No. 566] 



SPOTTING IN MICE 



79 



1. It may have been completely lost, failing to manifest 

 itself in his germ cells. 



2. It may have continued to exist and to be inherited 

 together with the dominant type of spotting. 



3. It may have been changed to a so-called "dominant" 

 type of spotting simply by the nature of modifying sup- 

 plementary factors which it encountered during ontogeny. 



The first two cases necessitate the origin of the " domi- 

 nant" spotting by a mutation in no way connected with 

 the previous recessive spotting. In the first case, more- 

 over, we should have to suppose the disappearance of the 

 recessive spotting character in a manner entirely con- 

 trary to any principle of Mendelian heredity. In the sec- 

 ond case the occurrence of the two types of spotting side 

 by side in the same litters of young would so complicate 

 the experiments that analysis would be difficult if not im- 

 possible, on Miss Durham's results. 



There is good reason to believe that the third possible 

 explanation is the correct one. It accounts for the for- 

 merly "recessive" type of spotting. It presupposes no 

 fundamentally different appearance of the two types 

 of spotting. Moreover, it is very probable that the al- 

 bino race brings in the modifying factors necessary to 

 give the apparent change in the type of spotting. The 

 addition of a factor as presupposed by the presence and 

 absence hypothesis is not proved by the results obtained 

 nor is it necessary to account for them. 



That the presence and absence hypothesis does not 

 apply to all cases of spotting is seen in the case of the 

 "blaze" mice in my experiments. Here, if F, animals 

 had been given me as a starting point for experimenta- 

 tion, I should conclude the spotting to be recessive, while 

 if F 2 spotted animals were given as a starting point the 

 conclusion would be inevitable, that spotting should be 

 considered dominant. Yet it is one and the same spot- 

 ting in both cases. It is certain that " self " and ' 1 1 ilaze 

 are alternative conditions, but it is equally certain that 

 they differ from each other rather as two degrees of a 



