58 Plant Genetics 



d) A cumulative factor, when added to another similar 

 factor, affects the degree of development of the character. 



Some examples of these types will make them clear, 

 those for complementary factors having been given 

 previously. 



2. INHIBITORY FACTORS. Recalling EAST'S experi- 

 ment with red-grained corn it will be remembered that 

 when both factors for red were present the grain was red, 

 but when either factor was absent the grain was white. 

 Later he crossed these strains with a new white strain, 

 and the result was surprising (3). The pure red strain 

 produced gametes carrying both the red factors, and 

 it would be expected that whatever such a gamete 

 mated with would result in red progeny; but when this 

 pure red was crossed with the new strain of white the 

 progeny were all white, although the hybrids certainly 

 contained both factors for red. The explanation which 

 first occurred to EAST, and which later experiments 

 confirmed, was that the new white strain contained 

 an inhibitory factor, which prevented the development 

 of red even though both the complementary factors for 

 red were present. Fig. 10 illustrates the situation and 

 shows why all the individuals of the F! generation are 

 white. It is interesting to note further the possibilities 

 of white and red in the F 2 generation. They would be 

 numerous, since we are dealing with trihybrid ratios 

 (see fig. n). This does not exhaust the possibilities, for 

 the cases given were homozygotes, each producing a 

 single kind of gamete. There remains for consideration 

 the heterozygote situation (see fig. 12). 



The possible mechanism of the inhibitory factor 

 is as follows. We have assumed that red is produced 



