Heredity and Sex (1913) 3
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
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Chapter III The Mendelian Principles of Heredity and Their Bearing on Sex
The modern study of heredity dates from the year 1865, when Gregor Mendel made his famous discoveries in the garden of the monastery of Briinn. For 35 years his paper, embodying the splendid results of his work, remained unnoticed. It suffered the fate that other fundamental discoveries have sometimes met. In the present case there was no opposition to the principles involved in Mendel's discovery, for Darwin's great work on ^'Animals and Plants" (1868), that dealt largely with problems of heredity, was widely read and appreciated. True, Mendel's paper was printed in the journal of a little known society — the Natural History Society of Briinn, — but we have documentary evidence that his results were known to one at least of the leading botanists of the time.
It was during these years that the great battle for evolution was being fought. Darwin's famous book on The Origin of Species" (1859) overshadowed all else. Two systems were in deadly conflict — the long-accepted doctrine of special creation had been challenged. To substitute for that doctrine the theory of evolution seemed to many men of science, and to the world at large, like a revolution in human thought. It was in fact a great revolution. The problems that bore on the question of how the higher animals and plants, and man himself, arose from the lower forms seemed the chief goal of biological work and thought. The outcome was to establish the theory of evolution. The circumstantial evidence that was gathered seemed so fully in accord with the theory of evolution that the theory became widely accepted. The acute stage was passed, and biologists found themselves in a position to examine with less haste and heat many other phenomena of the living world equally as important as evolution.
It gradually became clear, when the clouds of controversy had passed, that what I have ventured to call the '^ circumstantial evidence "on which the theory of evolution so largely rested, would not suffice as a direct proof of evolution. Investigation began to turn once more to that field of observation where Darwin had found his inspiration. The causes of variations and the modes of inheritance of these variations, the very foundations of the theory of evolution, were again studied in the same spirit in which Darwin himself had studied them. The return to Darwin's method rather than to Darwin's opinions marks the beginning of the new era.
In 1900 three botanists were studying the problem of heredity. Each obtained evidence of the sort Mendel had found. Happily, Mendel's paper was remembered. The significance of his discovery now became apparent. De Vries, Correns, and Tschermak brought forward their evidence in the same year (1900). Which of the three first found Mendel cannot be stated, and is of less importance than the fact that they appredated the significance of his work, and reaUzed that he had found the key to the discoveries that they too had made. From this time on the recognition of Mendel's discovery as of fundamental importance was assured. Bateson's translation of his paper made Mendel's work accessible to English biologists, and Bateson's own studies showed that Mendel's principles apply to animals as well as to plants.
The Heredity of One Pair of Characters
Mendel's discovery is sometimes spoken of as Mendel's Principles of Heredity and sometimes as Mendel's Law. The former phrase gives a better idea perhaps of what Mendel really accomplished, for it is not a little difficult to put his conclusions in the form of a law. Stated concisely his main discovery is this : — in the germ-cells of hybrids there is a free separation of the elements derived from the two parents without regard to which parent supplied them.
An example will make this more obvious. Mendel crossed an edible pea belonging to a race with yellow seeds to a pea belonging to a race with green seeds (Fig. 42). The offspring or first filial generation (Fi) had seeds all of which were yellow. When the plants that bore these seeds were self-fertilized, there were obtained in the next generation, F2, both yellow and green peas in the proportion of 3 yellows to 1 green (Fig. 42). This is the well-known Mendelian ratio of 3:1.
The clue to the meaning of this ratio was found when the plants of the second generation {F2) were selfbred. The green peas bred true ; but the yellows were of two kinds — some produced yellow and green seeds again in the ratio of 3 : 1 ; others produced only yellow peas. Now, if the yellows that bred true were counted, it was found that the number was but one-third of all the yellows.
Fig, 42. — Illustrating Mendel's cross of yellow (lighter color) and green (dark color) peas.
In other words, it was shown that the ratio of 3 yellows to 1 green was made up of 1 pure yellow, 2 hybrid yellows, 1 pure green. This gave a clue to the principles that lay behind the observed results.
Mendel's chief claim to fame is found in the discovery of a simple principle by means of which the entire series of events could be explained. He pointed out that if the original parent with yellow (Pi) carried something in the germ that made the seed yellow, and the original parent with green seeds (Pi) carried something that made the seed green, the hybrid should contain both things. If both being present one dominates the other or gives color to the pea, all the peas in the hybrid generation will be of one color — yellow in this case. Mendel assumed that in the germ-cells of these hybrids the two factors that make yellow and green separate, so that half of the germ-cells contain yellow-producing material, and half contain greenproducing material. This is Mendel's principle of separation or segregation. It is supposed to occur both in the male germ-cells of the hybrid flower, i.e. in the anthers, and also in the ovules. If self-fertilization occurs in such a plant, the following combinations are possible : A yellow-bearing pollen grain may fertilize a yellow" ovule or it may fertiUze a green" ovule. The chances are equal. If the former occurs, a pure yellow-seeded plant will result ; if the latter a hybrid yellow-seeded plant. The possible combinations for the green-producing pollen are as follows : A "green" pollen grain may fertilize a "yellow" ovule, . and produce a hybrid, yellow-seeded plant, or it may fertihze a "green" ovule, and produce a green-seeded plant. If these meetings are random, the general or average outcome will be : 1 pure yellow, 2 hybrid yellows, and 1 pure green.
It is now apparent why the pure yellows will always breed true, why the yellow-greens will split again into yellows and greens (or 1:2:1), and why the pure greens breed true. By this extremely simple assumption the entire outcome finds a rational explanation.
Fig. 43. — " Checker " diagram to show segregation and recombination of factors. In upper Kne, a black bearing gamete is supposed to unite with a white bearing gamete to give the zygotes shown in F\, each of which is heterozygous for black-white here represented as allelomorphs, etc.
The same scheme may be represented by means of the above checker" diagram (Fig. 43). Black crossed to white gives hybrid black, /^i, whose germ-cells are black or white after segregation. The possible combination of these on random meeting at the time of fertilization is shown by the arrows in F\ and the results are shown in the line marked F^. There will be one pure black, to two black-and-whites, to one pure white.
The first and the last will breed true, if self-fertilized, because they are pure races, while the black-and-whites will give once again, if inbred, the proportions 1:2: 1. A better illustration of Mendel's principles is shown in the case of the white and red Mirabilis jalapa described by Correns. This case is illustrated in Fig. 44, in which the red flower is represented in black and the pink is in gray. The hybrid, Fi, out of white by red, has pink flowers, i.e. it is intermediate in color. When these pink flowers are self-fertilized they produce 1 white, 2 pink, and 1 red-flowered plant again. The history of the germ-cells is shown in Fig. 45. The germ cell of the Fi pink flower segregates into white" and red," which by chance combination give the white pink, and red flowers of F2. The white and red flowers are pure ; the pink heterozygous, i.e. hybrid or mixed. In this case neither red nor white dominates, so that the hybrid can be distinguished from both its parents.
Fig. 44. — Cross between white and red races of Mirabilis Jalapa, giving a pink hybrid in Fi which when inbred gives, in Fa, 1 white, 2 pink, 1 red.
Fig. 45. — Illustrating history of gametes in cross shown in Fig. 44. A white and a red bearing gamete unite to form the pink zygote in Fi, whose gametes, by segregation, are red and white, which by random combinations give the F2 zygotes, etc.
Mendel tested his hypotheses in numerous ways, that I need not now discuss, and found in every case that the results coincided with expectation.
The Heredity of a Sex-Linked Character
We are now in a position to see how Mendel's fundamental principle of segregation applies to a certain class of characters that in the last chapter I called sexlinked" characters.
Diagram 35 (page 64) will recall the mode of transmission of one of these characters, viz. white eyes.
Let us suppose that the determiner for white eyes is carried by the sex chromosome. The white-eyed male has one sex chromosome of this kind. This sex chromosome passes into the female-producing spermatozoon.
Such a spermatozoon fertilizing an egg of the redeyed fly gives a female with two sex chromosomes — one capable of producing red, one capable of producing white. The presence of one red-producing chromosome suffices to produce a red-eyed individual.
When the Fi female produces her eggs, the two sex chromosomes separate at one of the two maturation divisions. Half of the eggs on an average will contain the ^^ white" sex chromosome, half the ^^red." There are, then, two classes of eggs.
When the Fi male produces his sperm, there are also two classes of sperm — one with the '^red" sex chromosome (the female-producing sperm), and one without a sex chromosome (the male-producing sperm) .
Chance meeting between eggs and sperm will give the classes of individuals that appear in the second filial generation {F2) . It will be observed that the Mendelian ratio of 3 red to 1 white appears here also. Segregation gives this result.
The explanation that has just been given rests on the assumption that the mechanism that brings about the distribution of the red- and the white-producing factors is the same mechanism that is involved in sex determination. On this assumption we can readily understand that any character that is dependent on the sex chromosomes for its realization will show sex-linked inheritance.
The reciprocal cross (Fig. 36) is equally instructive. If a white-eyed female is mated to a red-eyed male, all the daughters are red-eyed like the father, and all the sons are white-eyed like the mother. When these, Fi, flies are bred to each other there are produced redeyed females (25%), white-eyed females (25%), redeyed males (25%), and white-eyed males (25%). The explanation (Fig. 36 ; page 65) is in principle the same as for the other cross. If, for instance, we assume that the two X chromosomes in the white-eyed female carry the factors for white, all the eggs will carry one white-producing X. The red-eyed male will contain one X chromosome which is red-producing and passes into the female-producing sperm. The other sperm will not contain any sex chromosome, and hence lacks the factors for these eye colors. When the female-producing sperm, that carries the factor for red, fertilizes a white" egg, the egg will give rise to a female with red eyes, because of the presence of one red-producing chromosome. When the male-producing sperm fertilizes any egg, a white-eyed son will be produced, because the single sex chromosome which he gets from his mother is white-producing.
The production of four classes of individuals in the second generation works out on the same scheme, as shown in the diagram. The inheritance of white and red eyes in these cases is typical of all sex-linked inheritance. In man, for instance, color blindness, so common in males and rare in females, follows the same rules. It appears that hemophilia in man and night-blindness are also examples of sex-linked inheritance. These cases, as already stated, were formerly included under the term sex-limited inheritance, that implies that a character is limited to one sex, but we now know that characters such as these may be transferred to the females, hence the term is misleading. Their chief peculiarity is that in transmission they appear as though linked to the factor for sex contained in the sex chromosome, hence I prefer to speak of them as sex-linked characters.
If our explanation is well founded, each sex-linked character is represented by some substance — some material particle that we call a factor in the sex chromosome. There may be hundreds of such materials present that are essential for the development of sexlinked characters in the organism.
The sex chromosomes must contain, therefore, a large amount of material that has nothing whatever to do with sex determination ; for the characters in question are not hmited to any particular sex, although in certain combinations they may appear in one sex and not in the other.
What then, have the sex chromosomes to do with sex ? The answer is that sex, like any other character, is due to some factor or determiner contained in these chromosomes. It is a differential factor of such a kind that when present in duplex, as when both sex chromosomes are present, it turns the scale so that a female is produced — when present in simplex, the result is to produce a male.
In other words, it is not the sex chromosomes as a whole that determine sex, but only a part of these chromosomes. Hence we can understand how sex is determined when an unequal pair of chromosomes is present, as in lygseus. The smaller chromosome lacks the sex differential, and probably a certain number of other materials, so that sex-linked inheritance is possible here also. Moreover, in a type like oncopeltus, where the two sex chromosomes are alike in size, we infer that they too differ by the sex differential. If all the other factors are present, as their size suggests, sex-linked inheritance of the same kind would not be expected, but the size difference observable by the microscope is obviously too gross to make any such inference certain. We have come to see that it was a fortunate coincidence only that made possible the discovery of sex determination through the sex chromosomes, because the absence of the sex factor alone in the Y chromosome might have left that chromosome in the male so nearly the same size as the X in the female that their relation to sex might never have been suspected. When, however, one of the sex chromosomes began to lose other materials, it became possible to identify it and discover that sex is dependent upon its distribution.
The Heredity of Two Pairs of Characters
Mendel observed that his principles of heredity apply not only to pairs of characters taken singly, but to cases where two or more pairs of characters are involved.
An illustration will make this clear. There are races of edible peas in which the surface is round ; other races in which the surface is wrinkled. Mendel crossed a pea that produces yellow round seeds with one that produces wrinkled green seeds.
The result of this cross was a plant that produced yellow round peas (Fig. 46). Both yellow and round are therefore dominant characters. When these Fi plants were self-fertilized, there were produced plants some of which bore yellow round peas, some yellow wrinkled peas, some green round peas and some green wrinkled peas. These were in the proportion of 9:3:3:1.
The explanation of the result is as follows: One of the original plants produced germ-cells all of which bore determiners for yellow and for round peas, YR; the other parent produced gametes all of which bore determiners for green and for wrinkled, G^¥ (Fig. 47). Their combination may be represented :.
YR by GW = YRGW
The germ-cells of the hybrid plant YRGW produced germ-cells (eggs and pollen) that have either Y or G, and R or W. Expressed graphically the pairs, the so-called allelomorphs, are :
Y n
G W
and the only possible combinations are YR, YW, GR, GW. When pollen grains of these four kinds fall on the stigma of the same kind of hybrid plant whose ovules are also of the four kinds the following chance combinations are possible :
Fig. 46. — Illustrating Mendel's cross of yellow-round with green-wrinkled peas. The figures show the peas of F\ and F^ in the latter in the characteristic ratio of 9 : 3 : 3 : 1.
Fig. 47. — Illustrating the history of the gametes ot the cross represented in Fig. 46. The composition of the parents YR and GW and of the Fi hybrid YRGW is given above. The four classes of o^'^lles and of pollen are given in the middle of the figure. These by random combinations give the kinds of zygotes represented in the squares below. (One YR should be GR.)
In each combination in the table the character of the plant is determined by the dominant factors, in this case yellow and round, hence :
9 YR : 3 YW : 3 GR : I GW
This result works out on the assumption that there is independent assortment of the original determiners' that entered into the combination. The determiner for yellow and the determiner for round peas are assumed to act independently and to segregate from green and wrinkled that entered from the other parent. The 9:3:3:1 ratio rests on this assumption and is the actual ratio realized whenever two pairs of characters freely Mendelize.
The Heredity of Two Sex-Linked Characters
The inheritance of two sex-linked characters may be illustrated by an imaginary case in which the linkage of the factors to each other is ignored. Then the same case may be given in which the actual results obtained, involving linkage, are discussed.
The factors in the fruit fly for gray color, G, and for red eye, R, are both sex-linked, i.e. contained in the X chromosome. Their allelomorphs, viz., yellow color, Y, and white eye, W, are also sex-linked. When a gray red-eyed female is mated to a yellow white-eyed male, the daughters and sons are gray-red, GR. Their origin is indicated in the following scheme :
Gray-red? G R X — G R X
Yellow-white ^ Y W X— ...
„ \G R X Y W X Gray-red 9
^1 [G R X . . . Gray-red^
In the gray-red Fi female there will be the possibility of interchange of the G and F, and of the W and R factors, so, that gametes of four kinds will be formed, namely, GRX — GWX — YRX — YWX. For the moment we may assume free interchange of factors ; and therefore these four classes of eggs will exist in equal numbers.
In the gray-red Fi male there is but one X chromosome that contains the factors G and R. There will be then only one kind of female-producing sperm, viz., GRX ; and one kind of male-producing sperm, the latter containing no X, and therefore none of the factors in question. The chance meeting of these two classes of sperm and the four classes of eggs gives the following results :
Fi eggs GRX — GWX — YRX — YWX Fi sperm GRX
Females. Males.
GRXGRX gray-red.— GRX gray-red.
GRXGWX gray-red.— GWX gray-white.
GRXYRX gray-red.— YRX yellow-red.
GRXYWX gray-red.— YWX yellow-white.
All the females are gray with red eyes, since these are the dominant characters. There are four classes of males. These males give a measure of the kinds of eggs produced by the females, since the male-producing sperms, having no sex chromosomes, do not affect the sex-linked characters derived through the sex chromosome of the Fi female. The expected proportion is therefore :
90
HEREDITY AND SEX
GR9 GR$ GWS YR$ YW $
4 1111
These results are illustrated by means of Fig. 48, in which the yellow color of the fly is indicated by stippHng the body and wings, and the red eyes by black. The X chromosome is also marked and colored in the same way as the flies ; thus the two X's in the red-eyed gray female are half black (for red) and half gray ; the single X in the white-eyed yellow male is half white and half stippled.
Fig. 48. — Inheritance of yellow-white {$) and gray-red ( 9 ) of Drosophila. In Fi both sexes are gray-red. In F2 are produced 4 GR 9 — I GR $ — I GW $ — I YR $ — I YW $ .
In the Fi generation the X chromosomes are first represented as they came in (second line), i.e. with their original composition. The next line gives the three large classes that result, viz., 2 GR9 — 1 GR$ — 1 YW $ . But if free interchange takes place in the female, some of the eggs will have chromosomes like those in the fourth line, viz. YR and GW. Such eggs will give the classes represented in the lowest line, viz., 2 GR9~l GW$—l YR$ . Thus, as already explained, there results one kind of female and four kinds of males.
I said that the proportion 4:1:1:1:1 is the ideal result in the cross between the yellow-white and the gray-red flies. This ideal scheme is not realized because of a complication that comes in. The complication is due to linkage or a tendency to hang together of the characters that go in together. We must now take up this question. It is one of the most modern developments of the Mendelian theory — one that at first seemed to throw doubt on the fundamental idea of random assortment that gives Mendel's proportion 9:3:3:1. But I believe we can now offer a reasonable explanation, which shows that we have to do here with an extension of Mendelism that in no sense invalidates Mendel's principle of segregation. It not only extends that principle, as I have said, but gives us an opportunity to analyze the constitution of the germ-plasm in a way scarcely dreamed of two or three years ago.
The actual numbers obtained in the GR by YW cross are as follows. These are the figures that Dexter has obtained :
Gi^9
GR^
GW$
YR$
YWS
6080
2870
36
34
2373
The apparent discrepancy between the expected and the reaUzed ratios is due to the hnkage of the factors that went into the cross. For instance, the factors for gray and red that went in with one chromosome are Hnked ; hkewise their allelomorphs, yellow and white. As shown by the analysis, the Fi female offspring will have two sex chromosomes, one of each sort — one from the father, the other from the mother. But the male will have but one sex chromosome derived from the mother.
If in the germ-cells of the Fi females there were random assortment of the allelomorphs in the sex chromosomes, the ideal ratio of 4:1:1:1:1 would, as has been said, be reahzed. But if the red and gray factors tend to remain together since they go in together in the one chromosome, and the yellow and white in the other chromosome tend to remain together, and if the chances are about 84 to 1 that the factors that enter together remain together, the reaUzed ratio of 170 : 84 : 1 : 1 : 84 will be found.
Experiments show that, for these two factors, the chances are about 84 to 1 that the factors that go in together remain together; hence the departure from Mendel's ratios for these two pairs of characters. We may make a general statement or hypothesis that covers cases like these, and in fact all cases where linkage occurs : viz. that when factors He in different chromosomes they freely assort and give the Mendelian expectation ; but when factors lie in the same chromosome, they may be said to be linked and they give departures from the Mendelian ratios. The extent to which they depart from expectation will vary with different factors. I have suggested that the departures may be interpreted as the distance between the factors in question.
A Theory of Linkage
In order to understand more fully what is meant by linkage on the interpretation that I have here followed, it will be necessary to consider certain changes that take place in synapsis. The sex chromosomes (when two are present as in the female), Uke all other chromosomes, unite in pairs at the synaptic period. A recognized method of uniting is for like chromosomes to come to lie side by side.
Fig. 49. — Illustrating chiasma-type theory. 1 and 2, from Triton cristatus, 3-46, chromosomes of Batracoseps attenuatus. Note especially the chiasma shown in 13. (After Janssens).
Fig. 50. — Chromatin filaments in the amphitene stage from spermatocytes of Batracoseps. (After Janssens.)
Before they separate, as they do at one of the two maturation divisions, each chromosome may be seen to be spht throughout the length. Thus there are formed four parallel strands each equivalent to a chromosome — the tetrad group. At this time Janssens has found that cross unions between the strands are sometimes present (Fig. 49). In consequence a strand is made up of a part of one chromosome and a part of another. Whether this cross union can be referred to an earlier stage — at the time when the two like chromosomes come together, when they can be seen to twist around each other (Fig. 50) — is not certain ; but the fact of the existence of cross connections is the important point. A consequence of this condition is that the chromosomes that come out of the tetrad may represent different combinations of those that united to form the group. On the basis of this observation we can explain the results of associated inheritance. For, to the same extent to which the chromosomes that unite remain intact, the factors are linked, and to the extent to which crossings occur the exchange of factors takes place. On the basis of the assumption of the linear arrangement of the factors in the chromosomes the distance apart of the factors is a matter of importance. If two factors lie near together, the chance of a break occurring between them is small in proportion to their nearness. We have found that some factors cross over not once in a hundred times. I interpret this to mean that they lie very near together in the chromosome.
Other factors cross over to various degrees ; in the extreme cases the chance is one to one that they cross over. In such cases the factors lie far apart — perhaps near the ends of the chromosome.
The strongest evidence in favor of this view is found when the constant relation of the factors to each other is considered. If, for instance, we know the distance from A to B (calculated on the basis of crossing over) and from B to C, we can predict what A and C will do when they are brought into the hybrid from two parents. If a fourth factor, D, is discovered and its distance from A is made out, we can predict before the experiment is made what will take place when D and B or D and C are combined. The prediction has been fulfilled so many times and in so many ways that we feel some assurance that we have discovered here a working hypothesis of considerable interest. If the hypothesis becomes established, it will enable us to analyze the structure of the chromosomes themselves in the sense that we can determine the relative location of factors in the chromosomes. If, as seems not improbable, the chromosomes are the most important element in Mendelian inheritance, the determination of the linear series of factors contained in them becomes a matter of great theoretical interest ; for we gain further insight into the composition of the material on which heredity itself depends.
There is a corollary to this explanation of crossing over that has a very direct bearing on the results. In the male there is only one sex chromosome present. Hence crossing over is impossible. The experimental results show that no crossing over takes place for sex-linked factors in the male of drosophila.
Other factors, however, lie in other chromosomes. In these cases the chromosomes exist in pairs in the male as well as in the female. Does crossing over occur here in both sexes? Let me illustrate this by an example. In drosophila the factor for black body color and the factor that gives vestigial wings lie in the same chromosome, which we may call the second chromosome. If a black, long- winged female is crossed with a gray vestigial male, all the offspring will be gray in color and have long wings, since these are the dominant characters. If these Fi flies are inbred, the following classes will appear:
Gray Long Black Long Gray Vestigial
2316 1146 737
It will be noted that there are no black vestigial flies. Their absence can be explained on the assumption that no crossing over in the male, between the factors in the second chromosome, has taken place.
But if another generation (F-s) is raised, some black vestigial flies appear. With these, it is possible to test the hypothesis just stated. If, for instance, some of the long, gray, Fi females are mated to black vestigial males, the following classes are produced :
GL BL GV BV
578 1413 1117 307
The results are explicable on the view that crossing over takes place in the germ-cell of the Fi female, and that the chance that such will occur is as 1 to 3.
But if the long-winged, gray, i^i males are crossed to black vestigial females, only the following classes are produced :
BL GV
992 721
These results are in accord with the hypothesis that no crossing over takes place between the second chromosomes in the Fi male. Why crossing over should occur in the Fi female, and not in the Fi male, we do not know at present ; and until the synaptic stages in the males and females have been carefully studied, we must wait for an answer to the question.
Three Sex-Linked Factors
When three sex-Hnked factors exist in the same chromosomes, we have a method by means of which the ^'crossing-over" hypothesis may be put to a further test. Sturtevant has recently worked over the evidence for a case of this kind. He analyzed the data of the cross between a fly having gray color, red eyes, long wings, mated to a fly with yellow color, white eyes, and miniature wings. The relative location of these three factors is shown in the above diagram (Fig. 51, E, F, G, H). The Fi flies gave the expected results. These inbred gave the following F2 significant classes :
- The classes omitted are those that do not bear on the question in hand.
Fig. 51. — A-D, YW and GR that enter (A), crossing over to give YR and GW as seen in D. E-Ei, no crossing over. F-F\, crossing between WM and RL. G-Gi, crossing between YW and GR. H-Hi, double crossing over of YWM and GRL, to give YRM and GWL.
GRL YWM GWM YRL GRM YWL GWL YRM
2089 1361 17 23 887 817 5
In these results the classes where single crossing over is shown are GWM (17) and YRL (23) (Fig. 51, G, G') and GRM (887) and YWL (817) (Fig. 51, F, F').,
There are two classes, namely, GWL (5) and YRM (0) (Fig. 51, H, H'), which involve double crossing over. In order that they may take place, the two sex chromosomes in the female must break twice and reunite between the factors involved, as shown in the diagram. Such a redistribution of the parts of the homologous chromosomes would be expected to occur rarely, and the small number of double crossovers recorded in the results is in accord with this expectation.
In these questions of linkage we have considered some of the most recent and difficult questions in the modern study of heredity. We owe to Bateson and his collaborators the discovery of this new departure. In plants they have recorded several cases of linkage, and other authors, notably Correns, Baur, Emerson, East, and Trow have described further cases of the same kind. Bateson has offered an interpretation that is quite different from tlie one that I have here followed. His view rests on the assumption that separation of factors may take place at different times, or periods, in the development of the germinal tissues.
In a word, he assumes that assortment is not confined to the final stages in the ripening of the germ-cells, but may take place at any time in the germ-tract. It seems to me, however, if the results can be brought into line with the known changes that take place in the germ-cells at the time when the maternal and paternal chromosome unite, that we have not only a simpler method of dealing with these questions, but it is one that rests on a mechanism that can be studied by actual observation. Moreover, on purely a priori grounds the assumptions that I have made seem much simpler and more tangible than the assumptions of reduplication" to which Bateson resorts.
But leaving these more theoretical matters aside, the evidence from a study of sex-linked characters shows in the clearest manner that they, while following Mendel's principle of segregation, are also undeniably associated with the mechanism of sex. There is little doubt that sex itself is inherited in much the same way, since we can explain both in terms of the same mechanism. This mechanism is the behavior of the chromosomes at the time of the formation of the germcells.
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
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