MORGAN, COAT COLORS IN MICE 



113 



In the same way, the cress between yellow and gray would be repre- 

 sented as follows : If the yellow gamete is represented by Y and the 

 gray gamete by (Y Ch) M, the resulting yellow mice would have the 

 formula Y, (Y Ch), M ; and its gametes would be Y, (Y Ch), M, and 



no M or 0. The allelomorphs are then — — — > -~-l and the result- 

 ing gametes would be Y M, Y 0, (Y Ch) M (Y Ch) 0. The following 

 table gives the composition of the resulting hybrids. There result twelve 

 yellows to four grays. Now if the pure yellow combinations do not de- 

 velop, as Castle suggests, viz : YOYO, YOYM (twice), Y M Y M, 

 there will be left eight yellows and four grays, or 2 : 1. 



The "presence and absence" theory has been extensively used by modern 

 writers on Mendelian inheritance. Bateson states that it gives consistent 

 results in all cases so far studied, and he seems inclined, with some reser- 

 vations, to adopt it as a method applicable to Mendelian treatment in 

 general. Its usefulness and even necessity is apparent for many cases; 

 but it does not seem to me, therefore, that it is advisable to extend it to all 

 cases. If, as I have suggested, certain mutations result by or through the 

 loss from certain material bodies that ordinarily carry a group of such 

 bodies (such as gray, for example), the allelomorphs of the original body 

 will be the new body, lacking the particular factor in question, and not 

 the absence of the original body as a whole. The hypothesis of presence 

 and absence appears originally to have been invented to explain the gray 

 chocolate cross in mice. I have tried to show how the facts may be ex- 

 plained in a somewhat different way, so that the original combination for 

 gray is allelomorphic with the factor for chocolate, while presence and 



