MULTIPLE FACTORS 203 



modify the coat may produce 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. 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 

 F2 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 allelo- 

 morphic to white, hence the 3:1 ratio, but this clas- 

 sification 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 is a case in Drosophila that illustrates 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 color, but among the fights three 

 different but overlapping kinds may be detected. 

 Here, as in Castle's case, there is a chief factor (eosin) 

 for reduced pigmentation, which must be present if 

 any reduction in the color occurs at all, and another 

 factor (cream) that modifies the amount of pigmen- 

 tation only when the chief factor is present. 



In favor of the view that factors are constant are 



