HEREDITY 



in appearance, but it produces both the gray 

 and the black types. So the same gametic 

 formulae will account for both sets of facts, 

 if we suppose merely that the potency of A is 

 different in the two cases. In ordinary rats 

 (Mus norvegicus) A produces the gray coat in 

 a single dose; but in Mus rattus its potency 

 is less, two doses are required to produce the 

 gray coat. I am unable to frame any hypothe- 

 sis other than this which will account for the 

 reversal of dominance in one case as compared 

 with the other. 



Yellow color in mammals affords another 

 illustration of this same thing, reversal of 

 dominance. Black and brown are in most 

 mammals dominant over yellow in crosses, but 

 in mice the reverse is true. The differential 

 factor between black and yellow, if it is the 

 same in mice as in other rodents, must be in 

 one case potent enough to show itself if singly 

 represented in the zygote, whereas in the other 

 case it produces no visible effect unless doubly 

 represented in the zygote. Yellow certainly 

 seems to be a retrogressive variation from 

 gray, black, or brown. The pigment granules 



