HEREDITY 



whereas other yellow mammals are homozy- 

 gous recessives. 



In connection with this same case may pos- 

 sibly be found the explanation of the complete 

 absence of the yellow variation in rats. In 

 nearly all mammals kept in captivity yellow as 

 well as black varieties occur; this is true of 

 horses, cattle, swine, dogs, cats, rabbits, guinea- 

 pigs, and mice. In rats, however, a yellow 

 variety is unknown. We know that rats are 

 able to form yellow pigment, for all wild rats 

 do form yellow pigment in their agouti fur, yet 

 singularly enough no all-yellow rat has ever 

 been observed, so far as we have any record, 

 either wild or in captivity. A rat of this sort 

 would command a high price at the hands of 

 any fancier. Suppose the variation did occur 

 in a single gamete. If, as in most mammals, 

 it behaved as a recessive in crosses, it would 

 not become visible, and might be carried along 

 for untold generations without ever becoming 

 visible unless two yellow gametes met. But if, 

 as in mice, the yellow-yellow combination when 

 formed quickly perished, then the character 

 might never become visible. So the yellow 



96 



