324 Spotting in Mice and their Genetic Behaviour 



A marked discrepancy of the ratio again appears in this case ; as to 

 the reasons for such a result the writers cannot say, but think it to 

 be a chance deviation. 



The " Kasuri " Race carrying Self Factor in the Homozygous Condition. 



All of the specimens of the " Kasuri " race received from Mr Mito 

 proved, by the breeding test, to be doubly heterozygous, i.e. having the 

 genetic composition SS'Dd. If such mice are mated with' selfs of the 

 homozygous race having the constitution of SSdd, we should expect half 

 the number of the " Kasuri " young born to be homozygous in regard to 

 the self factor ; while we should expect one-third in the cross "Kasuri" x 

 "Kasuri," and " Kasuri " x heterozygous self of its reciprocal. But there 

 was often observed high mortality among the " Kasuri " young born 

 in these crosses, and, further, if they attained sexual maturity fre- 

 quently they fell into a sterile condition, more markedly on the female 

 side. Such an unfortunate occurrence, however, has been gradually 

 lessened in the course of the experiments. Probably this is connected 

 with an improvement in health by crossing with the strong European 

 breed, rather than with the elimination of the lethal factor or the like. 

 At present such sterility occurs occasionally, especially in the female, 

 and this is possibly one of the characteristics of the " Kasuri " race. The 

 case is somewhat similar to that observed in yellow mice. 



The sterility in the yellow mice seems connected with the abnormal 

 development of the fat tissue, but the case is somewhat different in the 

 " Kasuri " race. In the latter they often waste away gradually and at 

 last become sterile, though in some cases they grow quite plump like 

 the yellow mice. These conditions are also found, in a few instances, in 

 the " Daruma " race. 



Now we shall turn to the discussion of the experimental data on the 

 subject of this section. 



Out of seven " Kasuri " males obtained from the cross " Kasuri " x 

 homozygous self or its reciprocal, five proved to be homozygous as to the 

 self factor, and the rest (two) were heterozygous : of seven females having 

 a similar origin, one was homozygous and three were heterozygous, 

 but the rest (three) were doubtful, for the litters they gave were not 

 sufficient to decide their genetic composition. Out of twenty-one 

 males coming from the cross heterozygous self x " Kasuri " only one was 

 proved to be homozygous and eighteen were heterozygous, but the rest 

 (two) were doubtful : of nine females of the same origin, one was homo- 

 zygous and six were heterozygous, but the rest (two) were , doubtful. 



