322 Spotting in Mice and their Genetic Behaviour 



If such heterozygous Fi animals are back-crossed with selfs of pure 

 race the resulting young, without exception, are selfs, as is expected. 

 When back-crossed with piebalds, the expectation is equality of the 

 ratio of the two forms, namely, one self to one spotted. The actual 

 results thus obtained are represented in the following table : 



Self Piebald Totals 



Observed 77 82 159 



Expected ... 79-5 79*5 159 



An inspection of the above-mentioned data shows that the results 

 entirely agree with those obtained by other investigators, the spotting 

 being a recessive Mendelian unit to the self-coloured. 



Mating : " Kasuri " x Piebald. 



In attempting to separate phenotypically the " Kasuri " race from the 

 ordinary piebald, none of the authors distinguished it clearly, though 

 they were aware of the special silvered markings of the former type. 

 In the study made by the present writers, however, the distinction 

 between these two races was made without any great difficulty. Actu- 

 ally the " Kasuri " pattern is a sort of spotting, but this form is 

 characterized by fine silvered markings. With this appearance kept 

 in mind the two types of spotting may easily be separated from each 

 other, though in a few cases the former pattern fails to develop to its 

 markings. But such a trouble is not a matter confined only to this 

 case, it is almost equally met with in the case between " Kasuri " and 

 self, " Daruma " and " Kasuri," and " Daruma " and piebald as well as 

 between self and piebald. In these doubtful cases, of course, the breeding 

 test serves as a sole key by which to know their real nature. 



To understand the genetic composition of the " Kasuri " race the 

 writers often made the cross " Kasuri " x piebald reciprocally, for the 

 latter animal carries the pattern factors in the lowest condition. 

 Neither parent is self-coloured but has a sort of spotting, while among 

 the offspring of thele matings there appeared many selfs. Now, such 

 extracted selfs must be attributed in their origin to the " Kasuri " 

 parent, for no other carries the self factor, as was shown in the last 

 section. And as each sort raised is in almost an equal ratio the results 

 are similar to that when the double heterozygote is back-crossed to 

 the double recessive one in Mendelian inheritance. The following 

 allelomorphs assumed may suffice to illustrate these phenomena : 



I. A factor, S, is responsible for self and its allelomorph S' stands 

 for spotting of recessive nature. 



