded in F 2 . The purpose of the experiment, when it was begun, was to ascertain whether a gene changes under the influence of selection. It did in this case, apparently, for the plus selected line became so dark that only the ventral surface of the body was white, and the minus selected line became so light that only the head and tail were pigmented, and these only in part. But a further experiment showed conclusively that the genetic modification which had been secured by selection did not involve the hooded gene itself but only the residual heredity, consisting probably of an association of genes which modified the somatic effect of the hooded gene itself. Both the plus and the minus selected lines were crossed with the same wild rat, and from both crosses hooded individuals were recovered as recessives in F 2 , but in both cases the extracted individuals were less extreme in character than their hooded grandparent. The extracted plus individuals were still unmistakably plus, and the extracted minus individuals unmistakably minus in character, but both had lost a part of the modification secured by long-continued selection. A second and a third cross with the wild strain caused further loss of the modifications secured by selection and made it clear that those modifications resided not in the hooded gene itself but in associated genes which altered its somatic effects. Some critics have assumed that all genetic variability utilized in this experiment was present at the outset in the foundation stock, and this is undoubtedly true for the major part of it, but certainly not for all of it. For, in the course of the selection, a mutation occurred in the plus selected line, producing a group of individuals more strongly plus than had previously been

126

MAMMALIAN GENETICS

secured, and which thereafter bred true for this marked increase. This change apparently took place in the hooded gene itself, not in its modifiers, resulting in the allele known as Irish, in which only the belly is white, the rest of the coat being colored.* But it is quite possible that mutations in other genes also occurred which acted as modifiers of the hooded pattern, whatever other physiological function they may have had. Support for this view comes from an investigation made by Castle and Pincus on King inbred albino rats after they had been brothersister mated for 50 successive generations. The K i n g inbred rats were homozygous for the hooded gene, though they did not show it, for the reason that they were albinos. Castle and Pincus crossed a K i n g inbred male with a yellow self-colored rat. The offspring were gray selfs, except for a white belly patch, evidence of the presence of the hooded gene in heterozygous condition. Such F x females were backcrossed to the inbred albino male, their sire, and hooded individuals were thus obtained. This process was repeated through eleven successive generations, the same inbred male or his direct inbred descendants being used in each generation. The hooded progeny thus produced numbered more than 2000. In the earlier generations they were slightly plus in grade as well as larger than K i n g inbreds in consequence of heterosis, but from the fifth generation on, they were slightly and consistently minus in grade. They were also practically identical with K i n g inbreds in body weight and growth rate from the sixth generation on, as was to be expected in consequence of the repeated backcrossing to the inbred race, which should make them substantially identical with inbreds genetically, except for the single chromosome carrying the color factor which had been derived from the colored ancestor. (See Fig. 76.) A noteworthy occurrence in this experiment was an apparent minus mutation in a male of the fifth inbred generation, which, * A parallel occurrence, mutation of hooded to Irish (h to h 1 ) , has been observed by Curtis and Dunning in a colony of hooded rats.

VARIATION A N D SELECTION

127

though somatically normal, produced young of more minus character than the average of the race, and transmitted this peculiarity to three generations of his descendants. If mutations such as this occurred during the progress of a mass selection experiment, and in the direction of that selection, their We'iqWt in Grams — »

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150

ISO

FIG. 76. Growth curves for ages 30 to 150 days, of female K i n g inbred rats and of females of back-cross generations 3 to 5 reared under similar conditions.

occurrence might easily escape notice, but the consequence would be that the progeny of the mutant would be included in the selections made, and racial modification in the direction of selection would thus be accelerated. We must recognize, accordingly, that mass selection is able to modify a race not only within the limits of variability present in the material subjected to selection at the outset of the experiment, but also to the added extent that mutations in the direc-

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tion of selection may occur during the progress of the selection experiment. That gene mutations of one sort and another are of frequent occurrence, and may even be artificially produced, has been repeatedly demonstrated (as by Muller and others) since Johannsen promulgated his pure-line theory. This limits the sphere within which selection may be regarded as being without effect to self-fertilizing or closely inbred organisms in which no mutation of modifying genes is taking place. For if mutation is occurring in the direction of the selection, its products will be incorporated in the selected complex of genes, and the race will thus change progressively in the direction of selection. REFERENCES W. E. and J. C . P H I L L I P S , 191 I . Piebald rats and selection. Carnegie Inst. Wash. Publ. No. 195.

CASTLE,

•

-and G.

PINCUS,

1928. Hooded rats and selection.

J.

Exp. Zool. 50: 409.

and S. W R I G H T , 1921. A method of estimating the number of genetic factors in cases of blending inheritance. Science 54: 93 and 223. C U R T I S , M . R. and W . F . D U N N I N G , 1937. Mutations of the piebald gene in the rat. Jour. Hered. 28: 383.

CHAPTER XVIII M A J O R C O N T R I B U T I O N S OF F I S H G E N E T I C S like Drosophila, do not come immediately within the scope of this volume, they are, like Drosophila, important for the theoretical contributions which their study has made to the subject of genetics, and so can not be properly overlooked. The major discoveries which stand to the credit of fish genetics are ( i ) one-sided masculine sex-linked inheritance, i.e., the direct transmission of genes from father to son in the Y-chromosome, (2) the existence of crossing-over (exchange of genes) between the X and Y chromosomes, and (3) the existence in the same group of fishes of both systems of genetic sex determination, that which is found in Drosophila and mammals and that which is found in moths and birds. This last discovery has made clear the probable mode of origin in evolution of these contrary systems. Just as in the case of mammals our knowledge of inheritance comes chiefly from the study of the least valuable members of the class, these kept merely as pets, so among fishes we know most about the genetics of those which are kept in aquaria because of their small size and striking colors although their economic value is small. The kinds which have been found most useful are ( 1 ) the killifishes, Lebistes (the guppy or millions fish) from Venezuela, and Aplocheilus from the rice fields of Japan; (2) the top minnows, Platypoecilus (the platy fish), and Xiphophorus (the sword-bearing minnow), both from Mexico; and (3) the goldfish from China and Japan. All these genera belong to the same order of fishes, the Cyprinodontidae. Lebistes and Aplocheilus share the honors as regards greatest usefulness to genetics, for in them, independently in Denmark and Japan, were discovered the transmission of genes in .THOUGH FISH,

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the Y-chromosome resulting in one-sided masculine inheritance. To the Japanese investigator, Aida, belongs the credit for discovering also the occurrence of crossing-over between the X and the Y chromosomes in Aplocheilus, a fact later verified also for Lebistes by Winge. Lebistes is a small fish; females are about 5 cm long and males half that length. The males are rendered conspicuous by spots or blotches of black, blue, red, or yellow, arranged in quite definite patterns, the inheritance of which is governed by genes located in the sex chromosomes, X or Y . Genes in the X-chromosome are transmitted to female offspring but ordinarily produce no visible effect in them, since they lack the necessary activating male sex hormone. If, however, as occasionally happens, a female develops testicular tissue and thus becomes hermaphroditic, the genes for color markings may become effective, imperfect color markings being produced. Genes borne in the Y-chromosome are transmitted directly from father to son; never through females unless a crossover has first occurred from the Y to the X chromosome. Winge has described 18 genes which produce color markings in Lebistes males. Nine of these are borne in the Y-chromosome and three in the Xchromosome, without having been observed to cross over. Five have been found to cross over from Y to X or from X to Y or from X to X. Finally, one color gene is borne in an autosome, since it shows no sex linkage in transmission, though like the others, it is sex-limited in action (effective only in males). Several of the sex-linked genes are believed to be multiple alleles of a single gene differing in somatic expression. Winge thinks that a single principal gene for maleness is located in the Y-chromosome, and that a gene (maculatus) which produces a conspicuous black spot on the dorsal fin is closely linked with it or possibly identical with it, since the two are almost invariably associated. By selection Winge was able to reduce the size of the black fin spot, or in a few individuals to obtain its complete suppression; but in their descendants, when an outcross was made, the spot reappeared, so that Winge believes

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that the gene was not changed or eliminated by selection but only the association with that gene of modifying genes. In Aplocheilus, according to Aida, the wild type is brown in color as a result of black pigment produced by a dominant gene, B, associated with orange-red pigment produced by another dominant gene, R. Gene Β is carried in an autosome, R in a sex chromosome, either X or Y , between which, in male individuals, crossing-over occasionally takes place. Mutation of Β to the inactive state, b, leads to the production of a phenotype, red, bbRR. Mutation of red to the inactive state, r, produces the phenotype blue, BBrr. Both mutations combined produce white, bbrr. A cross between a white female and a homozygous red male produces heterozygous red offspring in both sexes, but the Fx females carry the red gene, R , in an X-chromosome, whereas the red males carry the R gene in a Y-chromosome. The essential difference between the X and the Y chromosomes in Aplocheilus would seem to be that X exerts a female-determining influence, whereas Y has a male-determining tendency. Either or both may carry the gene for red body color, and in heterozygous red males the R gene may cross over from Y to X or from X to Y . When a heterozygous red male carries the R gene in his X-chromosome, he will transmit it only to his daughters (unless a crossover occurs). When he carries the R gene in his Y-chromosome, he will transmit it only to his sons (unless a crossover occurs). Winge demonstrated that in Lebistes several different color genes normally borne in the Y-chromosome may cross over to the X-chromosome in heterozygous males, after which they follow the course of an X-chromosome in transmission. But there was one gene of the Y-chromosome (maculatus) which was never observed to cross over to X , and Winge concluded that this gene either itself has a specific male-determining influence or is closely linked with a gene having such an influence. Adopting a modified form of the hypothesis of Bridges as to genie balance in sex determination, Winge supposes that there are scattered among the autosomes numerous genes having

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an influence on sex determination, some of them favoring maleness, others femaleness. As a result of crossing over (or possibly of translocation) a particular autosome may come to have a net male influence, while its synaptic mate has an opposite (female) influence. Such a pair of autosomes would then become potential sex chromosomes in the absence of the usual X-Y pair. Winge was able to establish such a relationship experimentally by first securing an XX male, as indicated by markers borne in the X-chromosome of the parents. This male, when mated with ordinary females (also XX), produced only female offspring. Evidently the maleness of this individual, if genetic, was determined by genes other than those borne in a Y-chromosome. By mating him to his sisters, Winge was able to produce an XX race of Lebistes in which, however, both sexes occurred, the function of sex determination now having passed to what had previously been a pair of autosomes, and the X-chromosome itself having now become a paired autosome. By a similar process of selection guided by genes for color patterns borne in X and Y chromosomes respectively, Winge was able to establish a race in which females were heterogametic (XY) in constitution and males homogametic (YY). Such males, mated to ordinary females, produced only male offspring (XY). If these males were then mated with XY females, offspring were obtained three fourths of which were male, one fourth female (XX). Of the males, one third were apparently YY as expected, since when crossed with ordinary females (XX), they produced only male offspring. Winge's researches made possible an explanation of the curious fact that in the group of fishes to which Lebistes belongs contrary systems of genetic sex determination occur. In Lebistes and Aplocheilus the male is regularly heterogametic, as in Drosophila ( 2 XX,