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 F2 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 



XX 



X X ? 



2YO: 2M 



iXi 

 X K ? 



Parents 

 Gametes 



Fi 



Gametes 



F2 



XX M X 



$ $ d" 



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

 white-eyed male and red-eyed female; Fi, red-eyed males and females; F2, 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 ^ bears white 

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

 by F. (From Conklin, after Morgan.) 



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

 and white-eyed flies of both sexes in F2. But the male parent being 

 yellow, there will be no yellow flies produced in Fj and only yellow 

 males in F2. In the reciprocal cross (yellow female X white-eyed 

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

 in F2, while white-eyed flies wiU not appear until F2 and then only in 

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

 duction in F2 both of yellow-bodied males and of white-eyed males. 



