46 Plant Genetics 



These two cases are illustrations of what may be 

 called simple blending inheritance. There are other 

 cases of blending which are more complex and cannot 

 be explained so easily. 



EAST (4) , in crossing starchy and sweet corn, obtained 

 all starchy in the F x generation, followed by the usual 

 3 : i ratio in the F 2 . He says that starchiness is due to 

 the presence of a determiner which enables the corn to 

 mature starch grains. When this determiner is absent 

 starch grains cannot be formed, so that the carbo- 

 hydrates of the endosperm are left in the condition of 

 sugar, and the .result is a sweet corn. This is a simple 

 case of presence and absence, usually accompanied by 

 complete dominance. Occasionally, however, the cross 

 between pure sweet and pure starchy races gave semi- 

 starchy progeny. The hybrid therefore was inter- 

 mediate in this regard and would seem to represent a 

 case of blending inheritance, which of course means 

 incomplete dominance. This case, however, cannot be 

 explained easily by the presence and absence hypothesis 

 or any other hypothesis. The difficulty is that inherit- 

 ance, which usually shows complete dominance, occasion- 

 ally gives way to blending inheritance. The blend in 

 the four-o'clock was a constant behavior; in this 

 corn case it is the occasional behavior; so that the 

 mechanism proposed to explain the four-o'clock blend 

 cannot explain this inconsistent behavior of corn. 

 The best that can be said, apparently, is that some- 

 thing is interfering with the mechanism; and this 

 is about all that EAST concludes; he says "some- 

 thing is interfering with dominance; it is only 

 partial." 



