Heterozygosity at protein loci in inbred and outbred lines of chickens

Levels of heterozygosity at six polymorphic protein marker loci were determined by electrophoresis in 24 lines of poultry, encompassing 17 White Leghorn inbred lines (WLI) (with inbreeding coefficients, F, ranging from .946 to .988), five Australorp inbred lines (AusI) (with F values ranging from .924 to .961), and two randombred lines (one White Leghorn and one Australorp). Fixation was observed at one locus in WLI lines, and at two loci in the AusI lines. Segregation at the other loci was observed in the inbred lines of the two breeds. Observed heterozygosity in the inbred lines markedly exceeded the expectations under inbreeding theory. In White Leghorns, reproductive fitnesses for heterozygotes were superior to homozygotes in the inbred lines, but not in the control. Consequently, natural selection operating through associative overdominance appears to be responsible for the higher than expected heterozygosities in the inbred lines.