THE FACTOR HYPOTHESIS AS APPLIED TO PLANTS 257 



of dominance) and on the other hand that for cases of complete domi- 

 nance. 



Another example of simple blending inheritance is the case of 

 Adzuki beans, described by Blakeslee. In this bean the mottling of 

 the seed coat is dominant to the lack of mottling. In the hybrid 

 condition, however, the mottling is lighter than in the pure or homo- 

 zygous condition. Heterozygous plants, therefore, can be easily dis- 

 tinguished from homozygous plants, so that the 1:2:1 ratio is evident 

 on externa] inspection rather than the usual 3:1 ratio. 



III. THE FACTOR HYPOTHESIS 



Mendel concluded that each plant character depends upon a single 

 determiner. Inheritance, however, has proved to be a much more 

 complex phenomenon than indicated by Mendel's peas. Ratios have 

 appeared that were puzzling, and geneticists were forced to the conclu- 

 sion that there may be a compound determiner for a single character. 

 This conception is called the factor hypothesis, and the growing com- 

 plexity of genetics has developed in connection with this hypothesis. 

 With the consideration of factors instead of determiners one passes 

 from elementary to advanced genetics. Previously we have used the 

 word determiner, implying Mendel's idea that a single determiner is 

 responsible for the development of a plant character, and this 

 has been true of the examples of inheritance previously considered. 

 It is understood now, however, that a character is frequently deter- 

 mined by the interaction of two or more separately heritable factors, 

 and hence the factor hypothesis. The distinction between factors and 

 determiners should be made clear. In case only one factor is involved 

 in determining a character, there is no distinction between factor and 

 determiner; and in such a case the term factor should not be used. 



1. Complementary factors. — This is the simplest expression of 

 the factor hypothesis and it may be illustrated by some of East's work. 

 Crossing red-grained and white-grained corn he obtained all red in 

 the F! generation. This would suggest that the F 2 generation would 

 show 3 red to 1 white; but it showed 9 reds to 7 whites, which did not 

 suggest Mendelian inheritance. It is in accord with Mendel's law, 

 however, if we consider that two complementary factors are necessary 

 to produce the red character, and that each of these factors is inherited 

 separately. Such a situation would give a dihybrid ratio, as indicated 

 in Fig. 61. It will be seen that out of 16 progeny g will be red, for they 

 alone contain the complementary factors; the other 7 will be white. 



