Chromosome substitution effects associated with kappa-casein and beta-lactoglobulin in Holstein cattle.

Allelic variants of kappa-casein and beta-lactoglobulin were used to estimate chromosome substitution effects on transmitting abilities for yield traits among sons of two Holstein sires. Sire 1 was a heterozygote for both loci, whereas sire 2 was a heterozygote for beta-lactoglobulin only. Transmitting abilities for fat percentage and the exchange of protein and fat (protein yield minus fat yield) were influenced by kappa-casein genotype among offspring of sire 1. The B allele of kappa-casein was associated with a decrease in fat percentage, whereas fat yield was not affected; the result was 4.5 kg more transmitting ability for protein than for fat. Favorable chromosome substitution effects on transmitting abilities for fat percentage, protein yield, and the exchange of protein and fat were also observed for the A allele of beta-lactoglobulin among offspring of sire 1; there were consistent but not significant estimates for sire 2. These results suggest that different estimates could be obtained from offspring of different parents, perhaps from linkage disequilibrium between families. The chromosome substitution effects appeared to be in the direction opposite to the average correlation of milk fat and protein content; marker-assisted selection for increased yield of protein would result in decreased fat yield.