Principles of QTL Mapping

To date, most genetic progress for quantitative traits in livestock has been made by selection on phenotype or on estimates of breeding values (EBV) derived from phenotype, without knowledge of the number of genes that affect the trait or the effects of each gene. In this quantitative genetic approach to genetic improvement, the genetic architecture of traits of interest has essentially been treated as a ‘black box’ (Figure 1). Despite this, the substantial rates of genetic improvement that have been and continue to be achieved in the main livestock species is clear evidence of the power of quantitative genetic approaches to selection.