174 INHERITANCE IN RATS. 



agouti), the rest being gray (agouti). Both sorts included about equal 

 numbers of individuals with and without white spots. No difference 

 was observed in this respect between the progeny of spotted and of 

 unspotted parents. Table 158 shows the F 2 young grouped family by 

 family according to grade. Three of the four families are descended 

 from a single mutant grandparent; the fourth family is descended 

 from two different mutant grandparents which were bred simultane- 

 ously to the same wild male in the same cage. The 10 F 2 young of 

 this family may have been produced either by full brother and sister, 

 or by half-brother and half-sister; it is uncertain which. All other 

 F 2 young were produced by brother-sister matings. 



It will be observed that the F 2 young (table 158) which are white- 

 spotted are in no case hooded. Their range of variation does not fall 

 beyond that of the uncrossed mutant race. It is certain, therefore, 

 that the "mutant" condition is not hooded plus an independent Mendel- 

 ian modifier. It is a changed form of white-spotting, alternative to the 

 form of spotting found in the race from which it was derived (the plus- 

 selection series, generation 10). It is, without much doubt, also alter- 

 native to the self condition of wild rats, though fluctuation in grade ob- 

 scures the segregation, which may, very likely, be imperfect. This serves 

 to confirm the general conclusion that throughout the entire series of 

 experiments with the hooded pattern of rats we are dealing with quan- 

 titative variations in one and the same genetic factor. 



