Q4 Outline of Genetics 



certain arbitrary units), while the low pigmentation strain was 

 -2.63. Critical examination revealed the fact that the cross 

 between the +3.73 strain and the wild race brought a slight reduc- 

 tion in the amount of pigmentation as it appeared in the extracted 

 hooded "recessives" in the F2. Repeated recrossing of these 

 extracted individuals with the wild race finally resulted in extracted 

 hooded rats of the grade +3-04. No further reduction was pos- 

 sible in this way. These results could be explained by the follow- 

 ing assumptions. The hooded pattern is modified in degree of 

 pigmentation by a varying number of doses of cumulative factors 

 (as the ''mutationists" had previously maintained). The wild 

 race is characterized by having a certain number of doses of these 

 cumulative factors. The repeated crossings and extractions 

 mentioned above would eventually result in producing rats which 

 had the hooded pattern plus that number of doses of cumulative 

 factors which was characteristic of the germ plasm of the wild 

 race. Since it was found that repeated crosses with the wild race 

 could bring the degree of pigmentation down to +3-04 and no 

 lower, it was felt that +3.04 was the degree of pigmentation which 

 would be produced by that number of cumulative factors which 

 was characteristic of the wild race. 



The critical test of these assumptions could be made through 

 a similar manipulation of the low pigmentation strain. If the 

 assumptions were correct, the low pigmentation strain should be 

 raised finally to +3-04 by repeated crossing with the wild race. 

 Castle performed this experiment and got exactly this result, 

 one of the families from the low pigmentation strain (-2.63) being 

 finally brought up to -I-3.05. 



These results naturally caused Castle to change his views 

 on the matter, and served rather generally to establish the views 

 of the ''mutationists." The situation depended for its interpre- 

 tation upon the cumulative factor mechanism. (Here it was felt 

 that the cumulative factors were not primarily responsible for 

 the production of the character in question, but served merely 

 to modify the degree in which it expressed itself. Other cases of 

 the same sort have been encountered elsewhere, the mechanism 

 at play being commonly referred to in the literature as "multiple 

 modifying factors.") 



