160 



GENETICS AND EUGENICS 



To state the foregoing facts in another way, it will be ob- 

 served that the recessive sex-linked character in Drosophila, 

 when introduced in a cross by the male parent, disappears 

 entirely in Fi and reappears in F2 only in male individuals. 



Flies 



(S 



Chromosomes 



XX 



IXI 

 X X $ 



2YO: 2X11 



X ® 

 iXi 

 X 1 



XX XI X 



Parents 



Gametes 



Fi 



Gametes 



Fz 



Fig. 116. Sex-linked inheritance of white and of 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 

 X bears white eye. O indicates that an X is wanting; in recent publications Morgan replaces it by Y. 

 (From Conklin, after Morgan.) 



But if the recessive sex-linked character is introduced by the 

 female parent, it appears in Fi 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 22.) If 

 the white-eyed 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 Fi and only yellow males in F2. In the re- 

 ciprocal cross (yellow female X white-eyed male) yellow 



