The Factor Hypothesis 6i 



bring them together the result would be the appearance of a new 

 character; a few authors (notably Lotsy 5) believe that this is a 

 very important factor in the origin of species. 



The behavior of the red-grained and white-grained 

 corn serves as an introduction to the factor hypothesis 

 and as an illustration of one of the important t}'pes of 

 factor interactions. Complementary factors are always 

 to be recognized by the following behavior. A comple- 

 mentary factor interacts with a dissimilar factor to 

 produce a particular character. 



2. Supplementary factors. — A supplementary fac- 

 tor interacts with a determiner (or factor complex) to 

 modify the character produced by the latter. An excel- 

 lent illustration of this factor type can also be provided 

 by the inheritance of color in grains of corn (East 3). 

 Interacting with the factors R and C is a third factor P} 

 This P factor is inherited quite independently of the 

 other two, but whenever it is present the red color 

 becomes modified to purple. The behavior of this factor 

 is revealed when we consider the phenotypes to which 

 the following homozygotes belong. Corn of the formula 

 PPRRCC has purple grains, ppRRCC is red grained, 

 while PPRRcc and PPrrCC are both white grained. 

 From these facts, one can draw the following conclu- 

 sions: (i) when P, R, and C are all present a pur])le 

 grain results; (2) red color can appear only when P is 

 absent; (3) P itself is entirely ineffective in the absence 

 of either R or C. 



* In the literature on inheritance in corn, this factor is referred to 

 by the symbol Pr, its absence being denoted b>' pr (see work of Emerson 

 and others). It is to avoid possible confusion in the mind of the student 

 that it is referred to by the single letter in the present text. 



