DETECTION OF QUANTITATIVE TRAIT LOCI FROM FREQUENCY CHANGES OF MARKERALLELES UNDER SELECTION

A method was developed to estimate effects of quantitative trait loci (QTL) by maximum likelihood using information from changes of gene fre quency at marker loci under selection, assuming an additive model of c omplete linkage between markers and QTL. The method was applied to dat a from 16 molecular and coat colour marker loci in mouse lines derived from the F-2 of two inbred strains which were divergently selected on 6-week weight for 21 generations. In 4 regions of the genome, marker allele frequencies were more extreme than could be explained by sampli ng, implying selection at nearby QTL. An effect of about 0.5 standard deviations was located on chromosome 11, and accounted for nearly 10% of the genetic variance in the base population. QTL with effects as sm all as 0.2 phenotypic standard deviations could be detected. For typin g of a given number of individuals, the power of detection of QTL is v ery high compared to, for example, analysis of an F-2 population. The joint effects of linkage and selection were investigated by Monte Carl o simulation. Marker gene frequencies change little as a consequence o f selection at a QTL unless the marker and QTL are less than about 20 cM apart.