The Factor Hypothesis 67 



P gives f with P\\ without P. AppUed to the A that was 

 carried on, this becomes o^ Xf or /5V which lacks / and has C, R, 

 and P, and is therefore purple; and ^aX\ or of.n which lacks /, 

 has C and R, and lacks P, and is therefore red. 



Summarizing, f plus iV plus ^t^ or .7^,1 is colorless, 250 is red, 

 and 2^5^ is purple. The phenotypic ratio is, 27 purple 19 red: 220 

 white. 



Thus three factor types are at play in the inheritance of color 

 in grains of corn. Four distinct factors are interacting, but all 

 are inherited independently and quite in accordance with Men- 

 delian principles. 



Again, the exact physiological nature of the / factor is not 

 understood, but can be only guessed at. We have assumed that 

 color is produced when an enzyme is present to oxidize a chromo- 

 gen. Enzymes are sensitive; their activities may be affected or 

 completely checked by various agents. Assume that / is such 

 an agent, and the necessary mechanism is provided. When / is 

 present R is paralyzed, so that it cannot oxidize C. 



4. Cumulative factors. — These are considerably 

 different from the other types and will be considered in 

 another chapter under the caption "Inheritance of 

 quantitative characters." 



These four great factor types are really the only ones 

 encountered in genetics, each representing a distinct 

 type of interaction. In the literature of the subject 

 many other descriptive titles are given to factors, but 

 no fundamentally new mechanisms are introduced. 



A few more words might be said on the three factor types that 

 have already been considered. Of these three, the rarest type is 

 the inhibitory factor, complementary and supplementary factors 

 being quite common. 



It is not surprising to lind that true doniiiKint inhibitory 

 factors are rather rare, as is suggested by the following reasoning. 

 In nature, there has been for countless generations a struggle for 

 existence among the individuals of a species, with a survival of the 



