SEX-LINKED INHERITANCE 



435 



Usually no other sort of male is produced throughout the experiment 

 except these two, but occasionally 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 



Flies 



Chromosomes 



XX 



X X ? 



X 

 iXi 

 X 1 



XX X^ X 



? ? d' 



Parents 



Gametes 



Fi 



Gametes 



Fz 



S 



Fig. 88. — Reciprocal cross to that shown in Figure 87. Parents, red-eyed 

 male and white-eyed female; Fj, white-eyed males and red-eyed females ("criss- 

 cross inheritance" — Morgan); Fj equal numbers of red-eyed and white-eyed 

 individuals of both sexes. The distribution of the sex chromosomes is shown at 

 the right, as in Figure 87. {From Conklin, after Morgan.) 



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 F2 males possessing both dominant characters and 

 others possessing both recessive characters or gray-red and yellow- 

 white respectively, as actually observed in about one case in a hundred 

 by Morgan. 



It may add interest to the case to state parenthetically that in man 

 occur a number of sex-linked variations which are inherited in this same 

 curious fashion. Among them may be mentioned color blindness and 



