Sex-Linkage 



87 



FEMALE 



JJK 



X X 



MALE 



4^ 



FIGURE 12-6. Silhouettes of chromosomes of 

 Drosophila melanogaster as seen at mitotic meta- 

 phase. 



present twice in the female. Moreover, the 

 reverse cytological picture is observed in 

 poultry; here the males show that the homo- 

 logs are similar for each pair of chromosomes, 

 while the female has one pair that is hetero- 

 morphic, that is, one whose members are dif- 

 ferent from each other in appearance, one 

 member being similar to, and one different 

 from, the corresponding pair in the male. 



In moths, as in birds, it is also found that 

 males are XX and females XY. In human 

 beings, genetic and cytological evidence shows 

 XY to be male and XX to be female, as in 

 Drosophila. It might be mentioned that in 

 man a certain kind of red-green color-blind- 

 ness is sex-linked, due to a recessive allele, c, 

 present on the X and absent on the Y. Ac- 

 cordingly, color-blind women, X'X% who 



marry normal men, X^Y, have normal daugh- 

 ters, X^X^ and color-blind sons, X'^Y. The 

 classical bleeders' disease in human beings, 

 hemophilia type A, is also due to an X-linked 

 recessive gene, /?, absent from the Y. This is 

 a rare disease usually occurring in males; 

 recently, however, a few hemophilic women 

 have been discovered in England. These 

 homozygotes are viable; they are extremely 

 infrequent because they must have for parents 

 a hemophilic father, X''Y, and a heterozygous 

 mother, X^X'' (Figure 12-7). 



Let us consider certain additional experi- 

 ments performed with the sex-linked gene for 

 white eye in Drosophila} When white 99 

 (X«'X«') are crossed to dull red d" d" (X-^'Y) 

 and large numbers of progeny are scored, 

 almost all Fi are white sons (X"'Y) and dull 

 red daughters (X"'"X"'), as explained. But 

 one or two Fi per thousand do not show 

 this typical result of sex-linkage, but are 

 exceptional in being dull red-eyed sons or 

 white-eyed daughters (Figure 12-8 A). These 

 exceptional flies cannot be explained as the 

 result of careless scoring of phenotypes or 

 contamination by strange flies. Moreover, 

 they cannot be explained as being due to mu- 

 tation, since the mutation rate from m'+ to w 

 or the reverse is several orders of magnitude 

 lower in frequency than that with which the 

 two kinds of exceptional flies are obtained. 

 ^ Based upon work of C. B. Bridges. 



FIGURE 12-7. Pedigree showing a woman homo- 

 zygous for the Iwmophilia gene. 



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4-riii ^h-D 6 



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