Wright: Color Inheritance in Mammals 



429 



between various generations of hooded 

 stock and the mutant, wild and Irish 

 stocks respectively. In interpreting the 

 results, the strength or weakness of the 

 various assumptions made must, of 

 course, be born in mind. 



counts for more than variation of the 

 main factor, although it is to be noted 

 that hooding from plus rats in each 

 case does average a little more than 

 hooding from minus rats. The general 

 difference in residual heredity between 



From the table it appears that 

 with a given residual heredity the main 

 hooding factors derived from the plus 

 and minus series produce but little 

 difference in effect. Thus, finding the 

 weighted average for hooded rats derived 

 from the plus and the minus strains 

 when associated with the residual hered- 

 ity of the mutant, wild and Irish stocks, 

 resiDectively, we get the following results : 



Source of Residual heredity 



hooding factor Mutant Wild Irish 



Plus +3.79 +2.99 +0.51 



Minus +3.51 +2.15 -0.08 



Difference +0.28 +0.84 +0.59 



It appears that residual heredity 



1" Castle, W. E., and Phillips, J. C. 1914. 

 11 Castle, W. E. 1916. Loc.cit. 



the Irish and wild rats seems very 

 clear. This suggests that Irish rats 

 may possess the same hooding factor 

 as wild rats, but owe their white bellies 

 to minus selection or, more likely, to 

 crossing with hooded rats which had 

 been selected in a minus direction. 



In the case of the mutant rats, how- 

 ever, there would seem to be no room 

 for doubt that we are dealing with a real 

 variation of the hooding factor inter- 

 mediate between the level in wild or 

 Irish rats and that in hooded rats. 

 The mutants look like Irish rats in 

 having white on the belly but breed very 

 differently. They possess the residual 

 heredity of the plus selection series 



Loc. cit. 



