JAMES F. CROW 



Universify of Wisconsin 



Chapter 1 8 



Dominance 

 and Overdominance 



Since the first attempts to explain hybrid vigor and the deleterious effects 

 of inbreeding in Mendelian terms, there have been two principal hypotheses. 

 Both were advanced early, and though each has had its ups and downs in 

 popularity, both have persisted to the present time. The first hypothesis is 

 based on the observed correlation between dominance and beneficial effect 

 (or recessiveness and detrimental effect). Inbreeding uncovers deleterious 

 recessives, and typically results in deterioration. 



With hybridization, some of the detrimental recessives brought into the 

 hybrid zygote by one parent are rendered ineffective by their dominant 

 alleles from the other, and an increase in vigor is the result. If the number 

 of factors is large, or if there is linkage, the probability becomes exceedingly 

 small of a single inbred line becoming homozygous for only the dominant 

 beneficial factors. Consequently, there should be a consistent decrease in 

 vigor with inbreeding, and recovery with hybridization. This idea has been 

 called the dominance or the dominance of linked genes hypothesis. 



The alternative theory assumes that there is something about hybridity 

 per se that contributes to vigor. In Mendelian terms this means that there 

 are loci at which the heterozygote is superior to either homozygote, and that 

 there is increased vigor in proportion to the amount of heterozygosis. This 

 idea has been called stimulation of heterozygosis, super-dominance, over- 

 dominance, single gene heterosis, cumulative action of divergent alleles, and 

 simply heterosis. 



In accordance with the title of this discussion I shall use the words domi- 

 nance and overdominance for the two hypotheses. This leaves the word 

 heterosis free for more general use as a synonym for hybrid vigor (Shull, 1948). 



* Paper No. 434 from the Department of Genetics, University of Wisconsin. 



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