172 PHYSICAL BASIS OF HEREDITY 



to this evidence as showing the identity of the sex-deter- 

 mining mechanism of man and the vinegar fly. 



When two or more sex-linked characters are involved 

 at the same time, the situation is different only in so far 

 as crossing over may take place in the female. It will be 

 simpler to consider such a cross and its reciprocal in the 

 reverse order from that just given. If a female with 

 yellow wings {yy) and white eyes {wiv) is crossed to a 

 wild- type male, *^gray*^ wings (Z) and red eyes (TF), the 

 sons are yellow white and the daughters are gray red 

 (Fig. 71). When these are inbred there are four types 

 in F2 (ignoring sex), viz., the two original combinations 

 yellow white and gray red, and the two crossover com- 

 binations yellow red and gray white. They occur in the 

 following ratios : 



Yellow white Gray red Yellow red Gray white 



99 per cent. 1 per cent. 



In this case the Fj male acts as a double recessive, reveal- 

 ing the amount of crossing over in the F^ female. Since 

 neither his female-producing nor his male-producing 

 sperms carry factors that cover up the characters carried 

 by the four classes of gametes in the F^ female, all four 

 classes of her gametes are revealed in their numerical 

 proportions. Reciprocally, when a male with yellow 

 wings {y) and white eyes {w) \^ crossed to a wild- type 

 female (gray {YY) red {WW), both sons and daugh- 

 ters are gray red, because both get the dominating genes 

 for these characters carried by the X-chromosome 

 received from the mother. If these F-^^s are inbred (Fig. 

 72), the 2^2 females are gray red, since each contains an 

 X with the two dominant genes derived from the father 

 whose genes have remained completely linked, as there 

 is no crossing over in the male. On the other hand, 

 there are four kinds of F2 males : yellow white ; gray red ; 

 yellow red; gray white; because each male shows the 



