COLOR MUTATIONS IN MICE OF PEROMYSCUS 297 



are not far from the 'ochraceous tawny' of Ridgway's ''Color 

 Standards," while those of the 'a' strain approach more nearly 

 the 'clay color,' though the latter comparison is quite misleading. 



It soon developed also that many of the 'yellows,' both of the 

 'a' and '&' strains, were of a much less intense color and con- 

 tained more black hairs than those which had been first examined 

 (fig. 4) . These latter have, for the sake of convenience, been termed 

 'atypical yellows.' This expression is quite arbitrary, however, 

 since one may arrange among the offspring of 'yellow' parents 

 a graded series between the most 'typical' yellows and specimens 

 closely resembhng the paler and more buff-tinted individuals of 

 the wild type. Indeed, where we are dealing with the offspring 

 of heterozygotes, it is not in every case possible to distinguish 

 the pure recessive 'yellows' from the others. 



Regarding the genetic status of these 'atypical j^ellows' we 

 are at present far from clear. That they are not heterozygous 

 individuals, resulting from blended inheritance (imperfect domi- 

 nance), seems certain. This we conclude both from the fact 

 that such individuals have never resulted from the mating of 

 yellows with pure dominants, and from the fact that two of these 

 'atypical yellows' have never produced offspring of the wild type."^" 

 Unfortunately, the distinction between 'typical' and 'atypical' 

 individuals was not always recorded in our earlier entries, and 

 these cover perhaps the major part of our yellow stock. Our 

 records seem to show, however, that whereas very clear ('typical') 

 yellows tend to produce offspring like themselves, 'atypical' 

 offspring have occasionally been born to two perfectly 'typical' 

 parents. They have likewise resulted from the mating of a 

 'typical' yellow with a heterozygous animal whose yellow parent 

 was also 'typical.' In a number of instances, too, it is known 

 that 'typical' and 'atypical' mice have occurred among the off- 

 spring of the same parents, and even within the same brood. 

 That two 'atypical' individuals have ever produced 'typical' 

 young we have no clear evidence. On the contrary, abundant 



'"Unless we regard as atypical yellows certain of the 'doubtful' specimens 

 which appeared in the F2 generation of the yellow-pallid cross (see below). 



