110 PROCEEDINGS OP THE AMERICAN ACADEMY. 



homozyogous) results in the production of individuals showing the pig- 

 ment character only, whether that be total or partial. This is true not 

 only for mice and rats but for other mammals also. 



When two mice or rats, each having the character complete albinism 

 recessive, are mated, there result 25 per cent of albino individuals. This 

 relation sometimes occurs in single litters of young, but obviously can do 

 so only in lots of 4 or of some multiple of 4. In the long run, however, 

 the totals for a large number of such litters show a very close approxima- 

 tion to the Mendelian ratio of 3 pigmented mice to 1 albino. 



That, further, Mendel's law of segregation holds in the inheritance of 

 albinism, may be seen in cases where albinos are crossed with pigmented 

 heterozygotes having albinism recessive. In such crosses the expectation 

 is that pigmented and albino young will be produced in the ratio of 1 : 1. 

 This is found true experimentally, as an average result. In the writer's 

 experiments, it is found that of 13 litters of mice produced by heterozy- 

 gotes mated inter se, in which the number of young was either 4 or 8, 7 

 litters, or practically one half, show the precise Mendelian ratio of 3 pig- 

 mented mice to 1 albino; and of 25 cases taken also at random, in which, 

 from back-crossing of a heterozygote with an albino, an equality of these 

 two classes is expected (the selected litters containing an even number of 

 young), this ratio appears in 12 cases, or a trifle less than one-half. 



This result is, on mathematical grounds, about what we should expect. 

 For, in cases where equality of white and of pigmented offspring is ex- 

 pected in the long run, the chances for their distribution in individual 

 litters of four young each are as follows : 



All white : .... 

 3 white : 1 pigmented 

 2 white : 2 pigmented 

 1 white: 3 pigmented 



All pigmented . . . 



1 chance in 70 

 16 chances in 70 

 36 chances in 70 

 16 chances in 70 



1 chance in 70 



Consequently in litters of 4 we expect equality of albino and pigmented 

 young in scarcely more than half the individual cases. 



Similarly, when the litters consist of eight young each, the chances for 

 the distribution of white and pigmented animals in individual litters are 

 as follows : 



