MULTIPLE FACTORS 255 



a visible effect only in the presence of this chief 

 factor for hoodedness. The F2 from the crosses to 

 self-color indicate that such modifiers are really 

 present in the rats, since the hooded Fo are far more 

 variable than their hooded ancestors. The under- 

 standing of this point is so important that similar 

 relations of the same sort may be cited. If a choco- 

 late mouse {i.e., one that carries the factors for black 

 and for cinnamon) is mated to a white mouse carry- 

 ing the factors for gray (instead of those for black 

 and cinnamon) the Fi generation will be gray. In 

 the Fo there are three colored mice to one white 

 one, but there are several sorts of colored mice. 

 Color of any kind is dependent on the action of a 

 factor allelomorphic to white, hence the 3 : 1 ratio, 

 but this classification ignores the occurrence of 

 several kinds of colored mice which are due to 

 differences in other factors determining what kind 

 of color will develop. 



There are several cases in Drosophila that il- 

 lustrate the same point. Eosin is a light eye color. 

 Another factor called cream produces no effect on 

 other eye colors, but makes eosin still lighter. A 

 male pure for cream and for eosin bred to a red 

 female gives red eye color in Fi. The Fi's inbred 

 give three reds to one Hght eye coloi-, but among 

 the lights three different but overlai)])ing kinds 

 may be detected. Here, as in Castk^'s case, there 

 is a chief factor (eosin) for reduced ])igmentation, 

 which must be present if any reduction in the color 

 occurs at all, and another factor (cream) that 



