MARKER ASSISTED SELECTION FOR GENETIC-IMPROVEMENT OF ANIMAL POPULATIONS WHEN A SINGLE QTL IS MARKED

A Monte Carlo simulation study to evaluate the benefits of marker assi sted selection (MAS) in small populations with one marked bi-allelic q uantitative trait locus (QTL) is described. In the base generation, li nkage phase equilibrium between the markers, QTL and polygenes was ass umed and frequencies of 0.5 for the two QTL alleles were used. Six dis crete generations of selection for a single character measured on both sexes followed. An additive genetic model was used with the QTL posit ioned midway between two highly polymorphic markers. Schemes were simu lated with a distance of 10 cM between the QTL and either of the two m arkers and with the QTL explaining 1/8 of the total genetic variance i n the base generation. Values of 0.5, 0.25 or 0.1 were assumed for the heritability. Eight males and 16, 32 or 64 females were selected each generation with each dam producing four sons and four daughters as ca ndidates for the next generation. Animals were evaluated with a conven tional BLUP animal model or with a model using marker information. MAS resulted in substantially higher QTL responses (4-54%), especially wi th low heritabilities, than conventional BLUP but lower polygenic resp onses (up to 4%) so that the overall effect on the total genetic respo nse, although in the majority of cases favourable, was relatively smal l. With QTLs of larger size (explaining 25% of the genetic variance) c omparable results were found. When the distance between the QTL and th e markers was reduced to 2 cM, genetic responses were increased very s lightly with a heritability of 0.5 whereas with a heritability of 0.1 responses were increased by up to 10%, compared with conventional BLUP . Results emphasize that MAS should be most useful for lowly heritable traits and that once QTLs for such traits have been identified the se arch for closely linked polymorphic markers should be intensified.