Here is a summary of what I think we know so far. For anyone wishing to produce milk whose value is enhanced by possessing the best gene combinations, consider this information.
Beta Casein, one of three major types of milk proteins, is considered a “whey” protein, as it is not a curd-forming variety that would influence cheese yields. At this point it does not seem to matter whether you make hard cheeses from A2 milk or not, but of course it would show up in cheese forms (like cottage cheese) where whey is still present in the package we eat.
Beta Casein variants are most important to people with genetic backgrounds that have any of the auto-immune disorders, the most important being those producing autism or childhood-onset diabetes. The medical profession has concluded that the mutated Beta Caseins (forms A1, there are also B and a dozen other rare variants) aggravate the health and behavior of anyone afflicted with such ailments.
Beyond that, there does also seem to be evidence that much of the “lactose intolerance” diagnosed may actually be an allergic-like reaction to the A1 Beta Casein, as some studies that were done in places like Germany and Oregon have found previously diagnosed people able to drink A2 milk (or eat hard cheeses) without the expected discomfort.
In sire selection, you basically see three types of sires:
A2A2 the desired gene pairing for Beta Casein
A1A2 half good, half bad—milk will contain both variants
A1A1 the undesired gene pairing for Beta Casein
These are like most simple “polynomal” gene pairings, neither dominant or recessive. If you breed an A2A2 cow to an A2A2 bull, you are 99.9% certain of an A2A2 calf result. If you breed four A1A2 cows to A2A2 bulls, at random you would expect this distribution:
(two) A1A2 results and (two) A2A2 results = on average, 50% A2A2
A1A1 cows require at least two generations to produce any descendants that are A2A2, as in the first generation with A1A1 mated to A2A2, you get all A1A2 offspring.
As it is not possible to separate A1 from A2 in the milk such cows produce, the best you can do is utilize their milk for producing hard cheeses (the undesired A1 will pass out in the whey). I have actually seen farms bottling milk and making cheese, using two bulk tanks and segregating the milk according to whether cows were A2A2 or some lesser status.
There are actually two gene pairings that impact on yields of cheese produced form milk and the efficiency of cheese production: Kappa Casein and Beta Lactoglobulin. To the extent I understand the differences, Kappa Casein has the major effect on curd yields, while Beta Lac affects the efficiency of the processes, as to firmness and time of setting curds.
The desired Kappa Casein gene is the B variant. The desired Beta Lactoglobulin gene is also the B variant. The general assumptions are as follows:
the highest potential cheese yields, estimated up to 15% greater than
basic AA milk.
AB still pretty useful, estimated 7% greater potential cheese yields.
AA basic milk form, produces the expected average cheese yield (predicted from protein %).
If you ask the question, “are there genes that reduce cheese yields below normal?” the answer is YES, there is a mutated form of Kappa Casein, which is designated as E variant. ( The A gene is not a “negative” gene; cheese will form from A milk, it has just a “neutral” impact on yields. )
AE you will see a reduction in cheese yield,
and a resistance to produce firm curds.
EE such milk would not set curds at all.
Historically, before the more extreme genetic interventions caused by the focus on bottling milk volume from 1970 to 2000 (prior to “component pricing”) – we knew the following:
80% of little brown cows had at least one B
Brown Swiss 50% of big brown cows had at least one B gene
Holsteins only 20% of Black and White or Red & White cows had at least one B gene
The impact of Protein % on cheese yields
Higher protein breeds tended to have higher cheese yields, but the Jersey and the Brown Swiss were recognized as the leaders for cheese production, before the Kappa Casein gene was found as a contributor to cheese yields. Sire selection in favor of protein % remains important to the relative quantity of cheese-curd-forming caseins.
Brown Swiss milk could generally go straight from the cow barn to the cheese vat and produce a good product. Jersey milk used to be too high in butterfat % for the familiar varieties, thus some cream might have to be removed prior to cheese making. Thus the level of butterfat % also has an impact on cheese production, all based upon which cheese varieties you make.
In general, high milk volume, low bf% sires (perennially preferred in the non-component price southern milk orders and California) are least likely to have the B gene for Kappa Casein.