Casein SNP in Norwegian goats: additive and dominance effects on milk composition and quality

In this study, 575 goats were genotyped for 38 Single Nucleotide Polymorphisms (SNP) located within the four casein genes. Milk production records of these goats were obtained from the Norwegian Dairy Goat Control. Test-day mixed models with additive and dominance fixed effects of single SNP were fitted in a model including polygenic effects.Significant additive effects of single SNP within CSN1S1 and CSN3 were found for fat % and protein %, milk yield and milk taste. The allele with the deletion showed additive and dominance effects on protein % and fat %, and overdominance effects on milk quantity (kg) and lactose %. At its current frequency, the observed dominance (overdominance) effects of the deletion allele reduced its substitution effect (and additive genetic variance available for selection) in the population substantially.The selection pressure of conventional breeding on the allele with the deletion is limited due to the observed dominance (overdominance) effects. Inclusion of molecular information in the national breeding scheme will reduce the frequency of this deletion in the population.Under normal conditions, the milk of mammals contains 30-35 g of protein per liter [1]. In the milk of ruminants, more than 95% of these proteins are synthesized from six structural genes [2]. The two main whey proteins, α-lactalbumin and β-lactoglobulin, are encoded by the LALBA and LGB genes, respectively [3]. The four acid-precipitated proteins (caseins) - αS1-CN, β-CN, αS2-CN and κ-CN - are encoded by four tightly linked casein genes [2]. These four casein loci are found in the following order: CSN1S1, CSN2, CSN1S2 and CSN3 within 250 bp on caprine chromosome 6 [2,4-7]. In goats and other ruminants, casein represents about 80% of the total proteins [2].Casein genetic variants have been identified and characterized in different species (for a review see Ng-Kwai-Hang and Grosclaude [3]). Caroli et al. [8] have reported a comparison among casein genetic variants in catt