TH. DOBZHANSKY 



Columbia University 



Chapter 13 



Nature and 

 Origin of Heterosis 



Exploitation of heterosis in cultivated plants and animals is to date by far 

 the most important application of the science of genetics in agricultural prac- 

 tice. It is therefore unfortunate that few of the studies so far made on 

 heterosis go beyond crudely empirical observations and descriptions and 

 that little effort is being made to understand the underlying causes of the 

 phenomena involved. Such an understanding is needed particularly because 

 the advances of general genetics make it evident that several quite distinct, 

 and even scarcely related, phenomena are confused under the common label 

 of heterosis or hybrid vigor. 



In what follows, an attempt is made to indicate briefly what seem, to the 

 writer, promising lines of approach to a classification and study of the various 

 kinds of heterosis. The tentative nature of the classification here suggested 

 is fully realized. But it is believed that this classification may nevertheless 

 serve a useful function if it directs the attention of the students of heterosis 

 to factors which are only too often overlooked. 



MUTATIONAL EUHETEROSIS 



Perhaps the simplest kind of true heterosis — euheterosis — is that which 

 results from sheltering of deleterious recessive mutants by their adaptively 

 superior dominant alleles in populations of sexually reproducing and cross- 

 fertilizing organisms. 



Although only a small fraction of the existing species of organisms have 

 been investigated genetically, it is reasonable to assume that mutational 

 changes arise from time to time in all species, albeit at different rates. Fur- 

 thermore, a great majority of the mutations that arise are deleterious, and 

 lower the fitness of their carriers to survive or to reproduce in some or in all 



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