50 BREEDING CROP PLANTS 



basis of dominance and linkage. In comparing crosses with their 

 parents it is quite common to find that the F\ generation has a 

 higher value for nearly every growth character than has the aver- 

 age of the parents. Modern geneticists recognize that each 

 character is due to the interaction of many inherited factors. If 

 each growth factor gives as great an effect when heterozygous as 

 when homozygous or proves partially dominant when heterozy- 

 gous, it would be easy to explain heterosis by the actual physio- 

 logicial growth development which is a part of the normal 

 expression of a particular inherited factor. This explanation 

 was formerly advanced to account for heterosis but was con- 

 sidered unreliable, as it was difficult to account for the almost 

 universal decrease in vigor when such plants as maize were 

 selfed. This can be explained by the facts of linkage, as it is 

 possible to have a greater number of different growth factors 

 present in a heterozygous than in a homozygous individual. 

 The explanation has much in its favor. 



