136 PROCEEDINGS OF THE AMERICAN ACADEMY. 



to be lieterozygous with respect to these two characters, was back- 

 crossed with a chocolate individual. The expected result, on Mendelian 

 principles, is an equality of black and of chocolate young. The single 

 litter of three young obtained from this pair were two of them chocolate 

 and one black, hence fulfilling the expectation as closely as is possible 

 with an odd number of offspring. The following additional cases also 

 point to the same relation of black to chocolate. Bateson (: 03'', p. 83) 

 includes, among a series of matings supplied by a Mr. F. G. Parsons, two 

 cases in which black mice j^roduce chocolate young. These additional 

 cases are : 



" (8) bl.-w. 9 C (? DD) ... X bl.-w. (? a (DR) = 3 bl.-w., 1 br.-w. 

 (37) bl. 9 T>%D X a) (DR) x bl.-w. ^ a (DR) = 2 bl.-w., 2 bl., 1 br." 



Thus the same male gave, by two black females, in the one case 3 

 black-whites to 1 chocolate, and in the other, 2 blacks and 2 black- 

 whites to 1 chocolate, a total of 7 black to 2 chocolate individuals. If 

 in each case both black parents had the chocolate character recessive, 

 this result is approximately the 3 : 1 ratio expected on Mendelian prin- 

 ciples. Moreover, the black-white male ^ a was subsequently bred to 

 two chocolate females, with the result that there were 4 black, 4 choco- 

 late, and 6 albino young. 



As regards the transmission of pigment characters, the gametic unions 

 in this case were doubtless as follows : 



bl.-ch. + ch. = gametes of ^ bl.-wh. a [DR-R,] 



ch. -1- ch. = " 9 chocolate, 



2 bl. -I- 2 ch. = Fi. 



Both parents also had the character albinism recessive, but the albino 

 young resulting from the cross would contain one or both of the two 

 pigment characters latent, as could be shown by back-crossing them 

 with the chocolate parent. If the chocolate character be recessive, it is 

 expected that one-half the young will have the black, and one-half the 

 chocolate character. If the albinos possess the same equal distribution of 

 the pigment characters as do the pigmented animals, the ]\Iendelian 

 expectation is fully realized here. In the crosses given by Bateson, 

 black and chocolate are the only pigments produced, although albinos, 

 chocolates, and blacks are freely interbred. The albinos are doubtless 

 from two chocolate or two black parents, or from a chocolate and a black, 

 and thus do not contain the yellow element necessary to produce the gray 



