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THE AMERICAN NATURALIST [Vol. LIV 



skin in such small amounts as to leave the pelage clear 

 yellow. All three fundamental pigments are present in 

 yellow and it is essentially an agouti in which the dark 

 pigments are quantitatively restricted and reduced. 

 This gene acts as the dominant allelomorph of agouti, 

 light-bellied agouti, and non-agouti. Black mice, on the 

 other hand, represent no quantitative reduction in amount 

 of pigment but only the absence of the genes for yellow 

 and agouti. The sables contain a gene allelomorphic to 

 agouti and non-agouti. The evidence shows that this 

 gene is common to yellow, sooty, sable and black-and-tan 

 mice. This gene is a lethal. When it is present in both 

 gametes uniting to form a zygote it causes the death of 

 the zygote. This lethal gene (yellow) might conceivably 

 assume several forms and cause the differences noted 

 among the sable varieties. If such were the case, yellow 

 and sable varieties would be members of a series of mul- 

 tiple allelomorphs of a single gene. Such series have 

 been demonstrated in the gene for white eye in Droso- 

 phila by Morgan and his co-workers ; in the color gene ir* 

 the guinea pig by Wright (1915) ; and in the agouti gene 

 in the mouse itself as quoted previously. But if the 

 black-and-tan mouse or a sooty were due simply to a dif- 

 ferent form of the yellow gene the difference of their 

 black recessives from ordinary blacks would still remain 

 to be explained. Moreover yellow and the members of 

 the sable series are more closely related to each other 

 than as mere members of a multiple allelomorphic series. 

 They do contain an identical gene, and unlike multiple 

 allelomorphs can be changed one into the other more or 

 less completely. Their difference rather inheres in their 

 possession of modifying genes determining the quantita- 

 tive increase of black or brown pigments not only in con- 

 nection with the yellow gene itself but in connection with 

 the genes for black, brown and agouti. 



Such modifying genes can not be merely changes in a 

 distributive gene such as agouti or the gene for restric- 

 tion which causes yellow, for their presence has been 

 demonstrated in non-agouti, non-yellow animals. They 



