Variance component analysis of skin and weight data for sheep subjected torapid inbreeding

A variance component analysis was carried out on data from a 20-year experi ment in the rapid inbreeding of purebred and crossbred lines of three hill breeds of sheep. Parent offspring matings were made over several generation s to produce inbreeding coefficients in lambs of up to 0.59. The traits cho sen for analysis were the live weights at 24 and 78 weeks of age and th rat io of the densities of secondary and primary skin follicles. A complete mod el of intralocus allelic effects was carried out with both additive genetic variance and dominance variance. The latter was partitioned into component s arising from loci which were homozygous by descent and those that were no t. Inbreeding depression was fitted as a covariate. This model has not been attempted previously in livestock populations. Crossbred animals were foun d to exhibit more dominance variance than purebred animals. Though partitio ning of the dominance variance was possible in some of the data sets consid ered, estimation of the novel quadratic components was difficult and provid ed little evidence of homozygous dominance variance as distinguished from t he familiar random dominance variance (that arising in randomly mated popul ations). A pooled dominance model is proposed in which inbred dominance eff ects have the same variance as random dominance effects. For live weight th e results suggested that the genetic architecture involved many loci with d eleterious recessive alleles, but for the ratio of follicle density there w as no clear explanation for the results observed. (C) Inra/Elsevier, Paris.