H. K. HAYES 



University of Minnesofa 



Chapter 3 



Development of 

 the Heterosis Concept 



Hybrid vigor in artificial plant hybrids was first studied by Koelreuter in 

 1763 (East and Hayes, 1912). The rediscovery of Mendel's Laws in 1900 

 focused the attention of the biological world on problems of heredity and led 

 to renewed interest in hybrid vigor as one phase of quantitative inheritance. 



Today it is accepted that the characters of plants, animals, and human 

 beings are the result of the action, reaction, and interaction of countless 

 numbers of genes. What is inherited, however, is not the character but the 

 manner of reaction under conditions of environment. At this time, when 

 variability is being expressed as genetic plus environmental variance, one 

 may say that genetic variance is the expression of variability due to geno- 

 typic causes. It is that part of the total variance that remains after eliminat- 

 ing environmental variance, as estimated from studying the variances of 

 homozygous lines and Fi crosses between them. 



Early in the present century. East, at the Connecticut Agricultural Ex- 

 periment Station, and G. H. Shull at Cold Spring Harbor, started their 

 studies of the effects of cross- and self-fertilization in maize. The writer has 

 first-hand knowledge of East's work in this field as he became East's assist- 

 ant in July, 1909, and continued to work with him through 1914. In 1909, 

 East stated that studies of the effects of self- and cross-pollination in maize 

 were started with the view that this type of hiformation was essential to a 

 sound method of maize breeding. In addition to studies of maize, which is 

 normally cross-pollinated, East carried out studies in tobacco of crosses be- 

 tween varieties and species. This gave an opportunity of studying the effects 

 of self- and cross-pollination with a self-pollinated jilant. A 1912 j)ublication 

 of East and Hayes made the following statement: 



The decrease in vigor due to inbreeding naturally cross-fertilized species and the increase 

 in vigor due to crossing naturally self-fertilized species are manifestations of one phenome- 

 non. This phenomenon is heterozygosis. Crossing produces heterozygosis in all characters 

 by which the parent plants differ. Inbreeding tends to produce homozygosis automatically. 



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