HEEEDITY 



we must assign to it a different potency, or 

 power of dominance, so that it produces a 

 visible effect only when doubly represented in 

 the zygote. 



In guinea-pigs, rabbits, and mice we have 

 seen that the presence together in the same 

 zygote of two factors, A and B, in any com- 

 bination whatever, produces the gray or agouti 

 coat. The two factors are A, the agouti or 

 gray marking of the hair, and B, black pig- 

 ment in the fur. If A is lacking, the coat is 

 black; if B is lacking, it is brown, cinnamon, 

 or yellow. If both are lacking, it is either 

 brown or yellow. But if both are present, the 

 wild or agouti type is produced. So far as 

 the production of the agouti coat is concerned, 

 it makes no difference whether either factor is 

 singly or doubly represented in the zygote. 

 Each factor has potency enough to produce the 

 full effect either in a single or in a double 

 dose. Accordingly, as we noticed in an earlier 

 chapter, we can distinguish by their breeding 

 capacity, though not by their looks, four types 

 of agouti guinea-pigs or gray rabbits, viz. : 



92 



