The Genetic Factors in the Development of the Houseraouse, etc. 125 



From the AaGg mutant originated a strain of blacks (Aagg) and 

 it was the appearance of those blacks which gave me the first indi- 

 cation of a mutation having taken place. 



From the aaOg mutant originated a strain of aagg, albino mice 

 without the factor o. 



II. Loss of the factor F. 



In the same way as in the preceding case, I witnessed a second 

 case of the loss of a genetic factor in a gamete produced by an 

 animal otherwisely homozygous for that factor. 



A silvered (ABCDEfgH) (No 17) mouse was produced from two 

 black parents. Later these same parents gave another silvered black 

 young. In this case, as in the former, the family (which descended 

 in a straight line from that produced by the mutation just described) 

 was strictly inbred, so that these silvered mice had wo grandparents, 

 two great-grandparents, and so on, for at least twelve generations. 

 I looked up the great-grandparents of these silvered mice, and found 

 them both alive and healthy. I mated the greatgrandmother to the 

 silvered male, obtaining two litters of six young each, all young being 

 black. The great-grandfather was mated to a silvered female, and 

 later to two others. In all he gave five litters of young, twenty- 

 four in all, which were all black. In think these numbers are large 

 enough to feel sure that they both were homozygous ABCDEFgH. I 

 also tested the son of these two mice, the grandfather of the silvered. 

 (His female, the grandmother, was dead). Mated to two silvered 

 females he gave nine young, of which five were silvered. Here thus, 

 as in the first case, a mouse, homozygous for one factor must have 

 produced at least one gamete lacking this factor. 



III. Probable loss of the factor B. 



The third probable case of the spontaneous loss of a genetic 

 factor from a gamete produced by an animal homozygous for its 

 presence was the following. In an inbred family of agouti mice 

 (AABBCCDDEEFFGGHH) there was one day produced one yehow young. 

 This female was mated to other mice of her family, producing nothing 

 but agoutis, which, when mated inter se gave approximately 25 % of 

 yellow offspring (N" 18). As in both the other cases I wanted to 

 test the great-grandparents of this yellow mouse by mating them to 

 those new yellows, but unhappily the male (great-grandfather died 



