VI] MENDELIAN HEREDITY 75 



this is inconsistent with the hypothesis that allelo- 

 morphs exist always in pairs, one possessing a factor 

 lacking in the other. More correctly, then, each colour 

 is allelomorphic with its absence, but the presence of 

 a higher member of the series obscures or prevents 

 the development of the lower. This is expressed by 

 saying that grey is 'epistatic' over black and chocolate, 

 and black over chocolate. Since chocolate is the 

 lowest member of the series, it has been suggested 

 that its factor is indeed the pigment factor represented 

 in the case of the rats described above by the symbol 

 P, and that in other colours the special factors are 

 present in addition \ In grey mice yellow, black and 

 chocolate pigments are all present in the hairs, but 

 the factor for 'greyness' causes the yellow to be 

 restricted to certain parts of the hair. In black mice 

 both black and chocolate pigments are present, but 

 the black obscures the chocolate, and in chocolate 

 mice this pigment alone is present. 



The object of this rather special digression is to 

 show how the hypothesis of a series of colour-factors 

 acting together can completely coordinate the pheno- 

 mena of colour-inheritance, which very few years ago 

 seemed hopelessly confused and subject to no definite 

 rules. It is now possible to forecast with accuracy the 

 results of a pairing between individuals of diflPerent 



^ More recently a still lower member of the series, producing 

 ' orange ' in the absence of the other colour-factors, has been described 

 hy Hagedoorn. 







