DEVELOPMENT OF THE HETEROSIS CONCEPT 



57 



thetic seed progenies gave about the same forage yield. In this comparison, 

 heterosis continued through the second seed increase of the high yielding 

 synthetic. 



Other Studies with Maize 



Combining ability, that is ability to yield in hybrid combination, has been 

 shown by various workers to be an inherited character (Hayes and Johnson, 

 1939), (Cowan, 1943), (Green, 1948). It seems feasible to breed for high com- 

 bining ability as for other quantitative characters. In the breeding program 



TABLE 3.7 



FORAGE YIELDS OF POLY- 

 CROSSES COMPARED TO 

 TOP CROSSES OF THE 

 SAME CLONES* 



* After Tvsdal and Crandall. 



for the production of imj^roved inbred lines, it is often possible to select as 

 parents of crosses, select lines having high combining ability as parents of 

 crosses, in addition to selection for other characters that are desired. In 

 breeding for heterosis, however, it seems evident that genetic diversity of 

 parentage is equally as important as combining ability (see Hayes, and 

 Immer, 1942; Sprague, 1946b). 



All relatively homozygous, inbred lines in maize are much less vigorous 

 than the better Fi crosses. It is apparent that heterosis is of great impor- 

 tance in crosses with inbred lines of maize. 



Inbred lines that have undesirable characters may be easily imjjroved by 

 the application of any one of several methods of breeding. The breeder may 

 select for each problem the method or methods that seem to him most ap- 

 plicable. In breeding selfed lines the selection of parents that have comple- 

 mentary characters that together include the characters desired in the im- 

 proved inbred is a natural first step. Subsequent methods of breeding may 



