There are two entirely natural variants or forms of beta casein protein in the milk of dairy cows, called A2 and A1 beta casein
Proteins are chains made up of many amino acids.
The difference between A1 and A2 beta caseins is in a single amino acid, at position 67 in a strand of 209.
- Beta casein A1 has the amino acid Histidine.
- Beta casein A2 has the amino acid Proline.
The A1 variant beta casein in cow's milk is unique amongst all mammalian beta caseins, in having a histidine amino acid at this position. Other species milk contains beta casein that can be considered A2 like, as they have a proline amino acid at this equivalent position in their beta casein chains.
Water buffalo, yak, goat as well as human breast milk all contain the A2-like form of beta casein.
Beta casein variants in cow's milk
The trait for whether a cow produces A1 or A2 beta casein is carried in the genes it inherits from it's mother and father. As one gene is inherited from each parent for the trait of beta casein production each cow carries two genes determining what form of beta casein it produces.
Cows can have only the A1 or only the A2 production trait but most dairy cows carry two different traits for beta casein production, so produce both A1 and A2 beta casein in their milk.
The proportion of A1 to A2 beta casein in cow's milk varies between the breeds of cows.
For example in Holsteins, the most common of the US, Northern European and Australian dairy cows on average have the following beta casein traits:
- A quarter of cows carry traits for the production of A1 beta-casein, without the trait for A2
- Half of cows carry a combination of traits for the production of both A1 and A2 beta-casein
- A quarter of cows carry traits for the production of A2 beta-casein, without the trait for A1
This results in Holstein cows' milk containing on average a balance of 50% A1 beta casein and 50% A2 beta casein.
Other breeds such as Jersey and Guernsey cows can have a higher proportion of A2 beta casein in their milk. This is due to more Jersey and Guernsey cows carrying the A2 compared to the A1 beta casein production trait.
The one amino acid difference between the A1 and A2 beta casein types is due to a naturally occuring "single nucleotide polymorphism" or SNP in the DNA of the beta casein genes of the cow. This "SNP" can be easily detected by a non-invasive DNA test which accurately indicates which traits for beta casein production the cow carries.
Origins of Beta Casein Variants
A2 is recognised as being the original or ancestor beta casein gene from which the others evolved.
Research indicates that A2 may be regarded as the original or "progenitor" (direct ancestor) form of beta casein gene in the genus Bos, the genus of bovine animals that cattle belong to. A2 beta casein is found in all types of bovine animals (all western, African and Indian cattle including the water buffalo). A1 beta casein is found primarily in western dairy cows and is thought to have arisen by natural evolution.
The A1 and A2 beta-caseins have since given rise to a number of rare minor subvariants. The same amino acid difference at position 67 occurs between minor variants, which on the basis of the amino acid present at position 67 may be classified as 'A1 like' or 'A2 like'. Minor variants include A3, D and E, which like A2, contain a Proline at position 67; and B, C and F which, like A1, contain a Histidine and produce the same major digestion products as A1.
The A1 and A2 beta caseins are the usual forms of beta casein found in dairy cow's milk.
Evolution of the Beta Caseins in Cows' Milk from the Original A2 Type
More recent forms of beta casein