Polymorphisms of α-lactoalbumin, β-lactoglobulin and prolactin genes are highly associated with milk composition traits in Spanish Merino sheep

Traditionally, Merino Sheep has been used for wool and meat production. However, in Spain, it is used for manufacturing valuable artisanal cheeses. Therefore, milk production is becoming a very important trait to be considered. In this research, the effects of the different genetic variants of milk whey proteins alpha-lactoalbumin (LALBA) and beta-lactoglobulin (LGB), and prolactin hormone (PRL) on milk quality traits have been estimated in 1114 lactations from 396 genotyped Merino ewes of an experimental flock. Nucleotide substitutions g.133C > T (A and B alleles) of LALBA gene and g.1617T > C (A and B alleles) of LGB gene and a deletion, g.460_483del, of PRL gene (A and B alleles) were analysed. Hardy-Weinberg Equilibrium (HWE) and fixation index (F-IS) analyses detected that the genotyped Merino population have not been selected for the analyzed loci. Haplotype analyses of milk whey protein loci (LALBA and LGB) located at Ovis aries chromosome 3 (OAR3), indicate that alleles are independently inherited. So, we only studied the genetic effects of single genes on milk traits. Allele substitution and genotypes effects on milk yield and composition traits were estimated using a linear mixed model. Significant effects of LALBA *A (p. Ala(27)), LGB *A (p. Tyr(38)) and PRL *B (de124 bp) alleles on milk quality traits standardized to 120 days have been observed. LALBA *A allele is significantly associated with higher percentages of fat (FP; alpha = + 0.26%, P = 0.001), protein (PP; alpha = + 0.22%, P = 0.001), solids non fat (SNF; alpha = + 0.17%, P = 0.001) and total solids (TS; alpha = 0.42%, P = 0.0002). LGB *A allele shows significant positive effect on PP (alpha = + 0.136%, P = 0.032) and FP (alpha = + 0.140%, P = 0.070). PRL *B allele shows only a significant positive effect on PP (alpha = + 0.131%, P = 0.026). These positive effects could be of great interest for the cheese industry. Variants could be used as suitable markers to perform association analyses in commercial farms in order to implement a cheese yield selection program based on genotype information.