ASSOCIATIONS BETWEEN MAJOR HISTOCOMPATIBILITY COMPLEX GENES AND PRODUCTION TRAITS IN WHITE LEGHORNS

The frequency of the MHC haplotype B-15 had been found in a previous s tudy to be more than two times higher in a White Leghorn line selected for high egg production compared with the unselected control strain. To further evaluate these findings, matings were performed between chi ckens with the same heterozygous B genotypes, being combinations of th e most frequent haplotypes, i.e., B-15, B19, and B21. In total, more t han 1,300 observations from two generations were analyzed. In each gen eration, approximately one half of the chickens were derived from the line selected for total egg mass, the other half from the control stra in. The MHC genotypes were determined serologically. Additive and domi nance effects of B haplotypes on production traits were analyzed using an individual animal model. The estimation of genotypic values, toget her with the analysis of gene substitution effects, showed that the B- 15 haplotype was associated with early sexual maturity and low egg pro duction during the late production period, i.e., between 43 and 63 wk of age, whereas B19 was associated with later onset of sexual maturity . The association of B-15 with early sexual maturity would thus explai n the high frequency of the B-15 haplotype previously observed in a li ne selected for high early egg production. No dominance effect of the B system was observed for any of the traits, suggesting that the prese nt results were due predominantly to additive gene effects.