Genetic polymorphism of milk proteins: Relationships with production traits, milk composition and technological properties

Mutations within the milk protein genes lead to the existence of genetic variants which can be detected by electrophoresis. In the dairy cattle population, genetic variants are most commonly found for alpha(s1)-casein, beta-casein, kappa-casein and beta-lactoglobulin and frequency distribution of the different genetic variants varies according to breeds. Several findings emerging from the study of genetic polymorphism of milk proteins have been proposed to have practical applications for the purpose of improving the overall efficiencies for different sectors of the dairy industry. Because the different polymorphic forms of the milk proteins are controlled by autosomal genes which are inherited in accordance with the Mendelian mode of inheritance, selection of cows for a specific genetic type is feasible and it is possible to breed for a specific desirable variant. Hence, there is great interest in using the milk protein genes as marker genes for increasing milk yield, altering milk composition and improving the physicochemical properties of milk for the manufacture of various dairy products. The effect of certain genetic variants on production traits are, in the strictest sense, statistical effects only and the observed differences between the genetic variants are mere associations and in most cases little is known about the cause-and-effect relationships. The numerous studies regarding relationships between milk yield and genetic variants of alpha(s1)-casein, beta-casein, kappa-casein and beta-lactoglobulin are inconsistent for various reasons including: relatively small population sizes in many cases, breeds of cattle, very low Frequencies of some variants, methods of expressing yields, and most importantly, the rigour of statistical analysis to adjust for major factors which contribute more to milk production. Many studies have demonstrated that milk with the A variant of P-lactoglobulin contains higher concentrations of total proteins and beta-lactoglobulin, but lower amounts of casein and fat. The C variants of alpha(s1)-casein and the B variant of beta-casein are associated with high fat, protein and casein in the milk. At the K-casein locus, the B variant milk contains more fat, protein, casein, and kappa-casein than the A variant milk. The stability of the casein micelles and heat denaturation of beta-lactoglobulin are influenced by genetic variants of those proteins. Milk with the B variant of kappa-casein and beta-lactoglobulin is associated with better coagulating properties in having a shorter coagulation time, faster rate of firming and a firmer curd. In cheesemaking, it is also preferable to have milk with the B variant of kappa-casein and beta-lactoglobulin which gives higher yield of cheese having a better quality than the A variant milk.