MULTIPLE FACTORS 181 



one class may vary so much from each other as to 

 overstep the small differences separating the classes. 

 An accurate separation of the individuals into differ- 

 ent classes and a count of the number in each class is 

 then impossible, and it becomes so difficult to de- 

 termine the number of factors involved and the 

 effect of each factor (or, rather, factor-difference) 

 that such cases have at times been used in attempts 

 to disprove the factorial hypothesis. The problem is 

 likewise more difficult if more than two factor- 

 differences occur. This is true especially in those 

 cases where the effects of the different factors are 

 cumulative, for then classes are produced showing 

 characters intermediate in various degrees between 

 the characters of the most extreme classes, just as in 

 cases of incomplete dominance. It will be instructive 

 to consider several instances of crosses of the above 

 types, since, although definite ratios can not be 

 obtained, there are various characteristic effects 

 produced which show that multiple factors are re- 

 sponsible for the peculiarities of the results. 



The inheritance of black color in Drosophila has 

 already been described. Black is recessive to the 

 normal ("gray") color, but the heterozygous forms 

 are a little darker than the pure grays. Ebony is 

 another body color, similar in appearance to black, 

 but somewhat darker. It is similarly recessive to 

 gray, but the factor responsible for it is located in a 

 different chromosome (III) from that which carries 

 the factor for black (II). When black and ebony 

 are mated together we should expect gray flies in Fi. 



