82 HEREDITY AND SEX 



the distribution of the red- and the white-producing 

 factors is the same mechanism that is involved in sex 

 determination. On this assumption we can readily 

 understand that any character that is dependent on the 

 sex chromosomes for its realization will show sex-linked 

 inheritance. 



The reciprocal cross (Fig. 36) is equally instructive. 

 If a white-eyed female is mated to a red-eyed male, 

 all the daughters are red-eyed like the father, and all 

 the sons are white-eyed like the mother. When these, 

 FI, flies are bred to each other there are produced red- 

 eyed females (25%), white-eyed females (25%), red- 

 eyed males (25%), and white-eyed males (25%). The 

 explanation (Fig. 36 ; page 65) is in principle the 

 same as for the other cross. If, for instance, we 

 assume that the two X chromosomes in the white-eyed 

 female carry the factors for white, all the eggs will 

 carry one white-producing X. The red-eyed male will 

 contain one X chromosome which is red-producing 

 and passes into the female-producing sperm. The 

 other sperm will not contain any sex chromosome, and 

 hence lacks the factors for these eye colors. When the 

 female-producing sperm, that carries the factor for 

 red, fertilizes a " white" egg, the egg will give rise to a 

 female with red eyes, because of the presence of one 

 red-producing chromosome. When the male-produc- 

 ing sperm fertilizes any egg, a white-eyed son will be 

 produced, because the single sex chromosome which 

 he gets from his mother is white-producing. 



The production of four classes of individuals in the 

 second generation works out on the same scheme, as 

 shown in the diagram. The inheritance of white and 



