MORGAN, COAT COLORS IN MICE HI 



parable to that of the wild gray mouse, such mice should be crossed to a 

 yellow strain that contains no ticking factor and gives rise to some gray 

 mice. This test I have been unable as yet to apply. 



Castle has also suggested that melanistic forms of gray animals arise 

 through the loss of the ticking factor. In consequence, the colors are not 

 laid down seriatim and the darkest color becoming epistatic gives the 

 black color to the hair. The implication is that the yellow and the choco- 

 late are still present but obscured by black. By a further loss of black 

 the chocolate stains the hair and a chocolate animal results. Two diffi- 

 culties with reference to this view suggest themselves. It is assumed that 

 the loss of the ticking factor will lead to the uniform spread of the other 

 colors, but this is not self-evident or proven. I have tested a few black 

 mice and found no yellow present. 



Secondly, yellow itself may completely color the hair in a yellow 

 mouse that carries black or chocolate, and these latter pigments may even 

 sparsely develop. It might seem, therefore, that it is the yellow that 

 must disappear to produce a black mouse, and this, in fact, seems to be 

 realized in the black-chocolate ticked mouse that I have described. If 

 the ticking factor were then lost, pure black might result. It is possible 

 that black mice may have originated in this way ; but even were this true, 

 it still remains not clear why the black should then spread into the choco- 

 late region of the hair. Moreover, the sudden appearance of black animals 

 in nature without an intermediate black-chocolate condition indicates that 

 the transfer takes place at one step. These considerations seem to me to 

 show that our assumptions are still too crude to offer a reasonable inter- 

 pretation of the facts in hand. 



THE ASSOCIATION" HYPOTHESIS 



If we suppose that black, chocolate and yellow and the ticking factor 

 are carried by a common carrier in the gray mice, and that black is borne 

 by a different (i. e., not homologous) carrier in the black mouse, and 

 chocolate by a third carrier in the chocolate mouse, we can not give a 

 consistent hypothesis for the inheritance of these colors. For example, 

 when gray is crossed with black, the presence of the triune body gives 

 gray in the first generation. In the gametes of the gray hybrid, the gray 

 carrier finds no mate on the hypotheses. Its allelomorph is therefore its 

 absence. Similarly the black carrier has no allelomorph. The gametes 

 will then be GB, GO, BO, 00. There would result twelve grays, three 

 blacks, one white; but white mice do not result, as I have shown, from 

 this combination. Moreover, they would contain no color determiner; 

 and we know of no such mice. 



