The genetic polymorphism of the main bovine lactoproteins. Relationships with milk yield, composition, and cheese yielding capacity

Abstract

Four of the six main proteins of cow’s milk (αS1 - β -, and κ-caseins, and β-lactoglobubin) show, at the population level, at least two genetic variants identifiable by electrophoresis. A survey of this polymorphism has been carried out in 21 French bree ds of cattle in which the more frequent variants are αS1 -Cn B and C for αS1 -casein, β-Cn A2, A1 and B for β-casein, κ-Cn A and B for κ-casein, and β-Lg A and B for β-lactoglobulin. The structural genes of αS1 -, αS2 -, β-and κ-caseins are very closely linked, which means that the genetic unit of transmission is the haplotype, comprising one allele of each of the four loci. The β-lactoglobulin locus has a major effect on the concentration of this protein in milk (β-Lg A > β-Lg B) which has repercussions on the total concentration of proteins in lactoserum. Because those differences are balanced by complementary differences in the total concentration of caseins, allele β-Lg B has a favourable effect on casein concentration and on casein number (+ 2,5 to 3 %). As compared to variant κ-Cn A, variant κ-Cn B gives the milk better cheese making properties : shorter rennet clotting time and rate of firmness, firmer curd, and at least for certain types of cheese, higher yielding capacity (4 to 8 % difference between the milks of the two homozygotes). Variant β-Cn B has effects in the same direction as variant κ-Cn B. The usefulness and practicabilities of taking this polymorphism into account in milk selection remain to be assessed.