112 GENETICS AND EUGENICS 



sive. 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 Dr. C. C. Little, who 

 finds that in a total of over twelve hundred young produced 

 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 explana- 

 tion. The explanation of this ratio is to be found in the same 

 circumstance as is the total absence of pure yellow individu- 

 als. Pure yellow zygotes are indeed formed, but they 

 perish for some reason. A yellow individual produces gametes 

 of two sorts with equal frequency, viz., yellow and non- 

 yellow (let us say black). For, if yellow individuals are 

 mated w^ith black ones, half the offspring are black, half 

 yellow, as already stated. Now if yellow individuals are 

 mated with each other we expect three sorts of young to be 

 produced, numerically as 1:2:1, viz., 1 Y Y, 2 Y B, and 1 

 B B. But since observation shows that only two combina- 

 tions are formed which contain yellow to one not containing 

 yellow, and since further all yellows which survive are found 

 to be heterozygous (YB), it must be that the expected Y Y 

 individual either is not produced or straightway perishes. 

 As to which of these two contingencies happens we also have 

 experimental evidence. Dr. Little finds (confirming Cuenot), 

 that yellow mice when mated to black ones produce larger 

 litters of young than w^hen they are mated to yellow ones. 

 The average-sized litter contains something like 5.5 young 

 when the mate is a black animal, but only 4.7 when it is a 

 yellow animal. It is evident, then, that about one young one 

 out of a litter perishes when both parents are yellow, and this 

 undoubtedly is the missing yellow-yellow zygote. The 

 yellows which are left are heterozygous yellow-black zygotes, 

 and they are to those that perish as 2 :1. They are also to the 



