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Testing for the casein gene

The type of beta-casein protein in cows' milk is of increasing interest, and knowing which one their cows are producing is becoming valuable information for farmers - but how can they tell?

Beta-casein is one of the main proteins in cows' milk and it comes in several forms depending on the genetic makeup of the cow. The type of β-casein protein in milk is controlled by co-dominant variants of the β-casein gene. There are several alleles for this gene, but the most common are A1 and A2.

A cow that has an A1 and an A2 β-casein allele will produce milk containing both these proteins, whereas a cow with two copies of the A1 allele will produce milk containing only A1 β-casein and a cow with two copies of the A2 allele will produce milk containing only A2 β-casein.

So what's the difference, and why does it matter?

Get information sheet: A1 or A2?

The genetic difference

The A1 and A2 alleles are identical except for one nucleotide that causes the 67th amino acid in the β-casein protein to swap from hystidine to proline or vice-versa.

Testing for the β-casein gene

Livestock Improvement Corporation has developed a genetic test that farmers can use to find out the genotype of a cow and whether she will produce A1 or A2 β-casein in her milk.

For the genetic test, a sample of cells is taken from the cow - usually some hairs are pulled out, along with the skin follicle cells. The DNA from these cells is isolated and then millions of copies are produced by PCR.

The PCR machine is loaded as usual, with the addition of some specially designed primers. There are two types of primers, one complementary to the A1 allele and the other complementary to the A2 allele. Each type of primer also contains a fluorescent dye.

Some rare cases

The different alleles arose naturally through the process of genetic mutation. There are other alleles for the β-casein gene, including A3, B, and C (B and C are subvariants of the A1 allele), but all of these alleles are relatively rare. If they are present, the cow will produce correspondingly different β-casein proteins.

Why test the genes rather than the proteins?

Believe it or not, it is easier to determine the genotype of a cow than analyse the proteins in her milk (the phenotype). Gene testing requires only small samples of cells, which are much easier to store and transport than milk samples. Skin-cell samples also store better than milk samples, which are likely to "off", making protein structure very difficult to analyse.

Written by Sara Loughnane, NZ Science, Mathematics, and Technology Teacher Fellow, 2006.

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