Genetic and technological studies have revealed correlations between different individual, or combinations of, alleles of milk proteins and parameters such as cheese yield, the compositional characteristics of milk and total lactation yield. The kappa casein B allele is better for cheese-making, for example (Table 1): the mutation may alter the mechanism by which this protein stabilises the micelles of the caldum-sensitive caseins against precipitation. When rennin cleaves kappa casein between amino acids 105 and 106 the micelle is destabilised and clotting of milk into curds ensues. Therefore, selection for defined alleles or haplotypes could be desirable.

Until now selection for milk protein alleles was only possible by the analysis of the proteins in milk. For sire selection this required analysis of milk from daughters which is time consuming and expensive. However, phenotypic variation in the proteins arises from base substitutions in the DNA, as is illustrated in Figure 1 for the kappa casein gene.