296 F. B. SUMNER AND H. H. COLLINS 



The female parent of these yellows (P 9 15) when mated to 

 one of her j^ellow offspring bore one normal and three yellow 

 young. The male parent (P cf 59) was mated to nine other 

 females^ of the wild type, these being either wild mice or cage- 

 bred ones, unrelated to the yellow stock. Eight of these matings 

 resulted in the birth of tw^enty-four young, all of normal color. 

 The other female (a w^ild specimen) gave birth to four normals 

 and one yellow. She was evidently another heterozygous 

 (or 'mutating'?) individual, unrelated so far as we know, to the 

 male. 



INIatings between yellows of this strain yielded over fifty re- 

 corded offspring, the sexes being represented in about equal 

 proportions. All of these mice have been entered as 'yellows/ 

 though this characterization is subject to the qualification to be 

 discussed presently. 



Matings of yellows with those known (on the basis of parentage) 

 to be heterozygous^ gave 39 offspring, of which 21 are recorded 

 as 'yellows,' 11 as 'normal,' and 7 as 'doubtful.' If those of 

 the last class were all to be included among the 'normals,' we 

 should have a reasonably close approach to the 50:50 ratio. 

 Otherwise, there are too man}^ yellows. 



Only two matings are recorded between individuals, both 

 known from their parentage to be heterozygous. These gave 

 7 normal, 1 yellow, and 1 of uncertain type. 



It was early noted that the 'a' and '6' yellows differed quite 

 perceptibly in respect to their mean color tone. The second 

 strain is, on the average, of a richer color, there being fewer 

 black hairs in the pelage, and the pigment of the 'ticking' being 

 redder. Indeed, the term 'yellow,' as applied to the 'b' strain, 

 is in most cases a decided misnomer. The brighter specimens 



* These figures relate only to fertile matings. Throughout this work man}' 

 matings were made which yielded no results. 



^ Excluding the large number of related individuals which we know to have 

 been heterozygous only from the fact that they produced one or more yellow 

 offspring. The inclusion of these would, of course, be unwarranted unless it were 

 possible also to include such heterozygous individuals as gave rise to no recessive 

 offspring. There is no way of identifying these. 



