The Factor Hypothesis 57 



I. Complementary factors. — This is the simplest 

 expression of the factor hypothesis; it may be illustrated 

 by some of East's work (3). Crossing red-grained and 

 white-grained corn, this investigator obtained an Fi 

 progeny which was all red. This would suggest that the 

 F2 generation would show 3 red to i white; but instead 

 it showed 9 reds to 7 whites, which might seem to violate 

 the Mendelian method of inheritance. It is quite 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 inher- 

 ited separately. Such a situation would give a dihybrid 

 ratio, as indicated in fig. 9. It will be seen that, out 

 of the 16 individuals in the F2 checkerboard, 9 will be 

 red, for they alone contain both complementary factors; 

 the other 7 will be white. The situation is thus explained 

 by the dihybrid ratio; but, although only one character 

 is involved, that character depends upon two comple- 

 mentary factors. 



Another situation is worth noting. No. 6 of the 

 checkerboard is white because it contains only one of 

 the necessary factors; no. 11 is white for the same rea- 

 son, but its germinal constitution is just the opposite. 

 What would happen if these two were crossed ? There 

 is only one possibiUty, since each is a homozygote produ- 

 cing only one kind of gamete. The result would be red, 

 and thus a cross between two whites would produce only 

 reds. What would be the result if nos. 6 and 15 were 

 crossed, the former being a homozygote and the latter a 

 heterozygote ? It is obvious that the resulting progeny 

 would be one-half red and one-half white. The same 

 result would be secured in crossing nos. 11 and 14. A 



