Sex Linkage 



97 



When a miniature-winged female is mated with a wild-type 

 male (in contrast to the reciprocal cross just described) all the 

 females in the Fi are wild type but all the males are miniature. 

 The males receive an X chromosome from their mother but it 



Parents 



Pi gametes 



Wild-Type Female Miniature Male 

 X X 



F, 



Wild -T>^pe Females Wild Type Males 

 XX X Y 



Fi gametes /^ X 

 M- ] 



M ■ -M 



X Y 



XX XX X Y 



Wild-Type Wild-Type Wild-Type Miniature 



Female Female Male Male 



Homozygous Heterozygous 



Fig. 32. Diagram of sex-linked inheritance in a cross between a homo- 

 zygous wild-type female and a miniature-winged male of Drosophila melano- 

 gaster. The Fi consists of wild-type females and males, and the F2 falls 

 into a ratio of 2 wild-type females : 1 wild-type male : 1 miniature-winged 

 male. 



has a gene for miniature, and they receive merely a Y chromo- 

 some from their father. As Fig. 33 shows, the Fo consists of 

 wild-type females, miniature females, wild-type males, and 

 miniature males in equal proportions. Neither the Fi nor the F2 

 of this cross is the same as the corresponding generation from 

 the reciprocal cross. 



Because a recessive female crossed with a dominant male pro- 

 duces dominant females and recessive males this method of 

 inheritance is often referred to as "crisscross inheritance." This 



