Co-occurrence of kappa-casein B and beta-casein A1 alleles in Holstein bulls

Abstract

Kappa-casein (CSN3) variant B is a widely known marker of milk processing traits. Beta-casein (CSN2) is a source of the opioid-like peptide β-casomorphins-7 (BCM-7), which is yielded by the successive in vitro gastrointestinal proteolytic digestion of the CSN2 A1 variant but not A2. Epidemiological and animal model studies suggest that BCM-7 may be a potential risk factor in the etiology of some human diseases. Because breeding companies and dairy plants have been encouraged to use CSN3 BB bulls for insemination of cows for over 30 yr, it was hypothesized that bulls of CSN3 BB genotype are spreading the undesirable A1 allele of CSN2 in the population of Holstein bulls. Among 519 Holstein bulls, 109 bulls were selected as having the CSN3 BB genotype. These bulls were then genotyped for the CSN2 polymorphism. All bulls had an official breeding index PF (production and functionality). The frequency of undesirable allele A1 of CSN2 locus was relatively high at 31.7%. Among 109 bulls with the CSN3 BB genotype, although 50 bulls had the desirable CSN2 A2A2 genotype, 10 carried the undesirable CSN2 A1A1 genotype. These 10 bulls (CSN3 BB and CSN2 A1A1) had the lowest average PF breeding index. Bulls with CSN3 BB genotype and the CSN2 A2 variant should be preferred in breeding schemes because they simultaneously improve general breeding value and cheese-making properties and promote milk without negative implications on human health.