No. 594] 



COLOR MUTATIONS IN MICE 



841 



were white-bellied agoutis or indeed of any but the ex- 

 pected eight color varieties. This proves conclusively 

 that the hybridization of these two races does not neces- 

 sarily cause the white-bellied mutation to appear. 



The fact that the white-bellied mutation has occurred 

 in wild races of Mus musculus as shown by the capture of 

 white-bellied mice would indicate that this mutation does 

 not necessarily depend for its origin upon the presence in 

 the germ cell of any of the recognized color factors in a 

 heterozygous condition. 



The test by breeding of the white-bellied agoutis ob- 

 tained in the F 2 and F 3 generations has been attempted. 

 One of F 2 generation mutants has not yet been bred, but 

 the other F 2 and F 3 white-bellied agoutis have shown that 

 their color pattern was epistatic to gray-bellied agouti 

 and non-agouti. In these respects, the mutants have be- 

 haved in a manner identical with the gris a ventre blanc 

 mice of Cuenot or the white-bellied agoutis of Morgan. 



The observed facts therefore prove clearly the origin 

 of this epistatic white-bellied agouti mutation at three 

 different times in the course of this experiment. 



It is interesting to note that there has been no selection 

 in the direction in which the mutation occurred as was 

 the case in the experiments of Castle and Phillips (1914) 

 on hooded rats. 



Although the white-bellied agouti may easily be con- 

 sidered merely an increased state of activity of the agouti 

 factor beyond the gray-bellied stage, it is certain that it 

 arises, not as the product of a series of small gradual 

 changes, but suddenly and distinctly, without warning, and 

 that after its appearance it behaves at once as a mendeliz- 

 ing character. 



Taking into consideration the facts now recorded con- 

 cerning this particular mutation we can say that its origin 

 is apparently not solely dependent upon any of the known 

 genetic processes. Inbreeding, hybridization, selection, 

 none of them is indispensable to the occurrence of the 

 mutation. The underlying cause or causes are at present 

 clearly outside the bounds of analysis. 



