282 



EVOLUTION, GENETICS, AND EUGENICS 



the reverse of that described for Drosophila, so far as the sexes are con- 

 cerned; and this, interestingly enough, is exactly what we get. 



A typical instance of sex linkage of this sort is seen when a barred 

 Plymouth Rock cock is mated with a Black Lanshan hen (Fig. 71). 

 All offspring, both males and females, of the Fi generation are barred; 



Fig. 71. — Sex-linked inheritance of barred and unbarred (black) plumage in 

 poultry. P, parents, barred male, unbarred female; F,, barred males and females; 

 F 2 , males all barred, females in equal numbers barred and unbarred. (After 

 Morgan.) 



but when the individuals of F x are interbred (or the Fi males are bred 

 with any barred females), all males are barred and half of the females 

 are barred and half are black. Here we see that the recessive char- 

 acter, black, is linked with the female sex. If we cross a black female 

 with an Fi male we can get equal numbers of barred and of black males 

 and females. The reciprocal cross (Fig. 72) illustrates crisscross in- 

 heritance. Starting with a black male and a barred female, we get in 

 Fi barred males and black females. When the Fi individuals are in- 

 terbred we get half barred and half black males and females. 



