The Factor Hypothesis 59 



ratios were obtained because the genotypes were dif- 

 ferent. 



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 phenomenon of reversion. For ex- 

 ample, in the course of numerous breeding experiments 

 Bateson (i) obtained two strains of white sweet peas, 

 each of which when normally *'selfed" bred true to the 

 white color; but when these two were artilicialh' crossed 

 all of the Fi 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. 

 Further breeding, however, showed that this was just 

 such a case of complementary factors as we have been 

 considering. One of Bateson's white strains had one 

 of the factors for purple flower color and the other strain 

 had the other factor. 



It is interesting to note that if an investigator should cross 

 homozygote no. i with homozygote no. 11, the Fx and F2 results 

 would lead him to conclude that the red character was due to a 

 simple Mendelian determiner. R would remain a ''determiner" 

 until a strain of corn was discovered which lacked the C factor; 

 crosses with such a strain would reveal the real mechanism of the 

 situation, and thereafter R would be known as a "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 chemis- 

 try (2). The most prominent group of pigments in plants is the 

 group of anthocyanins, which are produced as follows. Plants 

 contain compounds called chromogens, which are colorless them- 

 selves, but which produce pigment when acted upon by certain 

 oxidizing enzymes or oxidases. This would provide a mechanism 

 to account for the behavior of complementary factors. If one of 

 East's white strains of corn contained a chromogcn capable of 



