Associations between milk protein genetic polymorphisms and milk production traits in Merino sheep breed

Genotypic effects of β-casein (CSN2), κ-casein (CSN3) and β-lactoglobulin (LGB) on milk yield, and fat, and protein percentages were estimated in Merino breed ewes by using an animal model. Casein genotype effects were estimated separately and then combined as a composite β–κ–CN genotype. The CSN2 gene was polymorphic (alleles A and G) in the exon VII in the Merino population analysed. The frequencies of the alleles A and G were 0.7644 and 0.2356, respectively. Although CSN2 gene polymorphisms were reported, there were not any previous studies relating these polymorphisms with milk production and composition traits. Sheep carrying the CSN2 *GG genotype presented the highest milk production yield. In relation to CSN3 gene, the most frequent genotype corresponded to the genotype, K3K3. The K1K4 and K3K3 genotypes were associated with the highest productions for milk yield and the genotypes K1K4, K2K2 and K3K5 were associated with the highest values for fat, protein and lactose percentages, respectively. The composite β–κ–CN genotypes, AGK3K5 and GGK3K3 were associated with the highest milk yield on the first milk recording and 120-day milk yield. Genotypes AGK2K3, AGK2K2 were associated with the highest fat and protein percentage, respectively. In addition, the LGB BB genotype had a favourable effect on milk yield.