Heredity and Sex (1913) 2
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
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Chapter II The Mechanism of Sex-Determination
In many species of animals and plants two kinds of individuals are produced in every generation. This process occurs with such regularity and persistence that our minds naturally seek some mechanism, some sort of orderlv machinerv, bv which this condition is brought about. Yet from the time of Aristotle almost to the present day the problem has baffled completely all attempts at its solution. However, the solution is very simple. Xow that we hold the situation in our grasp, it seems surprising that no one was keen enough to deduce it by purely theoretical reasoning. At least the general principles involved might have been deduced, although we can see that without an intimate knowledge of the changes that take place in the germcells the actual mechanism could never have been foretold.
The bodies of animals and plants are composed of millions of protoplasm-filled compartments that are called cells. In the middle of each cell there is a sphere, or nucleus, containing filaments called chromosomes fFig. 5).
At each division of a cell the wall of the nucleus is absorbed, and the thread-like chromosomes contract into rod-shaped, or rounded bodies (Fig. 6). Each chromosome sphts lengthwise into halves ; the halves are brought into relation with a spindle-shaped system of Unes, and move apart along these lines to opposite sides of the cell. The protoplasm of the cell next constricts to produce two daughter cells, each containing a group of daughter chromosomes.
Fig. 19. — Fertilization and polar-body formation of Nereis. The four smaller figures show entrance of sperm. The extrusion of the first polar body is shown in lower left-hand figure and of the second polar body in the two large right-hand figures. The last three also show the formation of the sperm asters, which is the beginning of the first cleavage spindle in the egg. (After F. R. Lillie.)
The egg is also a cell, and in its earlier stages contains the same number of chromosomes as do the other cells of the body; but after two peculiar divisions that take place at maturation the number of the chromosomes is reduced to half.
But before this time the egg-cells divide, like all the other cells of the body. In this way a large number of eggs is produced. After a time they cease to divide and begin to grow larger, laying up yolk and other materials. At this time, the chromosomes unite in pairs, so that their number seems to be reduced to half the original number. At the final stage in the maturation of the egg, two peculiar divisions take place that involve the formation of two minute cells given off at one pole — the polar bodies. In some eggs, as in the sea urchin, the polar bodies are given off while the egg is still in the ovary and before fertilization ; in other eggs, as in the frog, one polar body is given off before fertilization, the other after the sperm has entered ; and in other eggs, as in nereis (Fig. 19), both polar bodies are given off after fertilization.
The formation of the polar bodies is a true celldivision, but one that is unique in two respects. First, one of the cells is extremely small, as seen in Fig. 19. The smallness is due to the minute amount of protoplasm that it contains. Second, the number of chromosomes at each division is the half or haploid " number. There is much evidence to show that at one or at the other of these two divisions the two chromosomes that had earlier united are separated, and in this respect this division differs from all other cell-divisions. In consequence, the egg nucleus, 4:hat re-forms after the second polar body has been produced, contains only half the actual number of chromosomes cliaracteristic of all the other cells of the female.
In the formation of the spermatozoa a process takes place almost identical with the process just described for the female (Fig. 20). In their earUer history the germ-cells of the male divide with the full number of chromosomes characteristic of the male, which may be one less chromosome than in the female. The early germ-cells then cease to divide for a time, and begin to grow, laying up yolk and other materials. At this time the chromosomes unite in pairs, so that the number appears to be reduced to half. Later two divisions occur (Fig. 20, D-H), in one of which the united chromosomes separate. The male germ-cells differ, how ever, from the female, in that at each of these two divisions the cells are equal in size. Thus four spermcells are produced from each original cell, all four produce tails, and become spermatozoa.
Fig. 20. — A-B, somatic cell division with four chromosomes. C-H, the two maturation divisions to produce the four cells {H) that become spermatozoa. (After Wilson.)
At the time of fertilization, when the spermatozoon touches the surface of the egg, the egg pushes out a cone of protoplasm at the point of contact (Fig. 19), and, lending a helping hand, as it were, to the sperm, draws it into the egg. The projecting cone of protoplasm is called the fertilization cone. In a few minutes the head of the sperm has entered. Its tail is often left outside. The head absorbs fluid from the egg and becomes the sperm nucleus, which passes towards the center of the egg. Here it comes to lie by the side of the egg nucleus, and the two fuse. The walls of the combined nuclei dissolve away and the chromosomes appear. Half of these are derived from the father through the nucleus of the sperm, and half from the mother through the egg nucleus. If we count the paternal chromosomes, there are half as many of them as there are chromosomes in each cell of the body of the father. Presently I shall point out that this statement is not always true, and on this little fact, that it is not quite true, hangs the whole story of sex-determination.
What is the meaning of these curious changes that have taken place in the egg and sperm ? Why has the egg deliberately, as it were, twice thrown away its most valuable heritage — its chromatin material ? We do not know with certainty, but one consequence at least stands out clearly ! Before the egg gave off its polar bodies it had the full, or diploid, number of chromosomes. After this event it has only half as many. A similar reduction occurs in the sperm, excepting that no chromatin is lost, but is redistributed amongst four spermatozoa. Egg and sperm-nucleus each have in consequence the haploid or half number. By combining they bring up the number to that characteristic of the species.
The history of the germ-cells, that we have just traced, is the background of our knowledge of the process of heredity in so far as observable changes in the germ-cells have been made out. We owe to Weismann more than to any other biologist the realization of the importance of these changes. It is true that Weismann contributed only a part of the actual facts on which the interpretation rests. Many workers, and a few leaders, have laboriously made out the complete account. But Weismann, by pointing out the supreme importance of the changes that take place at this time, has furnished a stimulus that has acted like yeast in the minds of less imaginative workers.
We are now in a position to apply this knowledge to the interpretation of the mechanism by means of which sex is determined.
The Cytological Evidence
If we study by means of modern histological methods the body cells of the male of the insect, Protenor belfragei, we find, when each cell is about to divide, that a group of chromosomes appears like that shown in Fig. 21, A. There are twelve ordinary oval chromosomes, and one much larger than the rest. This group of chromosomes is characteristic of all divisions of the cells of the body, regardless of whether the cells belong to muscle, skin, gland, ganglion, or connective tissue. The early germ-cells of the male, the so-called spermatogonia, ' ' also have this same number. It is not until a later stage in their development that a remarkable change takes place in them. When this change occurs the thread-like chromosomes unite in pairs. This is the synapsis stage — the word means to fuse together.
It is the most difficult stage to interpret in the whole history of the germ-cells. In a few forms where the changes that take place have been seen to best advantage it is found that chromosomes are in the form of long threads and that these threads unite in pairs to make thicker threads. When the process is completed, we find half as many threads as there were before. This statement is not quite true. In the case of the male protenor, for instance, there are twelve ordinary chromosomes and one large one. The twelve unite in pairs at synapsis, so that there are six double chromosomes, but the large one has no mate (Fig. 21, B). When the others have united in synapsis, it has taken no part in the process, hence the reduced number of chromosomes in the male is seven — the seventh is the sex chromosome.
Two divisions now follow each other in rapid succession (Fig. 21, C, D). In the first division (C) each chromosome divides — seven go to one pole and seven to the other pole. Two cells, the primary spermatocytes, are produced. Without resting, another division takes place (D) in each of these two cells. It is the second spermatocyte division. Each of the six ordinary chromosomes divides, but the large sex chromosome does not divide, and, lagging behind the others, as shown in the figure (D), it passes to one pole. Each secondary spermatocyte produces, therefore, two cells — one with six, the other with seven chromosomes. These cells become spermatozoa {EE'), the ones with seven chromosomes are the female-producing spermatozoa, the ones with six chromosomes are the male-producing spermatozoa. These two classes of spermatozoa are present in equal numbers.
Fig. 21.
If we study the body cells of the female protenor, we find fourteen chromosomes (Fig. 22, A). Twelve of these are the ordinary chromosomes, and two, larger than the rest, are the sex chromosomes. At the synapsis stage all of the chromosomes unite in pairs, including the two sex chromosomes. When the process is finished, there are seven double chromosomes (Fig. 22, B).
When the egg sends off its two polar bodies, the chromosomes divide or separate. At the first division seven chromosomes pass out (C), and seven remain in the egg. At the next division the seven chromosomes in the egg divide again, seven pass out and seven remain in the egg (Z)). Of these seven, one chromosome, recognizable by its large size, is the sex chromosome.
All the eggs are alike {E). there is only one kind of egg, but there are two kinds of sperm. Any egg that is fertilized by a sperm carrying six chromosomes produces an individual with thirteen chromosomes. This individual is a male.
Any egg that is fertilized by a sperm carrying seven chromosomes produces an individual with fourteen chromosomes. This individual is a female.
In another species of insect, Lygseus bicrucis, the male differs from the female, not in having a different number of chromosomes as in protenor, but by the occurrence of a pair of different-sized chromosomes.
Fig. 23.
The body cells of the male have twelve ordinary chromosomes and tw^o sex chromosomes — one larger, X, than the other, Y (Fig. 23, A).
After synapsis there are six double chromosomes and the two sex chromosomes, called X and Y (Fig. 23, D).
At the first spermatocyte division all the chromosomes divide (C). The two resulting cells have eight chromosomes, including X and Y. At the second division [D) the double chromosomes again divide, but X and Y do not divide. They approach and touch each other, and are carried into the spindle, where they separate from each other when the other ordinary chromosomes divide. Consequently there are formed two kinds of spermatozoa — one containing X and the other Y (Fig. 23, E).
In the body cells and early germ-tract of the female of lygseus (Fig. 24, A), there are twelve ordinary chromosomes and two sex chromosomes, X and X. After reduction there are seven double chromosomes, the two X's having united when the other chromosomes united (B). Two divisions take place (C, D), when the two polar bodies are formed, leaving seven chromosomes in the egg (E) . Each egg contains as a result only one X chromosome.
Any egg of lygseus fertilized by a sperm carrying an X chromosome produces a female that contains two X's or XX. Any egg fertilized by a sperm containing a Y chromosome produces a male that contains one X and one Y, or XF.
Another insect, Oncopeltus fasciatus, represents a third type in which the chromosome groups in the male and in the female are numerically alike and alike as to visible size relations.
In the body cells of the male there are sixteen chromosomes (Fig. 25, A). After reduction there are nine chromosomes — seven in a ring and two in the middle (B). The seven are the fused pairs or double chromosomes ; the two in the middle are the sex chromosomes that have not fused.
Fig. 26.
The evidence for this interpretation is circumstantial but sufficient.
At the first reduction division all nine chromosomes divide (C). Just before the second division the two central chromosomes come together and remain in contact {DD'). All the double chromosomes then divide, while the two sex chromosomes simply separate from each other, so that there are eight chromosomes at each pole (DE).
In this case all of the spermatozoa (EE') contain eight chromosomes. There is no visible difference between them. Nevertheless, there is reason for believing that here also there are two kinds of sperm. The principal reason is that there are all connecting stages between forms in which there is an unequal pair, as in lygseus, and forms with an equal pair, as in oncopeltus. Another reason is that the two sex chromosomes behave during the synapsis stages as do the X Y chromosomes in related species. Moreover, the experimental evidence, of which I shall speak later, leads us to conclude that the determination of sex is not due only to a difference in size of X and Y. The sex chromosomes must carry a host of factors other than those that determine sex. Consequently it is not surprising that in many species the sex chromosomes appear equal or nearly equal in size. It is a fortunate circumstance for us that in some species there is a difference in size or an unpaired sex chromosome ; for, in consequence, we are able to trace the history of each kind of sperm in these cases ; but it is not essential to the theory that X and F, when present, should be visibly different.
Fig. 27.
In the female of oncopeltus sixteen chromosomes occur as in the male (Fig. 26, A). The reduced number is eight double chromosomes {E). At one of the two polar divisions eight chromosomes pass out, and eight remain in the egg (C). At the second division also eight pass out, and eight remain in the egg (Z)).
I shall pass now to a fourth condition that has only recently come to light. It is best shown in some of the nematode worms, for example, in the ascaris of the horse. Here the sex chromosomes are generally attached to other chromosomes. In this case, as shown by the diagram (Fig. 27, A), there is in the male a single X attached to one of the other chromosomes. At the first spermatocyte division it does not divide (C), but passes over bodily to one pole, so that two kinds of cells are produced. At the second spermatocyte division it divides, in the cell that contains it, so that each daughter cell gets one X (D). Two classes of sperm result, two with X {E), two without (E').
In the female there are two X's, each attached to a chromosome (Fig. 28). After the polar bodies are given off, one X only is left in each egg (C, D, E). Sex is determined here in the same way as in the insects, described above, for there are two classes of sperm and but one class of eggs.
The discovery of the sex chromosome and its relation to sex is due to several investigators. In 1891 Henking first described this body, and its unequal distribution, but was uncertain even as to its relation to the chromosomes. Paulmier (1899), Montgomery (1901), Sinety (1901), gave a correct description of its behavior in spermatogenesis. McClung (1902) confirmed these discoveries, and suggested that the accessory, or odd chromosome, as it was then called, had some relation to sex, because of its unequal distribution in the sperms. He inferred that the male should have one more chromosome than the female, but he gave no evidence in support of this suggestion, which as we have seen is the reverse of the actual conditions. Stevens (1905) made out the relations of the XF pair of chromosomes to sex and Wilson in the same year (1905) the correct relation of the accessory chromosome to sex. The results described above for the insects are for the most part from Wilson's studies on the chromosomes ; these for ascaris from the recent work of Sophia Frolowa, which confirms in the main the work of Boveri, Gulick, Boring, and Edwards.
A case similar to ascaris has been described by Stevens for the mosquito, in which there is an X and a F in the male, each attached to another chromosome. In the guinea pig also, there seems to be in the male an X and a F, attached to another pair of chromosomes. Finding these cases so widely distributed, it seems not unlikely that in other cases, where an unpaired X or an X and a F have not been detected, they are parts of other chromosomes.
The whole history of the sex chromosomes of ancyrocanthus, a nematode worm, is strikingly shown in a recent paper by Carl Mulsow (Fig. 29 and 29a, A). This is a typical case in which the male has one less chromosome than the female, as in protenor. The case is striking because the chromosomes can be seen and counted in the living spermatozoa. Some sperm have six, some have five chromosomes. The spermnucleus can be identified in the egg after fertilization because it lies nearer the pole opposite to the polar bodies. The entering sperm nuclei show in half of the fertilized eggs six chromosomes and in the other half five chromosomes.
An interesting confirmation of these conclusions in regard to the relation between sex and the sex chromosomes was found in another direction. It has long been known that the fertiUzed eggs of aphids or plant lice produce only females. The same thing happens in near relatives of the plant lice, the phylloxerans.
Fig. 29. — 1 and 2 are spermatogonia ; 3, growth period ; 4-7, prophases ; 8, equatorial plate of first division, 9-10 ; 11, spermatocytes of second order ; 12-13, division of same; 14-16, the four cells or spermatids that come from the same original cell, two with 5, two with 6 chromosomes; 17, spermatids; 18, mature sperm; 19, living sperm. (After Mulsow.)
In these insects a study of the chromosomes shows that the male has one less chromosome than the female. At the first maturation division in the male (Fig. 30), all the chromosomes divide except one, the X chromosome, and this passes to one cell only. This cell is also larger than the sister cell. The small cell lacking the X degenerates, and does not produce spermatozoa. The large cell divides again, all of the chromosomes dividing. Two functional spermatozoa are produced, each carrying one sex chromosome. These spermatozoa correspond to the female-producing spermatozoa of other insects.
In the sexual female there is an even number of chromosomes — one more than in the male. They unite in pairs. When the two polar bodies of the sexual egg are formed, all the chromosomes divide twice, so that each egg is left with one sex chromosome.
Fig. 29a. — 20 and 21, oogonia (equatorial plate) ; 22, growth period ; 23, before fertilization; 24-25, entrance of sperm; 26-31, prophases of first division ; 32-33, formation of first polar body ; 34-36, extrusion of same and formation of second polar body; 37, two pronuclei ; 38-41, union of pronuclei ; 42-45, cleavage. (After Mulsow.)
It is now evident why only females are produced after fertilization. The female-producing sperm alone is functional.
Fig. 30. — Diagram of chromosomes in Phylloxera carycecaulis. Top line, somatic cell of female with 6 chromosomes and somatic cell of male with 5 chromosomes. Second line, stages in first spermatocyte division producing a rudimentary cell (below) with two chromosomes. Third line, second spermatocyte division into two equal cells. Fourth line, sexual egg (3 chromosomes) and two polar bodies ; and two functional, femaleproducing sperm with three chromosomes each.
The Experimental Evidence
The experimental evidence, indicating that there is an internal mechanism for sex determination, is derived from two sources — from experimental embryology, and from a study of the heredity of sex-Hnked characters.
The evidence from embryology shows that the chromosomes are the bearers of materials essential for the production of characters. The evidence from heredity shows that certain characters follow the sex chromosomes.
It has long been taught that the hereditary factors are carried by the nucleus. The evidence for this was found in fertilization. When the spermatozoon enters the egg, it carries in, as a rule, only the head of the spermatozoon, which consists almost entirely of the nucleus of the original cell from which it comes. Since the male transmits his characters equally with the female, it follows that the nucleus is the source of this inheritance.
The argument has not been regarded as entirely conclusive, because the sperm may also bring in some of the protoplasm of the original cell— at least that part lying immediately around the nucleus. In addition a small body lying at the base of the sperm head seems also to be brought in by the male, and according to some observers it becomes the center about which the entire division system or karyokinetic spindle develops.
The most convincing evidence that the chromosomes are the most important elements in heredity is found in some experimental work, especially that of Boveri, Baltzer, and Herbst. Under certain circumstances in the sea-urchin two spermatozoa may enter a single egg. They both unite with the egg nucleus (Fig. 31). Each brings in 18 chromosomes. The egg contributes 18 chromosomes. There are in all 54, instead of 36 chromosomes, as in normal fertilization.
Fig. 31. — Dispermy and its effects in egg of sea urchin. (After Boveri.)
Around these chromosomes a double system of threads develops with four poles. The chromosomes become unequally distributed on the four spindles that develop. Each chromosome then divides, and half of each goes to the nearest pole. To some of the poles many chromosomes may pass, to other poles fewer.
In order to simplify the case let us imagine that each sperm has only four chromosomes and the egg nucleus only four. Let us represent these by the letters as shown in Fig. 32. Any one of the four cells that is produced at the first division of these dispermic eggs may contain a full complement of the chromosomes, or only some of them. The possibilities for four chromosomes are shown in the diagram. Any cell that does not contain at least these four chromosomes is shaded. One case is present in which all the four cells contain a complete assortment. If normal development depends on an embryo containing in every cell at least one of each kind of chromosome, then in our simple case only one group of four cells has this possibility.
Fig. 32. — Diagram illustrating the irregular distribution of the chromosomes in dispermic eggs in an imaginary case with only four kinds of chromosomes, a, b, c, d. There are here three sets of each of these in each egg. The stippled cells are those that fail to receive one of each kind of chromosome. (After Boveri.)
Boveri found that such dispermic eggs produce normal embryos very rarely. He calculated what the chance would be when three times 18 chromosomes are involved. The chance for normal development is probably not once in 10,000 times. He isolated many dispermic eggs and found that only one in 1,500 of the tetrad type developed normally.
Boveri went still further in his analysis of the problem. It had been shown for normal eggs that if at the two-celled stage the cells are separated, each forms a perfect embryo. This is also true for each of the first four cells of the normal egg.
Boveri separated the four cells of dispermic eggs and found that the quadrants not infrequently developed normally. This is what we should anticipate if those cells can develop that contain one of each kind of chromosome.
The evidence furnishes strong support of the view that the chromosomes are different from each other, and that one of each kind is necessary if development is to take place normally.
The evidence that Baltzer has brought forward is also derived from a study of sea-urchin eggs. It is possible to fertilize the eggs of one species with sperm of another species. The hybridizing is greatly helped by the addition of a little alkali to the sea water.
Baltzer made combinations between four species of sea-urchins. We may take one cross as typical. When eggs of strongylocentrotus are fertiUzed with sperm of sphserechinus, it is found at the first division of the egg that, while some of the chromosomes divide and pass normally to the two poles, other chromosomes remain in place, or become scattered irregularly between the two poles, as shown in Fig. 33. When the division is completed, some of these chromosomes are found outside of the two main nuclei. They often appear as irregular granules, and show signs of degeneration. They are still present as definite masses after the next division, but seem to take no further part in the development.
Fig. 33. — 1 and la, chromosomes in the normal first cleavage spindle of Sphajrechinus ; 2, equatorial plates of two-cell stage of same ; 3-3a, hybrid, Sphaerechinus by Strongylocentrotus, spindle at two-cell stage ; 4-4a, same equatorial plates; 5-5a, hybrid, Strong, by Sphaer., cleavage spindle in telophase ; 6, next stage of last ; 7, same, two-cell stage ; 8, same, later ; 9, same, four-cell stage ; 10, same, equatorial plate in two-cell stage (12 chromosomes) ; 11, same, from later stage, 24 chromosomes. (After Baltzer.)
Baltzer has attempted to count the number of chromosomes in the nuclei of these hybrid embryos. The number is found to be about twenty-one. The maternal egg nucleus contains eighteen chromosomes. It appears that only three of the paternal chromosomes have succeeded in getting into the regular cycle — fifteen of them have degenerated.
Baltzer thinks that the egg acts injuriously in this case on the chromosomes of foreign origin, especially on the fifteen that degenerate, so that they are eliminated from the normal process.
The embryos that develop from these eggs are often abnormal. A few develop as far as the pluteus stage, when a skeleton appears that is very characteristic for each species of sea-urchin. The plutei of these hybrids are entirely maternal. This means that they are exactly like the plutei of the species to which the mother belongs.
The conclusion is obvious. The sperm of sphserechinus has started the process of development, but has produced no other effect, or has at most only slightly affected the character of the offspring. It is reasonable to suppose that this is because of the elimination of the paternal chromosomes, although the evidence is not absolutely convincing.
Let us now examine the reciprocal cross. When the eggs of sphserechinus are fertilized by the sperm of strongylocentrotus, the division of the egg and of the chromosomes is entirely normal. All the chromosomes divide and pass to the poles of the spindle. The total number (36) must, therefore, exist in each cell, although in this case they were not actually counted.
The pluteus that develops has peculiarities of both maternal and paternal types. It is hybrid in structure.
Both parents have contributed to its formation. It is not going far from the evidence to infer that the hybrid character is due to both sets of chromosomes being present in all of the cells.
Fig. 34. — 1. The chromosomes of the egg lie in the equator of the spindle, the chromosomes of the sperm lie at one side. 2. A later stage, showing all the paternal chromosomes passing to one pole. 3 (to the right). A later stage, a condition like the last. There is also a supernumerarj' sperm in the egg (to left, in another section.) 4. Same condition as last. 5. Pluteus larva that is purely maternal on one side and hybrid on the other. (After Herbst.)
The evidence that Herbst has brought forward is of a somewhat different kind. It supplements Baltzer's evidence and makes more probable the view that the chromosomes are essential for the development of the characters of the individual.
Herbst put the eggs of sphaerechinus into sea water to which a httle valerianic acid had been added. This is one of the many methods that Loeb has discovered by which the egg may be induced to develop parthenogenetically, i.e. without the intervention of the spermatozoon. After five minutes the eggs were removed to pure sea water and the sperm of another species, strongylocentrotus, was added. The sperm penetrated some of the eggs. The eggs had already begun to undergo the changes that lead to division of the cell — the sperm entered ten minutes late. The egg proceeded to divide, the sperm failed to keep pace, and fell behind. Consequently, as shown in Fig. 34, the paternal chromosomes fail to reach the poles when the nuclei are re-formed there. The paternal chromosomes form a nucleus of their own which comes to lie in one of the two cells. In consequence one cell has a nucleus that contains only the maternal chromosomes ; the other cell contains two nuclei, one maternal and the other paternal. In later development the paternal nucleus becomes incorporated with the maternal nucleus of its cell. There is produced an embryo which is maternal on one side and hybrid on the other. Herbst found in fact that half-and-half plutei were not rare under the conditions of his experiment.
This evidence is almost convincing, I think, in favor of the view that the chromosomes are the essential bearers of the hereditary qualities. For in this case, whether the protoplasm of the embryo comes from the egg or the sperm also, the fact remains that the half with double nuclei is hybrid. Even if the spermatozoon brought in some protoplasm, there is no reason to suppose that it would be distributed in the same way as are the paternal chromosomes.
Evidence from Sex-Linked Inheritance
The experimental evidence based on sex-linked inheritance may be illustrated by the following examples.
The eyes of the wild fruit-fly, Drosophila ampelophila, are red. In my cultures a male appeared that had white eyes. He was mated to a red-eyed female. The offspring were all red-eyed — both males and females (Fig. 35). These were inbred and produced in the next generation red-eyed females, red-eyed males, and white-eyed males (Fig. 35). There were no whiteeyed females. The white-eyed grandfather had transmitted white eyes to half of his grandsons but to none of his granddaughters.
Equally important are the numerical proportions in which the colors appear in the grandchildren. There are as many females as the two classes of males taken together ; half of the males have red eyes and half have white eyes. The proportions are therefore 50 % red females, 25 % red males, 25 % white males.
Only white-eyed males had appeared at this time. It may seem that the eye color is confined to the male sex. Hence the origin of the term sex-limited inheritance for cases like this. But white-eyed females may be produced easily. If some of the red-eyed granddaughters are bred to these white-eyed males, both white-eyed females and males, and red-eyed females and males, appear (Fig. 37). The white eye is therefore not sex-limited but sex-linked.
With these white-eyed females it is possible to make the reciprocal cross (Fig. 36). A white-eyed female was mated to a red-eyed male. All of the daughters had red eyes and all the sons had white eyes. These were then inbred and gave red-eyed males and females, and white-eyed males and females in equal numbers (Fig. 36).
Fig. 35. — Sex-linked inheritance of white and red eyes in Drosophila. Parents, white-eyed $ and red-eyed 9 ; -^i. red-eyed $ and 9 ; F2 redeyed 9 , red-eyed $ and white-eyed $ . To right of flies the history of the sex chromosomes XX is shown. The black X carries the factor for red eyes, the open X the factor for white eyes ; the circle stands for no X.
The heredity of this eye color takes place with the utmost regularity, and the results show that in some way the mechanism that is involved is closely bound up with the mechanism that produces sex.
Other combinations give results that are predictable from those just cited. For instance, if the Fi red-eyed female from either of the preceding crosses is mated to a white-eyed male, she produces red-eyed males and females, and white-eyed males and females, as shown in
Fig. 36. — Reciprocal cros.s of Fig. 35. Parents, white-eyed 9 and
red-eyed $, (criss-cross inheritance). Fi, red-eyed 9. white-eyed $.
F', red-eyed 9 find $ ; white-eyed 9 and $. To right, sex chromosomes (as in Fig. 35).
Fig. 37 (upper two lines). If the Fi red-eyed male from the first cross (Fig. 35) is bred to a white-eyed female, he will produce red-eyed daughters and whiteeyed sons. Fig. 37 (lower two lines).
The same relations may next be illustrated by another sex-linked character.
A male with short or miniature wings appeared in my cultures (Fig. 38). Mated to long-winged females only long-winged offspring were produced. When these were mated to each other, there were produced
Fig. 37. — Upper series, back cross of Fi 9 to white ^ . Lower series back cross of Fi red-eyed $ to white 9 .
long-winged females (50%), long-winged males (25%) and miniature-winged males (25%).
It is possible to produce, in the way described for eye color, miniature-winged females.
When such miniature-winged females are mated to long-winged males, all the daughters have long wings, and all the sons have miniature wings (Fig. 39). If these are now mated, they produce, in equal numbers, long-winged males and females and miniature-winged males and females.
The same relations may again be illustrated by body color.
Fig. 38. — Sex-linked inheritance of short ("miniature") and long wings in Drosophila. Parents, short-winged $ , long-winged 9 . Fi long-winged $ and 9 • F2 long-winged 9 and $ and short-winged $ . Sex chromosomes to right. Open X carries short wings.
A male appeared with yellow wings and body. Mated to wild gray females he produced gray males and females. These mated to each other gave gray females (50%), gray males (25%), and yellow males (25%).
As before, yellow females were made up. Mated to gray males they gave gray females and yellow males.
These inbred gave gray males and females and yellow males and females, in equal numbers.
These cases serve to illustrate the regularity of this type of inheritance and its relation to sex. In the fruit fly we have found as many as twenty-five sex-linked factors. There are other kinds of inheritance found in these flies, and at another time I shall speak of some of these ; but the group of sex-linked factors is of special importance because through them we get an insight into the heredity of sex.
Fig. 39. — Reciprocal cross of Fig. 38. Parents, long-winged Z and short-winged 9 . Fi long-winged 9 . short-winged Z • F2 long-winged 9 and Z , short-winged 9 and $ . Sex chromosomes as in last.
In the next chapter, when we take up in detail Mendelian heredity, I shall try to go further into the explanation of these facts. For the present it will suffice to point out that the cases just described, and all like them, may be accounted for by means of a very simple hypothesis. We have traced the history of the sex chromosomes. If the factors that produce white eyes, short (miniature) wings, and yellow body color are carried by the sex chromosomes, we can account for these results that seem at first sight so extraordinary. The history of the sex chromosomes is accurately known. Their distribution in the two sexes is not a matter of conjecture but a fact. Our hypothesis rests therefore on a stable foundation.
At the risk of confusion I feel bound to present here another type of sex-linked inheritance. In principle it is like the last, but the actual mechanism, as we shall see, is somewhat different. Again I shall make use of an illustration. If a black Langshan hen is mated to a barred Plymouth Rock cock, all the offspring will be barred (Fig. 40). If these are inbred, there are produced barred females and males, and black females. The numerical proportion is 50 per cent barred males, 25 per cent barred females, and 25 per cent black females.
The black hen has transmitted her character to half of her granddaughters and to none of her grandsons. The resemblance to the case of the red-eyed, whiteeyed flies is obvious, but here black appears as a sexlinked character in the females;
The converse cross is also suggestive. When a barred hen is mated to a black cock, all the daughters are black and all the sons are barred (Fig. 41). When these are inbred, there are produced black males and females and barred males and females in equal numbers. Again, the resemblance of the reciprocal cross to one of the combinations for eye color is apparent. In fact, this case can be explained on the same principle as that used for the flies, except that in birds it is the female that produces two kinds of eggs ; she is heterozygous for a sex factor while the male produces only one kind of spermatozoon.
Fig. 40. - Sex-linked inheritance in fowls. Upper line black Langshan hen and barred Plymouth Rock cock. Second Hue F, barred coc^ and hen. Third line, Fo, three barred (cock, hen, cock) and one black (hen). (Cuts from " Reliable Poultry Journal.")
Fig. 41. — Reciprocal cross of Fig. 40. Upper line, black cock and barred hen. Second line, h\, barred cock and black hen. Third line, Fi, barred hen and cock, black cock and hen. (Cuts from " Reliable Poultry Journal.")
We lack here the certain evidence from cytology that we have in the case of the insects. Indeed, there is some cytological evidence to show that the male bird is heterozygous for the sex chromosome. But the evidence does not seem to me well established ; while the experimental evidence is definite and has been independently obtained by Bateson, Pearl, Sturtevant, Davenport, Goodale and myself. However this may be, the results show very clearly that here also sex is connected with an internal mechanism that is concerned with other characters also. This is the mechanism of Mendelian heredity. Whether the chromosomes suffice or do not suffice to explain Mendelian heredity, the fact remains that sex follows the same route taken by characters that are recognized as Mendelian.
To sum up : The facts that we have considered furnish, I believe, demonstrative evidence in favor of the view that sex is regulated by an internal mechanism. The mechanism appears, moreover, to be the same mechanism that regulates the distribution of certain characters that follow Mendel's law of inheritance.
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Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.
Cite this page: Hill, M.A. (2024, April 23) Embryology Heredity and Sex (1913) 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Heredity_and_Sex_(1913)_2
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