INVESTIGATION OF THE INHERITANCE OF BIRTH-DEFECTS IN SWINE BY COMPLEXSEGREGATION ANALYSIS

The inheritance of birth defects was investigated by analysing a large data set with more than 30 000 litters. The data were recorded as 0/1 traits in connection with the evaluation of an anomaly index for AI b oars in the German state of Bavaria. The overall incidence of defects was 2.07%, and the frequencies of the individual traits varied from 0. 04% (hermaphrodism) to 0.69% (splayleg).Monogenic, digenic, polygenic, and mixed models of inheritance were compared using Maximum Likelihoo d methodology. Integrations necessary to calculate likelihoods were pe rformed numerically (Gauss-Hermite), and a derivative-free algorithm ( Powell method) was used for maximization. The most likely mode of inhe ritance was determined based on likelihood ratio tests. The inclusion of common litter environments improved the model fits remarkably. Omit ting c(2) effects were shown to lead to false detection of major genes . In most cases, the polygenic model fitted the data best. Heritabilit ies estimated on the underlying scale ranged from 0.10 to 0.25, and c( 2) effects accounted for 20-40% of the liability. Major gene effects o f about one standard deviation of liability were found for scrotal her nia in the offspring of DL boars and for splayleg in the litters of DL and PI boars. Allele frequencies of the defect-causing gene were 0.02 for scrotal hernia and 0.25 for splayleg, respectively. The results w ere discussed in the light of previous investigations, and further ana lyses were suggested.