EFFECTS OF HETEROZYGOSITY OF MAJOR HISTOCOMPATIBILITY COMPLEX HAPLOTYPES ON PERFORMANCE OF LEGHORN HENS SHARING A COMMON INBRED BACKGROUND

Four sets of diallel crosses between partially congenic lines of Leghorns differing in their MHC haplotypes (B blood group) were made in 4 successive yr. Hens heterozygous for B haplotypes resulting from the crosses were then compared with those representing the homozygous congenic parent lines. Heterozygotes superior in performance to the average of their parental homozygotes were found to occur predominantly for survivor egg production, egg weight, and viability to 40 wk of age. Body weight of adult hens showed least heterosis. Surprisingly few crosses produced heterozygous embryos with superior hatchability, although heterotic combinations did exist. Additive differences between parent homozygotes were of greater magnitude than dominance effects measured as average excess of heterozygotes over homozygote averages of parents. A control genotype heterozygous for the B2/B(Q) combination was included in all tests and proved heterotic in most cases. This same genotype also had been found to confer resistance to Marek's disease under experimental inoculation.