98 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the wild house mouse or tlie wild brown rat is crossed with an albino, 

 heterozygotes are produced in which the character, total pigmentation, 

 is dominant, and complete albinism is recessive. In the case of the 

 spotted varieties of mice and rats it is also found that the pigmented 

 condition is dominant over complete albinism. In both cases the hetero- 

 zygotes produce, on interbreeding, the Mendelian proportions of pig- 

 mented and of albino individuals. This is true, furthermore, where two 

 heterozygotes are mated, of which the one had a wild totally pigmented 

 mouse as the colored parent, and the other a spotted or partially pig- 

 mented one. To illustrate : the writer bred house mice to albinos and 

 obtained gray heterozygotes. Black-white mice were also bred to the 

 same stock of albinos, and gray heterozygotes were obtained similar 

 in outward appearance to those from the other cross. The following dia- 

 gram shows this parentage : 



P $ bl.-wh. 9 wh. ^ house mouse (gray) 



Fi S gr.(DR • R) 9 gr.(D • R) 



Fa 7 wh. 19 gr. 



The heterozygotes of generation F i were then interbred, and as each 

 contained complete albinism recessive, it is expected that 25 per cent of 

 the young of generation F 2 would be albinos. The actual result was 

 that, of 26 young produced, 19 were gray, and 7 were albinos. The al- 

 binos are thus but .5 of an individual in excess, so that, within the 

 possibilities of experimentation, the expectation is realized. Some of 

 the pigmented mice showed small flecks of white on tail and feet, but 

 unfortunately no exact record of this was kept. 



A different result is to be expected on Mendelian principles, if, instead 

 of both animals of generation F 1 having complete albinism recessive, only 

 one is of this nature. If we postulate that the other parent shall be a 

 homozygote, two conditions are now possible which will allow the occur- 

 rence of three coat characters in generation F 2 : either (1) the heterozy- 

 gote must produce gametes representing respectively the spotted and the 

 albino conditions, while the homozygote's germ cells contain only the char- 

 acter total pigmentation ; or (2) the gametes of the heterozygote must 

 represent in equal numbers the characters total pigmentation and albinism, 

 while those of the homozygote represent only the spotted character. If 

 matings such as these be made, the offspring will be of two sorts, half of 



