384 MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 



and white offspring approximately in the Mendelian ratio, 3:1. The exact numbers 

 recorded are 198 gray : 72 white, or 26.6 per cent, albinos. According to Mendelian 

 principles the grays of this second filial generation should consist in part of pure grays, 

 which would not transmit the albino character, and in part of hybrid grays like their 

 parents, — the first filial generation, — which would transmit alike the pigmented 

 and the albino characters. This Cuenot demonstrated to be actually so, for certain 

 pairs formed by random selection of the grays gave only gray offspring (189 individ- 

 uals); the remaining pairs produced albino as well as gray offspring, and in the 

 expected ratio, 3 grays : 1 albino. The precise numbers recorded are 162 grays : 57 

 albinos, or 26 per cent, albinos. 



Cuenot's observations are fully substantiated by our own experiments, the 

 results of which may be summarized as follows: Wild gray mice were crossed with 

 albino mice; the offspring, sixty-four in number, were all gray like the wild parent, 

 though a single litter of three young, which died without attaining their full growth, 

 were of a somewhat lighter gray than the wild parent. Certain of the cross-breds 

 were paired together, and produced 66 offspring, 42 of which were gray, 24 white. 

 This is a considerable deviation from the expected ratio, 3:1, but it should be 

 remembered that the total number is relatively small. The result is of the nature 

 expected, in that both gray and albino offspring are produced, and of the former a 

 larger number than of the latter. 



To determine whether the grays are, as expected, of two sorts, one hybrid, the 

 other pure, six pigmented individuals have been crossed with the parental white 

 stock. Three of the six have thus far produced only pigmented offspring, indicating 

 that they are pure; the other three have produced both gray offspring and white 

 offspring, showing that they are hybrids. The two sorts of offspring produced in 

 the case last mentioned should, according to Mendelian expectation, be equally 

 numerous. The numbers thus far recorded are 35 pigmented individuals: 23 

 albinos, a result agreeing with expectation in that both gray and white offspring are 

 produced, though these are not in the exact proportions demanded by Mendel's laws. 



If we combine the results of this cross with those obtained by interbreeding 

 hybrids of the first filial generation, we get for the whole a close agreement between 

 expectation and observation. The expectation is 78.5 gray : 45.5 white; the ob- 

 served result is 77 gray : 47 white. 



White mice obtained by one or the other of these crosses have repeatedly been 

 bred together, but without the occurrence of a single exception to the expected Men- 

 delian result, the offspring being invariably albinos. 



A further test of the Mendelian hypothesis as applied to albinism in mice was 



