WHITE SPOTTING IN MICE 



489 



The probable existence of additional genes in mice of 

 the nature of modifying factors has been stressed be- 

 cause it is felt that animals which can be easily bred in 

 the laboratory should be thoroughly explored genetically, 

 in order to find out characters not known at present. As 

 the number of genes approaches the number of chromo- 

 somes, the probabilities of finding linkage become greater 

 and it is through the investigation of linkage and the 

 localization of the hereditary determiners that the most 

 exact knowledge concerning the nature of the hereditary 

 material can be secured. 



Inheritance of Belly Spotting 

 Before concluding this discussion of modifiers of white 

 spotting in mice some evidence may be added which bears 

 on the appearance of a small amount of white spotting 

 on the bellies of mice which are genetically self as re- 

 gards IF and s. Piebald spotting has been regarded as 

 the recessive allelomorph of self or uniform coloration, 

 and all the evidence from crosses between these two 

 varieties supports this view. It has been noticed, how- 

 ever, that the heterozygotes resulting in F : are not al- 

 ways exact duplicates of the self parents. Where large 

 numbers have been bred, investigators have always 

 found Fj animals with some white spotting, usually con- 

 sisting of a patch of white on the belly not exceeding 12 

 per cent, of the ventral surface. 



Records of experiments in the present investigation 

 disclosed this apparent imperfection of dominance, and 

 all animals resulting from the cross of piebald with self 

 have been graded according to the per cent, of white 

 spotting exhibited. A tabulation of these records shows 

 that out of 51 pure piebalds bred to self mice, 36 pro- 

 duced only self mice with no white hairs; 15 produced 

 some perfect self and some with white hairs. Of these 15 

 parents only 7 produced more than one young showing 

 any white spotting. 557 young resulted from all piebald 

 X self matings, of which 524 showed no trace of spotting 



