4i8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



The situation is thus explained by the dihybrid ratio, but although 

 only one character is involved that character depends upon two com- 

 plementary factors. 



Another situation is worth noting. No. 6 of the diagram is white 

 because it contains only one of the necessary factors; No. ii is white 



Fig. 79. — Diagram illustrating behavior of complementary factors in cross 

 between red-grained and white-grained corn. R and C must both be present to 

 produce red-grained corn. {From Coulter and Coulter.) 



for the same reason, but its germinal constitution is just the opposite. 

 What would happen if these two are crossed? There is only one 

 possibility, since each is a homozygote producing only one kind of 

 gamete. The result would be red, and thus a cross between two whites 

 would produce only reds. What would happen from crossing Nos. 6 

 and 15, the former being a homozygote and the latter a heterozygote ? 



