GORDON E. DICKERSON 



UnivarsHy of Missouri 



Chapter 21 



Inbred Lines for Heterosis Tests? 



The justification for considering heterosis tests in breeding work rests on the 

 mode of action and interaction of the genes responsible for genetic variability 

 in the material available to the geneticist. The nature of this genetic variabil- 

 ity may vary widely between species or populations in response to differences 

 in the degree of inbreeding and kind of selection, natural or imposed, that 

 has characterized the population over an extended period. For any given 

 trait or combination thereof, structure of genetic variation will depend upon 

 how consistent, intense, and prolonged selection has been. 



It follows that choice of the system of mating and selection appropriate 

 for most rapid improvement in economic attributes of any given plant or 

 animal population should be guided by as complete knowledge of the kind 

 of genetic variation in the population as analysis of all available data affords. 

 The discussion which follows is an attempt (1) to interpret the evidence 

 presently available concerning the sort of genetic system which underlies 

 important economic traits, using swine as the example; and (2) to compare 

 expected effectiveness of several alternative breeding methods. 



NATURE OF GENETIC VARIATION IN ECONOMIC TRAITS 



Types of association between the genotype and its phenotypic expression 

 have been classified logically as intra-allelic and inter-allelic. The former 

 includes all degrees of dominance or levels of expression for the heterozygote 

 relative to the corresponding homozygotes. The concept of heterozygote ad- 

 vantage or overdominance differs from the usual ideas of dominance in that 

 each gene is visualized as exerting certain dominant favorable effects lacking 

 in its allele. Inter-allelic gene action or epistasis includes all effects of a gene 

 in one set of alleles on the expression of genes in other sets of alleles. Comple- 

 mentary, inhibiting, duplicate dominant, and duplicate recessive gene inter- 

 actions are extreme examples. 



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