422 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Fig. 80 illustrates the situation and shows why all the individuals of 

 the Fi generation are white. It is interesting to note further the 

 possibilities of white and red in the F2 generation. They would be 



^ — ^ 



Red 



R^ 



White 



White 

 Fig. 81. — Diagram showing some possible combinations in F2 when Fi of 

 Figure 80 is inbred. Individual on left end of upper set red-grained, because R 

 and C both present and / absent; other individuals in upper set white, because 

 lacking C or i? or both; individuals in lower set with inhibitory factor and there- 

 fore white, whatever other combinations of factors they may contain. {From 

 Coulter and Coulter.) 



numerous, since we are dealing with trihybrid ratios (see Fig. 81). 

 This does not exhaust the possibilities, for the cases given were 



honiozygotes, each producing a single kind of 

 gamete. There remains for consideration the 

 heterozygote situation (see Fig. 82). 



The possible mechanism of the inhibitory 

 factor is as follows. We have assumed that red is 

 produced only when the enzyme is present to 

 oxidize the chromogen. Enzymes are very sensi- 

 tive; their activities may be affected or com- 

 pletely checked by various agents. Suppose that 

 I of the diagram be such an agent and the neces- 

 sary mechanism is apparent. When / is present R is paralyzed, so 

 that it cannot oxidize C. 



3. Supplementary factors. — A supplementary factor is one that is 

 added to a dissimilar factor, with the result that a character is modified 

 in some way. 



