The bovine β-casein (CSN2) gene has been shown to span a region of 8·5 kb, containing nine exons and eight intervening introns (Bonsing et al. 1988; Martin et al. 2002). The exons range in size from 24 to 498 bp; the introns, however, are much larger and account for 85% of the gene. Twelve genetic variants in the coding sequence of the β-casein gene have been reported (Farrell et al. 2004). The A2 allele of the β-casein gene has been associated with a higher milk production (Lin et al. 1986; Bech & Kristiansen, 1990) while the B variant has been associated with an increase in protein content and better cheesemaking properties (Marziali & Ng-Hang-Kwai, 1986). The β-casein gene codes for a protein of 209 amino acids with varying regions at codons 67, 106 and 122. The A1 variant differs from A2 at position 67, where a histidine replaces a proline (Lien et al. 1992). The β-casein A2 variant has histidine and the A3 variant has glycine at position 106 (Lien et al. 1992); the β-casein A2 variant has serine at position 122 and the β-casein B variant has arginine at this codon (Stewart et al. 1987; Damiani et al. 1992).
Single Nucleotide Polymorphism in the Promoter Region of the IGF-1 Gene is Associated with Milk Production in Holstein and Jersey Cattle – Is the Aspect of Present Research Still Relevant in the Era of Genomic Selection?
