FUTURE OF THE SCIENCE OF BREEDING 159 



back with the whites they give equal numbers of whites and 

 blacks. Moreover, the whites which appear, whether from the 

 mating between two blacks or from the mating between black 

 and white, never throw blacks again but behave like the whites 

 of the pure white strain. The inference from these facts can 



"R cce 



S S L ve- 



FIG. 1. Diagram showing the three types of zygotic combination 

 which it is possible to produce from the two sorts of gametes, white and 

 black. The black and white circles shown inside each bird represent the 

 nature of its output of gametes. 



only be that the first-cross birds form two kinds of gametes, 

 and two only. Further, the numerical proportions given above 

 force us to conclude that the two kinds of gametes are formed 

 in equal numbers. In other words, when the duality of the 

 hybrid zygote is resolved, half of the resulting gametes are like 

 those peculiar to one of the original parental strains, and half 

 are like those peculiar to the other. 



