VAEIATIONS IN POTENCY 



remain in a lower oxidation stage in yellow than 

 in black or brown. We suppose that in the 

 yellow animal something is wanting which 

 makes that further oxidation possible. This 

 hypothesis would fully account for the observed 

 recessive nature of yellow in the case of all 

 mammals except mice. But here the capacity 

 to form black or brown pigment is regularly 

 present in the yellow individual but is held in 

 check. We may suppose, therefore, that the 

 differential factor, that which converts yellow 

 into brown or black, must in this case be doubly 

 represented in the zygote in order to produce 

 brown or black fur, whereas in most mammals 

 a single dose is effective. Accordingly, if the 

 unmodified black or brown factor is represented 

 only once in the zygote, and the yellow modi- 

 fication is represented once, the latter will 

 show, since the former is singly ineffective. 

 The animal accordingly is a heterozygous yel- 

 low, capable of producing also black or brown 

 offspring. But mice are peculiar in that they 

 cannot exist in the doubly deficient condition 

 of a pure yellow zygote, consequently all 

 yellow mice are heterozygous dominants, 



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