Advances in molecular genetics have generated genetic maps showing highly polymorphic markers evenly spaced over the 19 chromosomes of the species. Consequently, the role that individual gene or marker identifications can play in breeding schemes is enhanced, compared to classical quantitative genetics methods relying only on measurements of performances.1-6-1
For single-locus traits, the objective is to change gene frequency at the locus of interest by selecting the gene itself (when possible) together with nearby marker loci, a process termed marker-assisted selection (MAS). The process depends on the marker loci hitchhiking the genes of interest. This procedure has been illustrated in the elimination of the deleterious halothane gene from maternal lines, achieved in the 1980s by using biochemical markers. The pig linkage map offers several similar possibilities. Major genes for meat quality and resistance to disease are areas of particular interest.
Polygenic traits are under the control of quantitative trait loci (QTL) and the environment. Several QTL have been mapped1-7-1 and may be exploited in selection. When all sources of gain are cumulated, considerable increases in responses may be expected from MAS. This implies that the relevant QTL can be hitchhiked by the markers, as in the single-locus case examined earlier. In situations of statistical independence among loci, marker QTL associations can be detected only within families, and lower gains in selection accuracy are obtained. The gains then result from more exact coancestry for segments of the genome including QTL.
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