DISTRIBUTIVE COLOR FACTORS. 



35 



The sooty yellows in my stock are most of them homozygous for black, i, e., 

 BB. The question arose, how would such sooty forms behave if crossed with 

 clear yellow lacking B. Experimental evidence shows that sooty yellows hav- 

 ing only one dose of B are not as dark as those with two doses. 



Thus 7451 sooty (grade +li) when crossed with sooty yellow female 7452 

 (+1|) gave sooty yellow young, the average grade of which was+1. The 

 same male when crossed with a brown ( 9 7730) gave yellow young, the grades 

 of which averaged +.31. The writer has never obtained a sooty yellow or sable 

 that did not carry the factor for the production of black pigment (B) in at least 

 half its gametes. Several hundred sooty yellows or sables have been obtained. 



Yellows having one dose of B when crossed inter se should give (besides 

 yellows of their own type) those lacking B entirely as well as those having two 

 doses of B. If the ''sooty" character segregates, 25 per cent of the yellow 

 animals should approximate the grade of their "sooty" grandparent, 50 per 

 cent should resemble their parents, and 25 per cent should be clear yellow like 

 their other grandparent. A table follows, showing the results of two families 

 in this experiment. The numbers of young are as yet small. 



Sooty yelloiv x broivn. 



It will, then, be seen that when the sooty grandparent is of low grade, +1, 

 nearly clear yellow, his ''sooty" character is to all intents and purposes lost in 

 the cross with brown. It fails to reappear in F2, so far as the animals obtained 

 are concerned. When the grade of the sooty grandparent is 5 times as great, 

 that is, +1.25, it will be seen that the average grade of Fi is also about 5 times as 

 great (that is, 0.31 as compared with 0.06). Also in F2 darker forms are 

 obtained which may approach the degree of sootiness seen in the grandparent. 



Miss Durham (1911) is able to distinguish clearly between " sooty yellows " 

 and "sables." In this connection she characterizes "sables" as produced in 

 crossing yellows with blacks or browns. They have a dark black or brown 

 streak down the middle of the back, while the rest of the mouse is yellow. 

 Generally the hairs in the dark streak show a ticked pattern independent of 

 whether the animal carries agouti or not. She has had certain animals (sables) 

 which were dark-pigmented except for a yellow belly. Such animals always 

 molted out sables. She says "sables" are not to be confused with sooty 

 yellows, which result from mating yellow with black, and are dirty color all 

 over with no pattern. She finds that yellows crossed inter se may give "sables," 

 and that "sables" crossed inter se may give yellows, and that matters are 



