Casein is the main protein in milk. There are three types of casein, one of which is beta casein. Beta casein comes in two main variants called A1 and A2. The order of amino acids in each variant is identical except for the 67th amino acid in the chain (see diagram below). In A1 beta casein, the 67th amino acid is histidine (His), while in A2 beta casein it is proline (Pro). Scientists believe that A2 was the original form of beta casein

Cows' milk that contains A1 beta casein is called A1 milk, while milk that contains A2 beta casein is called A2 milk. In New Zealand, 30% - 40% of dairy cows produce pure A2 milk. The rest produce either pure A1 milk or milk that is a mixture of both A1 and A2. From these figures, it can be calculated that the current frequency of the A1 allele is around 37%.

There are several breeds of dairy cow. Each breed has a combination of desirable characteristics that has been obtained through selective breeding. For example, the proportion of fat or protein in the milk, the amount of milk produced by the females, the breeding success of the females. Different breeds produce different types of milk in relation to beta casein. Some breeds produce mainly A1 milk, while other breeds produce mainly A2 milk. Recent research has suggested that there may be health benefits to humans in drinking A2 milk, as opposed to drinking A1 milk.

Discuss the genetics involved in the inheritance of the alleles responsible for the A1 and A2 casein  proteins in the milk of dairy cows. In your discussion, consider the following:

• how the allele for A1 milk would have entered the gene pool
• the pattern of inheritance shown by A1 and A2 milk
• possible explanations for the current frequency of the A1 allele in the gene pool.