The Genetic Factors in the Development of the Housemouse, etc. ng 



Factor L. 



Tliis factor is again one which is present in all the wild rodents, 

 and wliich has been widely studied. Its presence is necessary to 

 make '.in animal solid coloured, and those animals which lack it are 

 partially albinistic. 



CuENOT has already shown that the difference between partially 

 albinistic and solid-coloured mice was not due to one factor only, but 

 that several factors are necessary to make the pigmented area extend 

 over the whole animal. I am just now working out the interrelation 

 of these different factors in the brown rat, and am finding that the 

 analysis of these factors is an exceedingly difficult one, as these factors 

 are very numerous. A I have already said, it must not be lost sight 

 of, tliat, however much these factors may have analogous functions, 

 they may be fundamentally different. In the experiments of Nilsson- 

 Ehle, where four factors were found, all tending to deepen the colour 

 of the grain, it is very probable that the way in which each of these 

 factors contributes to this colour is fundamentally different from that 

 in which all the others act. In the gradations of partial albinism in 

 rodents the interrelation of the different factors is such, that there 

 exists one factor, which we will call L, whose absence is necessary, 

 when the coat is to be marked with white. Two animals, therefore, 

 heterozygous for L, and some of those other factors produce, when 

 mated, only 25% of partially albinistic young, the more or less 

 extended markings of which depend upon presence or absence of those 

 other factors. All animals having L are completely pigmented, no 

 matter wheter they have or lack the other factors. It is therefore 

 possible to study the inheritance of L, independantly from that of 

 those other factors. It can never be seen whether a fully pigmented 

 mouse or rabbit contains or lacks any or all of these latter factors, 

 other than by mating it to an animal lacking L (a spotted) and 

 breeding the second generation. It can therefore be said, that all 

 fully pigmented rodents possess the factor L that all partially albinistic 

 animals lack it. Loss of l, therefore, produces quite a different result 

 in animals differing in the presence or absence of these other factors. 

 It is probably for this reason that the distribution of white patches 

 on mice with the least amount of white is essentially the same as 

 that in the rabbit and the cavy, but very different from that in the 

 brown rat. A rabbit, cavy, black rat or mouse, with a very small 

 amount of white in the coat, almost invariably has some white on 



