INFERENCES ABOUT VARIANCE-COMPONENTS AND SELECTION RESPONSE FOR BODY-WEIGHT IN CHICKENS

Response to selection for body weight at 40 days was analyzed using le ast squares, a 'REML/BLUP' approach, and finally using Bayesian method s. The last two methods were implemented using an animal model that in cluded a term accounting for a covariance among full-sibs (f effect), other than the additive genetic. The data, which originate from the St ryno breeding station in Denmark, comprised 6 900 recorded individuals from 200 sires and 720 darns and covered eight generations of selecti on. The base population was formed from a population with a long histo ry of selection for body weight. The least squares procedure yielded a total phenotypic change of 390.4 g. The estimate of total genetic cha nge based on REML/BLUP was 356.4 g and the Bayesian approach produced an estimate (mean of the marginal posterior distribution) ranging from 358.3 to 368.0 g, depending on the prior distribution assumed for the variance components. This corresponds to a response per generation of about 45 g, or 2.65% of the mean of the base population. The Bayesian approach was implemented using the Gibbs sampler. The REML estimates of heritability and of the proportion of the variance due to the f eff ect were 0.25 and 0.029, respectively. The corresponding values obtain ed from the Bayesian analysis were approximately 0.26 and 0.030, regar dless of the prior used. A likelihood ratio test indicated that the va riance component due to the f effect should be included in the model. We speculate about the possible mechanisms that can lead to the f effe ct.