378 EVOLUTION, GENETICS, AND EUGENICS 



It is obvious that the resulting progeny would be one-half white and 

 one-half red, although both parents are white. The same result would 

 be secured in crossing Nos. ii and 14. A cross between Nos. 14 and 

 15, both of which are heterozygotes, would result in 3 whites and i red, 

 the ordinary 3 : i ratio. These illustrations show how differently the 

 same phenotype may behave in inheritance. In each case two whites 

 were crossed, that is, the same phenotypes, but three different ratios 

 were obtained because the genotypes were different. 



The striking feature of this situation is that one can cross two 

 whites and get a red. This gives an insight into the so-called phenome- 

 non of reversion. For example, in the course of numerous breeding 

 experiments Bateson obtained two strains of white sweet peas, each 

 of which when normally "selfed" bred true to the white color; but 

 when these two were artificially crossed all the progeny had purple 

 flowers, like the wild Sicilian ancestors of all cultivated varieties of 

 the sweet pea. This appeared to be a typical case of reversion. Fur- 

 ther breeding, however, showed that this was just such a case of com- 

 plementary factors as we have been considering. One of Bateson's 

 white strains had one of the factors for purple and the other strain had 

 the other factor. 



Complementary factors have been defined and the method of their 

 inheritance described, but is there any mechanism to explain the 

 situation ? A suggestion may be obtained from plant chemistry. 

 The most prominent group of pigments in plants is the group of antho- 

 cyanins, which are produced as follows. Plants contain compounds 

 called chromogens, which are colorless themselves but which produce 

 pigments when acted upon by certain oxidizing enzymes or oxidases. 

 This is a sufficient mechanism for the behavior of complementary 

 factors. If one of East's white strains of corn contained a chromogen 

 capable of producing red but lacked the necessary oxidase it would 

 remain colorless. If the other white strain contained the oxidase but 

 no chromogen it would remain colorless. In crossing them, however, 

 chromogen and oxidase would be brought together and a red-grained 

 hybrid would be the result. Inbreeding such red-grained individuals 

 of course would give red and white progeny in a ratio of 9:7, as 

 explained in connection with East's corn. This seems to be the explana- 

 tion of the behavior of complementary factors in many cases of color 

 inheritance. 



Where other characters are involved the mechanism must be some- 

 what different. In some cases the two factors may be the enzyme 



