THE FACTOR HYPOTHESIS 151 



ter may appear; (2) Supplementary, when a factor is 

 added to a dissimilar factor already effective, with the 

 result that a character is modified or changed in some 

 way; (3) Lethal, so-called since they "cause the early 

 death of those gametes or zygotes in which such a fac- 

 tor is not balanced by a normal one" (Conklin). 



It will be profitable to consider a few illustrations 

 of the factor hypothesis in some detail since it helps to 

 explain both the reappearance of old types and the for- 

 mation of new ones. 



3. COMPLEMENTARY GENES 



In the course of numerous breeding experiments 

 Bateson obtained two strains of white sweet peas, 

 Lathyrus, which, when normally self-fertilized, each 

 bred true to the white color. When these two strains 

 were artificially crossed, however, the progeny all had 

 purple flowers like the wild ancestral Sicilian type of 

 all cultivated varieties of sweet peas. 



Here was apparently a typical instance of "rever- 

 sion" which would have delighted Darwin's heart, but 

 according to the factor hypothesis the true explanation 

 is this. The character of purple color is dependent 

 upon two independent genes which, though separately 

 heritable, are both required to produce it. Each of 

 these white strains of sweet peas possesses one of these 

 genes which can produce colored flowers only when 

 united with its complement, a proof of which appeared 

 upon interbreeding hybrid purples from such a cross. 

 In short, the color purple depends upon the action of 



