E. J. WELLHAUSEN 



Rockefeller Foundation, Mexico City 



Chapter 27 



Heterosis in a New Population 



Data recently presented by Mangelsdorf and Smith (1949) indicate that corn 

 was being grown in what is now southwestern United States and Mexico at 

 least four thousand years ago. The corn grown in these prehistoric times was 

 both a pod corn and a pop corn of relatively low yield capacity. Today in 

 this same area an enormous variation exists. Direct derivatives of the ancient 

 low yielding pod-pop type still can be found on a very limited scale in certain 

 areas of Mexico, but these low yielding ancient corns now have been replaced 

 largely by more vigorous and productive types. 



Tremendous changes have been brought about in both type and yield 

 capacity since ancient times. The modern varieties of Mexico have a yield 

 capacity many times more than the ancient types. On the high plateau of 

 Mexico a variety known as Chalqueno, whose pedigree in part can be traced 

 back to an ancient pop corn, has yielded up to 125 bushels per acre. If the 

 various evolutionary processes and the kinds of gene actions involved in the 

 development of such high yielding varieties from the low yielding prehistoric 

 types were known, we would certainly have a better understanding of the 

 phenomenon of heterosis. 



It is the purpose of this chapter to present, first, what seems to have been 

 involved in the development of the modern, relatively high yielding varieties 

 over a period of about four thousand years; and second, a discussion of the 

 methods used and results obtained in the further improvement of some of the 

 modern varieties in a short period of six years. 



Perhaps the title might best have been "Heterosis in an Old Population" 

 in the sense that the Mexican corns as a whole are much older than those in 

 the United States. However, from the standpoint of modern corn breeding, 

 it is a new population in that it involves new material on which to try the 

 modern techniques of corn breeding developed in the United States. The suc- 



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