38o 



EVOLUTION, GENETICS, AND EUGENICS 



but we shall not attempt to list all of the proposed types. A simple 

 illustration of the better known types is as follows : 



a) A complementary factor is added to a dissimilar factor to pro- 

 duce a particular character. 



b) An inhibitory factor prevents the action of some other factor. 



c) A supplementary factor is added to a dissimilar factor with the 

 result that the character is modified in some way. 



d) A cumulative factor, when added to another similar factor, 

 affects the degree of development of the character. 



Some examples of these types will make them clear, those for 

 complementary factors having been given previously. 



R 



© X 



Pure Red Parent 



Gamete 



Gamete 



White Parent with 

 Red Inhibitor 



Fig. 70. — Diagram illustrating behavior of inhibitory factor. {From Coulter 

 and Coulter.) 



2. Inhibitory factors. — Recalling East's experiment with red- 

 grained corn it will be remembered that when both factors for red 

 were present the grain was red, but when either factor was absent the 

 grain was white. Later he crossed these strains with a new white 

 strain, and the result was surprising. The pure red strain produced 

 gametes carrying both the red factors, and it would be expected that 

 whatever such a gamete mated with would result in red progeny; but 

 when this pure red was crossed with the new strain of white the pro- 

 geny were all white, although the hybrids certainly contained both 

 factors for red. The explanation which first occurred to East, and 

 which later experiments confirmed, was that the new white strain con- 

 tained an inhibitory factor, which prevented the development of red 

 even though both the complementary factors for red were present. 



