Genetic Insights Into the Β-Casein (Csn2) Locus in Holstein-Friesians: A Step Towards A2 Milk Production

Milk plays a major role as protein source in human diet. Among the casein fractions, β-casein, encoded by the CSN2 gene, is characterized by the presence of several polymorphic variants. The most predominant types in dairy cattle are A1 and A2. Phylogenetic analyses confirm that the A2 variant is the original, while A1 is a result of a mutation that can lead to the release of a biologically active peptide (BCM-7) during digestion. The present study aims to assess the allele and genotype frequencies of the CSN2 locus in 30 Holstein-Friesian cows bred at the Academic Technological Complex of Trakia University –Stara Zagora, and to analyze the genetic equilibrium of the population according to the Hardy–Weinberg principle. The analysis was performed with the BovineSNP50 v3 Bead Chip and established the presence of 3 genotypes -A1A1, A1A2 and A2A2. The results exhibit an allele frequency of the A1 allele = 0.4 and of the A2 allele = 0.6, with a χ² test confirming that the population is in Hardy–Weinberg equilibrium (χ² = 0.37, df = 1, p > 0.05). The witnessed high proportion of heterozygous individuals (A1A2) provides an opportunity for selection towards the A2A2 genotype. These results are fundamental for the development of breeding programs aiming the production of A2 β-casein milk, which has a growing consumer demand and potential health benefits.