Book - Evolution and Genetics 8

From Embryology

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 8 Sex-Linked Inheritance

When we follow the history of pairs of chromosomes we find that their distribution in successive generations is paralleled by the inheritance of ]Mendelian characters. This is best shown in the sex chromosomes (fig. 44) . In the female of Drosophila there are two of these chromosomes that are called X-chromosomes ; in the male there are also two, but one differs from those of the female in its shape, and in the fact that it carries none of the ordinary genetic factors. It is called the K-chromosome.

Morgan 1925 fig44.jpg

Fig. 44. Diagram showing the distribution of the sex chromosomes from parents to offspring.


The inheritance of a pair of characters whose genes lie in the A^-chromosomes is shown in figures 45 and 46, illustrating crosses between a white-eyed and a red-eved individual.

The first of these represents a cross between a white-eyed male and a red-eyed female (fig. 45, top row) . The X-chromosome in the male is represented by a bar, (w), the I^-chromosome is bent. In the female the A"-chromosomes are W and W. Each egg of such a female will retain one X (with W) after the polar bodies have been thrown off. In the male there are two classes of sperm — the female producing, carrying JT (with w) ; and the male producing, carrying the I^-chromosome. Any egg fertilized by an X-bearing sperm w^ill produce a female with red eves, because the A"-chromosome (W) from the mother carries the dominant factor for red. Any egg fertilized by a I^-bearing sperm will produce a male with red eyes because he gets his X-chromosome (W) from his mother.

When these two Fi flies (second row) are inbred the following combinations are expected. Each egg will contain a red-eye producing X, ( W ) , or a white-eye producing X, (w) , after the polar bodies have been extruded. The male will produce two kinds of sperms, of which the female-producing will contain a red-eye producing X. Since any egg may by chance be fertilized by any sperm, there will be the four classes of individuals shown in the bottom row of the diagram. All the females will have red eyes, because irrespective of the two kinds of eggs of the female, all the female-producing sperm carry a (w) X, Half of the males have red eyes, because half of the eggs had each a red-producing X-chromosome. The other half of the males have white eyes, because half of the eggs had each a white producing A^-chromosome. Evidence from other sources shows that the I^-chromosome of the male is indifferent, so far as these Mendelian factors are concerned.

Morgan 1925 fig45.jpg

Fig. 45. Cross of a red-eyed female and a white-eyed male of the vinegar fly, showing sex-linked inheritance.

The reciprocal experiment is illustrated in figure 46. A white-eyed female is mated to a red-eyed male (top row) . Each of the mature eggs of such a female contains one white-producingX-chromosome, represented by the open bar in the diagram. The redeyed male contains female-producing X-bearing sperm, that carry the factor for red-eye color, and male-producing I^-chromosomes. Any egg fertilized by an A"-bearing sperm will become a red-eyed female because the X-chromosome that comes from the father carries the dominant factor for red eye color. Any egg fertilized by a I^-bearing sperm will become a male with white eyes because the only X chromosome that the male contains comes from his mother and is white-producing.

When these two F^ flies are inbred (middle row) the following combinations are expected. Half the eggs will contain each a white-producing X-chromosome and half a red-producing. The female-producing sperms will each contain a white X- and the male producing sperms will each contain an indifferent I^-chromosome. Chance meetings of eggs and sperms will give the four F2 classes (bottom row) . These consist of white-eyed and red-eyed females and white-eyed and red-eyed males. The ratio here is 1 : 1 and not three to one (3:1) as in other Mendelian cases. But Mendel's law of segregation is not transgressed, as the preceding analysis has shown; for, the chromosomes have followed strictly the course laid down on Mendel's principle for the distribution of factors. The peculiar result in this case is due to the fact that the F^ male gets his single factor for eye color from his mother only, and it is contained in a body (the X-chromosome) that is involved in sex determination, while the mate of this body, the Y chromosome, is indifferent with regard to these factors.

Morgan 1925 fig46.jpg

Fig. 46. Cross of a white-eyed female and a red-eyed male of the vinegar fly, showing sex-linked inheritance.

In human inheritance there are characters that show this same kind of transmission. Color-blindness, or at least certain kinds of color-blindness, appears to follow the same scheme. A color-blind father transmits through his daughters his peculiarity to half of his grandsons, but to none of his granddaughters (fig. 69). The result is the same as in the case of the white-eyed male of Drosophila. Colorblind women are rather unusual, which is expected from the method of inheritance of this character, but in the few known cases where such color-blind women have married normal husbands all thel^i sons inherit color-blindness from the mother (fig. 70). Here again the result is the same as for the similar combination in Drosophila.


In man the sex formula appears to be XX for the female and XO or XY for the male, and since this is essentially the same as that in Drosophila, the explanation of sex-linked inheritance is the same. According to de Winiwarter there are 48 chromosomes

Morgan 1925 fig47.jpg

Fig. 47. a, spermatogonimn cell of white man, showing 48 chromosomes, including the small F-chromosome; 6, same of negro; c and d. Primary spermatocytes, side views, showing X- and F-chromosomes. (After Painter.)

in the female and only 47 in the male, the I^-chromosome being absent. After the extrusion of the polar bodies there should be 24 left in the e^^. In the male at one of the maturation divisions the single X chromosome passes to one pole. In consequence there are two classes of sperms in man ; female-producing containing 24 chromosomes, and male-producing containing 23 chromosomes. If the factor for color blindness is carried by the X-chromosome its inheritance in man works out on the same chromosome scheme and in the same wav as does white eve color for any other sex-linked character) hi Drosophila, for the O-sperm in man would be equivalent to the I^-sperm in the fly.

Morgan 1925 fig48.jpg

Fig. 48. Primary spermatocytes of man, side views of spindle. The X- and the F-chromosomes are separating in advance of the others. (After Painter.)

Painter's later evidence (fig. 47), showing that there is a small I^-chromosome in the male that acts as the mate of the A"-chromosome (fig. 48), is very convincing. Whether in man the male is XO or XY the explanation of sex-linked inheritance is the same since the factors involved are carried by the X chromosomes.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures