22 



THE INHERITANCE OF COLOR IN MICE. 



In the cross between heterozygous black animals and the extracted brown 

 recessives, where equality of black j^oung and young lacking black are expected, 

 the following result has been obtained : 



If brown animals are crossed inter se, only brown young are expected. 

 The observed facts coincide with the expectation, 822 brown young having 

 been thus obtained. 



It is, then, clear that the presence of the factor for the production of black 

 pigment and its absence result in a pair of allelomorphic unit characters. In 

 all the cases above mentioned the heterozygous black animals were indistin- 

 guishable from the homozygous blacks, so far as external appearances go. The 

 dominance of black is thus apparently complete. Miss Durham (1911), how- 

 ever, has recorded the occurrence of a distinct heterozygous form ("chocolate 

 lilac") in crosses between pink-eyed black ("blue lilac") and pink-eyed brown 

 ("champagne") mice. The writer has made similar crosses with animals of 

 these color varieties, but is unable to confirm Miss Durham's results. 



Pink-eyed black mice vary in depth and intensity of pigmentation, which 

 fact will be discussed more fully under the head of distributive factors, but the 

 writer has been unable to detect any greater degree of brownness in the pink- 

 eyed blacks which are heterozygous ("chocolate lilacs ") than in those which are 

 homozygous. While it is possible that microscopic examination of the hair 

 of such heterozygous pink-eyed black animals may reveal a greater relative 

 amount of brown than occurs in the homozj-gous form of the same color variety, 

 it seems of doubtful propriety to call the heterozygous form a distinct color 

 variety. Furthermore, if the distinction between homozygous and heterozy- 

 gous pink-eyed blacks were a good one, it should be recognizable in the hetero- 

 zygotes produced by crossing pink-eyed black agouti with pink-eyed brown 

 agouti types. Miss Durham, however, mentions no such difference between 

 homozygous and heterozygous pink-eyed black agoutis, and I have been unable 

 to find any such difference in my own experiments. 



Black, in its method of inheritance, forms one of the best examples of a 

 mendelian unit character. It is, moreover, clearly a positive character which 

 is dominant over its absence, or chemically expressed it is a higher oxidation 

 stage epistatic to a lower stage and independent of the latter in inheritance. 



DISTRIBUTIVE COLOR FACTORS. 



We have seen that three factors act as agents in the formation of the 

 pigments of mice. We may now consider the factors concerned in controlling 

 the distribution of pigments thus formed. It is obvious that theoretically the 

 distribution of pigment may be controlled in two general ways: (1) by the 



