VARIATIONS IN POTENCY 



1. A A B B, which breeds true, since it forms game- 



tes all AB; 



2. ABB, which produces agouti young and black 



ones in the ratio 3:1, since it forms 

 gametes A B and B ; 



3. A A B, which produces agouti young and yellow 



ones in the ratio 3:1, since it forms 

 gametes A B and A; 



4. A B, which produces agouti, black, and yel- 



low young in the ratio 9:3:4. For 

 the gametes formed by this sort are of 

 four kinds, A B, A, B, and neither 

 A nor B. 



Now in rats we have no evidence that the 

 factor B has ever been lost, a matter to which 

 we shall presently return ; but the agouti factor 

 is apparently frequently wanting in ordinary 

 rats, which are then black. For ordinary rats, 

 then, the known combinations of A and B seem 

 to be three, viz. : 



A A B B = the pure gray (wild type) ; 



ABB = heterozygous gray, which produces off- 

 spring 3 gray : 1 black. This type is ob- 

 tained by crossing black with wild gray; 



B B = pure black. 



Now in Mus rattus, as we have seen, the 

 middle or heterozygous type is black, not gray 



93 



