MARKER-ASSISTED SELECTION IN AN OUTBRED POULTRY BREEDING NUCLEUS

The value of using a marker for a quantitative trait locus (QTL) affec ting a sex-limited trait in an outbred poultry breeding nucleus was st udied. Marker and QTL were in linkage equilibrium in the base populati on. The recombination rate between marker and QTL was 0 . 05. A closed nucleus with 9000 chickens per generation was deterministically simul ated. The genetic model contained polygenes and a QTL linked to a mark er. Genetic effects explained proportionately 0 . 3 of the phenotypic variance before selection. Under selection, polygenic variance reached an equilibrium and QTL variance decreased continuously over time. Coc ks were selected in two steps. First the best cocks of each full-sib f amily were selected (within-family selection) while final selection to ok place after information on full-sibs was available. Hens were selec ted after they had completed production. The effect of using marker in formation in estimating breeding values was studied in an ongoing bree ding programme. Transmission of marker alleles was always traceable. C umulative response over five generations increased proportionately by 0 . 06 to 0 . 13 if a marker linked to a QTL that explained 0 . 2 of t he genetic variance was used. Cumulative response increased up to 0 . 28 if the QTL explained 0 . 8 of the genetic variance. Additional resp onse due to the use of a marker increased with increasing intensity of within-family selection of cocks, increased with increasing variance explained by the QTL and was higher if within-family selection of cock s was carried out after rather than before their sibs had complete rec ords.