160 GENETICS 



another similar factor, affects the degree of expression 

 that a character is given; 



(4) An inhibitory factor, which prevents the action 

 of some other factor, and so on. 



It will be profitable to consider the factor hypothe- 

 sis in some detail, since it helps to explain both rever- 

 sion and the formation of new types. 



a. Bateson's Sweet Peas 



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 reversion, 

 but according to the factor hypothesis the explanation 

 is this. The character of purple color is dependent 

 upon two independent factors which, though sepa- 

 rately heritable, are both required to produce it. Each 

 of these white strains of sweet peas possesses one of 

 these factors 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 two complementary factors which follow 

 the behavior of a dihybrid. (See Chap. VTI, par. 10.) 



The gametic formulae for the two strains of white 

 sweet peas used in this experiment are Cp and cP y 

 respectively. C stands for a color factor without 



