VARIATIONS IN POTENCY 



is crossed with brown, the crossbreds are apt 

 to develop in their coats more brown pigment 

 granules than do homozygous or pure blacks. 

 Nevertheless, we have no reason to question the 

 entire purity of the gametes, both dominant 

 arid recessive, formed by such cross-bred black 

 animals. It is the dominance, not the segrega- 

 tion, which is imperfect. 



In other cases still the dominance may be 

 entirely reversed in character, owing to varia- 

 tion in the potency of a unit-character. Thus 

 in most rodents the gray or agouti pattern- 

 factor of the hair, A, is dominant. A cross of 

 black with homozygous gray, in rats, mice, or 

 rabbits, produces only gray offspring, which 

 in F 2 produce three grays to one black. But 

 the so-called black rat, Mus rattus, a species 

 distinct from the one which has given rise to 

 the varieties kept in captivity, behaves in a 

 different way, as shown by Morgan ('09). 

 When crossed with its gray variety, the roof 

 rat, Mus alexandrinus , it produces only black 

 offspring, and in F 2 , three blacks to one gray. 

 If we suppose the gray coat in this case to 

 be due to the same factor as in other rodents, 



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