DOMINANCE 



nant individuals to one recessive. Accordingly 

 we should expect yellow mice, if, as stated, 

 they are invariably heterozygous, to produce 

 three yellow offspring to one of a different 

 color, but curiously enough they do not. They 

 produce two yellows (instead of the expected 

 three) to every one of a different color. About 

 the ratio there can be no reasonable doubt. 

 It has been determined with great accuracy 

 by my pupil, Mr. C. C. Little, who finds that 

 in a total of over twelve hundred young pro- 

 duced by yellow parents almost exactly two- 

 thirds are yellow. Instead of the regular 

 Mendelian ratio 3:1, we have then in this case 

 the peculiar ratio 2:1, and this requires ex- 

 planation. The explanation of this ratio is to 

 be found in the same circumstance as is the 

 total absence of pure yellows. Pure yellow 

 zygotes are indeed formed, but they perish for 

 some unaccountable reason. For a yellow in- 

 dividual forms gametes of two sorts with equal 

 frequency, viz. yellow and non-yellow (let us 

 say black). For, if yellow individuals are 

 mated with black ones, half the offspring are 

 black, half yellow, as already stated. 



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