Animal breeding theory leans heavily on the concepts and 
methodology of quantitative genetics, which themselves were developed on the 
assumption of many genes with small effects. At least until recently, there 
was no cause to question the validity of this assumption with respect to 
production traits in domestic livestock. The possible identification 
of major genes influencing production was nevertheless kept under continual 
review, and received some impetus from studies of polymorphisms in general 
and of blood antigens in particular. However, there has been little room to 
challenge the generality of a conclusion by Neimann-Sorenson and Robertson 
(1961), in their case with specific reference to blood groups and dairy 
production that such approaches had a low predictive value i.e. the influence 
of specific loci on production could not readily be detected. However, 
this does not mean that genes with major effects do not exist, and recent 
work on the halothane and K-88 loci in pigs and on the Booroola gene in 
sheep suggests the need to re-examine the possibilities.

R. C Roberts, C. Smith

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 6. Round tables, , 420–438, 1982
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