80 PROCEEDINGS OF THE AMERICAN ACADEMY. 



albino young obtained by the writer from all litters in which the pig- 

 mented parent or parents were of house-mouse and albino ancestry, and 

 include the lots previously mentioned as well as several not elsewhere 

 noted. 



Table C shows that in a total of 1 23 young there is an excess of about 

 11 albinos (9 per cent) over the expected number, and this preponderance 

 of the recessive character is seen to be especially marked in generation 

 F.3 (from ^ ^ gr. 48, 49, 56, mated with 9 9 gr. 50, 51, 52), where it is 

 about 11 per cent. The additional lots, however, reduce this excess to 

 8 per cent in the total, and it would doubtless be found that larger 

 numbers would show still closer approximation to the theoretical result, 

 as indeed the experiments of Cuenot ( : 02) show. 



Table D is of more particular interest, since it gives the results of back 

 crossing with an albino a pigmented animal one of whose parents was an 

 albino. Consequently, by Mendelian formula (6), the pigmented mouse 

 is assumed to produce in the long run equal numbers of gametes bearing 

 the pigment and the albino characters respectively. It is evident that if 

 such an animal be bred to an albino, all the gametes it produces that bear 

 the albino character will unite with those of a similar sort furnished by 

 the albino mate, and albino young will result. Further, all the gametes 

 it produces that bear the pigment character will also unite with those of 

 the albino, but pigmented young will be produced, each of which will 

 contain the albino character recessive. It is thus possible to determine 

 how many gametes of each sort are produced by the heterozygote at each 

 union, or, at least, how many of them unite with those of a second 

 animal to form new individuals. 



In the case of the heterozygous male, No. 51, thirty young were 

 obtained in eleven litters by white females. Instead of the anticipated 

 equality of pigmented and albino mice, the former class stands to the latter 

 as 2 :1. Most of the litters are small, three consisting each of but a 

 single individual, and but one of these is an albino. Two litters consist 

 each of two young, and in the one case both are pigmented, while in the 

 otlier there is one i^igmented and one albino young. This last litter is 

 the only one of the eleven in which the expected equality of the two 

 classes is realized. There are three litters of three young each. One of 

 these is of albinos only. The other two taken together give an equality 

 of the two classes, the excess in the one exactly counterbalancing the 

 . shortage in the other. Two litters, each of four young, contain no albinos 

 at all, while the single litter of six contains four pigmented to two albino 

 young, a ratio again of 2 : 1. 



