SEX-LINKED INHERITANCE IN DROSOPHILA 161 



males will be produced in Fi and yellow flies of both sexes in 

 F2, while white-eyed flies will not appear until F2 and then 

 only in the male sex. In either of the reciprocal crosses we 

 expect the production in F2 both of yellow-bodied males and 



Flies 



Chromosomes 



X0 



6 



iXi 



Parents 



Gametes 



<S ? 



X IX ? 



Fi 



Gametes 



M M X0 m Fz 



Fig. 117. Reciprocal cross to that shown in Fig. 116. Parents, red-eyed male and white-eyed female; 

 Fi, white-eyed males and red-eyed females (" criss-cross inheritance " — Morgan); Fs, equal numbers 

 of red-eyed and white-eyed individuals in both sexes. The distribution of the sex-chromosomes is 

 shown at the right, as in Fig. 116, (From Conklin, after Morgan.) 



of white-eyed males. Usually no other sort of male is pro- 

 duced throughout the experiment except these two, but occa- 

 sionally there is produced a male both yellow-bodied and 

 white-eyed, or one which is gray-bodied and red-eyed, like 

 wild flies. How do these arise ? If in Fi females the paired 

 X's were to exchange loads in part, so that G and R came 

 to be attached to the same X and g and r to the other X, 

 and if each of the eggs having such a constitution were to be 

 fertilized with a sperm which lacked X (male determining 

 sperm), this would make possible the production of Fo males 

 possessing both dominant characters and others possessing 

 both recessive characters or gray-red and yellow-white 



