Sex Chromosomes and Sex-Linked denes 



95 



FEMALE 



MALE 



A tf 



xx * 



figure 7-5. Silhouettes of chromosomes of 



Drosophila melanogaster as seen at mitotic 

 metaphase. 



one pair whose members are morphologically 

 different, one being similar to, one different 

 from, the corresponding pair in the male. 



As in birds, the male of moths, butter- 

 flies, and some amphibians and reptiles is 

 XX and the females, XY. In human beings, 

 genetic and cytological evidence shows XY 

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

 Drosophila. So in different species different 

 sexes make two kinds of gametes; that is, 

 different sexes are heterogametic with re- 

 spect to sex chromosomes. 



In man, a certain kind of red-green color- 

 blindness is sex-linked due to a recessive 

 allele, c, present on the X and absent on 

 the Y. Accordingly, color-blind women 

 (X'X C ) who marry normal men (X r Y) have 

 normal daughters (X r X r ) and color-blind 



figure 7-6. Pedigree showing a woman homo- 

 zygous for the gene for hemophilia. 



m^5~i 



r 



6 



b 



sons (X'Y). The classical bleeder's disease 

 in human beings, hemophilia type A, is also 

 due to an X-linked recessive gene, h, absent 

 from the Y. This rare disease usually occurs 

 in males. Recently, however, a few hemo- 

 philic women have been discovered in Eng- 

 land. These homozygotes are extremely rare 

 because, barring mutation, they must have a 

 hemophilic father (X'Y) and a heterozy- 

 gous mother (X"X*) — (Figure 7-6). 



A. PHENOTYPES 



P, White O x DuM w-* 



B. GENOTYPES 



WW w 



X X x X Y 



F, TYPICAL 



EXCEPTIONAL 



X Y 



w w 

 X X 



H 



[w wl 

 X xj 



figure 7-7. Nonmutant exceptions in crosses 

 involving eye color in Drosophila. 



Nondisjunction 



Certain additional experiments have been 

 performed with the sex-linked gene for white 

 eye in Drosophila.- When white females 

 (X"X") are crossed with dull-red males 

 (X" Y) and large numbers of progeny are 

 scored, not all F ( are white sons (X"Y) 

 and dull-red daughters (X w+ X w ) as ex- 

 pected according to sex-linkage. One or 

 two flies per thousand F t are exceptional 

 dull-red-eyed sons or white-eyed daughters 

 (Figure 7-7). These exceptional flies can- 

 not be explained away as the result of care- 

 less scoring of phenotypes or contamination. 

 Moreover, they cannot be explained as being 

 due to mutation, since the mutation frc- 



- Based upon work of C. B. Bridges. 



