NEO-MENDELISM IN PLANTS 417 
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 external] 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. 
Tt 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; 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. 79. It will be seen that out of 16 progeny 9g will be red, for they 
alone contain the complementary factors; the other 7 will be white. 
