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THE AMERICAN NATURALIST [Vol. XLVIII 



simplest tests is the following: Yellows were bred to 

 chocolates. The combination gave yellow and agonti off- 

 spring, when certain yellows are used, and yellow and 

 chocolate offspring when other yellows are used. Mixed 

 litters of yellow, agouti and chocolate do not appear. 

 Xow when yellow and agouti appear in a given litter (as 

 above) the yellow parent must have carried agouti. If 

 her yellow gene "repels" the agouti gene, then none of 

 the yellow daughters should contain agouti genes, con- 

 sequently if such yellows are next bred to chocolate the 

 offspring should be only yellow and chocolate (or black) 

 and never yellow and agouti. This, in fact, is what my 

 experiments have shown. In the two following tables the 

 results of crossing yellows by chocolates are given by 

 litters. The yellows that were used at first were for the 

 most part heterozygous for gray white-belly, hence in 

 the earlier litters yellows and grays were generally ob- 

 tained. The yellow offspring of these earlier litters were 

 for the most part used in the later experiments, hence 

 the later litters are made up of yellows and chocolates. 

 The records (not given here) showed in every case that 

 yellow mice from litters of yellow and gray gave, when 

 bred to chocolate, only yellows and chocolates. 



TABLE II 



Li I 1 !! 1 1 UH I ill 



TABLE III 

 Yellow $ by Chocolate c 



