Abstract
Relative fitness of the genotypes at the B locus determining blood groups in chickens was studied in two closed lines and their cross-breds. These lines, which were derived from one population and have been closed more than ten generations, were maintained by reciprocal recurrent selection at the Takikawa Livestock Breeding Station. Both lines possessed five alleles at the B locus, respectively. However, in one line one of them was extinct in 1959.
Comparisons among the expected total, observed total and selected frequencies of the genes and the genotypes were performed for each generation. There was no obvious tendency in gene frequencies. On the other hand, the comparisons of geno-typic frequencies revealed apparent selective advantage of heterozygotes. The observed frequency of heterozygous genotypes in each total population was higher than the expected frequency. The proportion of heterozygotes increased again after selection. Thus, both natural and artificial selections favored the heterozygotes at the B locus. These results suggest that the B blood group in chickens is an example of balanced polymorphism.
Direct comparisons of the characters under selection between the genotypes at the B locus showed, however, that the superiority of heterozygotes was not explicable by any single character studied here.
