434 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



only in male individuals. But if the recessive sex-linked character is 

 introduced by the female parent, it appears in Fj in male individuals 

 but in F 2 in both sexes. 



Suppose now a cross is made between two races, each of which 

 possesses a different sex-linked recessive character, as for example 

 white eye and yellow body. (See Table II, p. 436.) If the white-eyed 



Flies 



Chromosomes 



FIG. 87. Sex-linked inheritance of white and red eyes in Drosophila. Parents 

 white-eyed male and red-eyed female; F t , red-eyed males and females; F 2 , red- 

 eyed females and equal numbers of red-eyed and white-eyed males. A black 

 X indicates an X chromosome bearing the gene for red eye, a white j bears white 

 eye. indicates that X is wanting; in recent publications Morgan replaces it 

 by Y. (From Conklin, after Morgan.) 



parent is a female, there will be produced white-eyed males in Fj 

 and white-eyed flies of both sexes in F 2 . But the male parent being 

 yellow, there will be no yellow flies produced in F x and only yellow 

 males in F 2 . In the reciprocal cross (yellow female X white-eyed 

 male) yellow males will be produced in F x and yellow flies of both sexes 

 in F 2 , while white-eyed flies will not appear until F 2 and then only in 

 the male sex. In either of the reciprocal crosses we expect the pro- 

 duction in F 2 both of yellow-bodied males and of white-eyed males. 



