No. 635] 



WHITE SPOTTING IN MICE 



477 



spotted types probably carry modifying genes which 

 have a pronounced effect in increasing the amount of 

 pigment present. 



Many crosses were made between Type "A" and pie- 

 balds with the object of determining whether the genes 

 W and s were linked or independent. All offspring from 

 this cross were also graded for variation in amount of 

 white spotting. The distribution of the 443 offspring 

 (Table II, Cross, and Fig. 3 solid line) indicates the cor- 

 rectness of the ranges already established for three types 

 of spotting, since black-eyed whites, " spotted" (Type 

 "A" and piebald) and selfs resulted in the expected ratio 

 of 1:2:1. The range of the black-eyed whites was from 

 100 to 56 per cent, white ; and " spotted" from 50 to per 

 cent, white. The mean of the black-eyed whites was 84.7 

 per cent. ± .6 of white dorsally with a standard deviation 

 of 10.3 ± .5, which values correspond closely to those cal- 

 culated for the cross black-eyed white X piebald (Table 

 II, Cross 1). The mean of the mixed spotted was at 11.6 

 per cent. ± .5 compared with 12.5 per cent. ± .6, the 

 mean of the piebalds in Cross 1. It may be inferred from 

 this that the residence in the same Type "A" zygote of 

 the genes W and self has had per se no darkening effect 

 on the black-eyed whites and spotted subsequently ex- 

 tracted. 



Self 



Data on the ranges of variability of white spotted mice 

 would not be complete without some reference to the 

 variability exhibited by mice which by all tests which 

 have been applied to them are genetically self mice, i.e., 

 lacking the genes at present known to cause white spot- 

 ting. The heterozygote between self and piebald is gen- 

 erally regarded as self, that is, self is supposed to be 

 completely dominant to piebald. But in a number of 

 cases in these experiments heterozygotes between self 

 and piebald have exhibited a small spot of white on the 

 belly, covering never more than 12 per cent, of the ven- 

 tral surface. This occurrence has been remarked by 

 Little in the case of Type mice resulting from the 



