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Genetic Polymorphism in Caseins of Cow's Milk. I. Genetic Control of _s-Casein Variation

Dairy Cattle Research Branch, Animal Husbandry Research Division, ARS Agricultural Research Center, Beltsville, Maryland

M. P. Thompson

Milk Properties Laboratory, Eastern Utilization Research and Development Division, ARS Wyndmoor, Pennsylvania

ABSTRACT

Caseins from milk of individual cows were studied by starch-gel-urea electrophoresis. The results indicate that _s-casein, the calcium sensitive component of -casein, occurs in at least three major forms. Family studies indicate that this variation is controlled by three allelic autosomal genes with no dominance. Each allele is responsible for the production of one of the three forms of _s-casein which are designated A, B, and C in order of decreasing mobility. Milk from individual cows may contain any one or any two of these. The six possible types and their corresponding genotypes are: A (A/A), B (B/B), C (C/C), AB (A/B), AC (A/C), and BC (B/C).

Milk samples from 1,378 cows showed the following distribution of _s-casein types: Ayrshire 98 B; Brown Swiss 192 B, 11 B/C; Guernsey 188 B, 180 BC, 32 C; Holstein 2 A, 81 AB, 5 AC, 410 B, 44 BC; and Jersey 44 B, 21 BC, 2 C. Of 68 crossbreds tested, 67 were B and one was BC. These data do not necessarily reflect the frequencies that would be found in random samples of the breeds. Sire groups and herds were often selected in an attempt to find certain variants.

The reason for the preponderance of the B allele is not apparent. The possibility of some selective advantage is being investigated.