The protein in milk has a beta-casein component.
Some time ago that component mutated. A1 and A2 are it's two varieties. A2 component is what is found in all mammals including humans. Also all indigenous breeds of cattle in India produce the same A2 protein. However most of the Holstein breed, among other breeds of cows form milk with the A1 variety. This breed of cows is generally found in dairy farms as it is known to have the highest milk production per lactation cycle.
By correlation it was found that the A1 type of milk is the one that is not good for human body. Rise in cases of Autism, lactose intolerance and in general lower immunity have been attributed to A1 milk. In fact there is a company by the name of A2 Corp that first bought this to the notice of the world. Govts never took them seriously but all the dairy farmers of New Zealand saw the truth in their work and converted all their cattle to the A2 breed.
Even US and Europe have started weeding out the A1 producing breed, but supposedly they are being dumped in developing countries like India.
However, according to a Feb 2012 paper by NBAGR that I read, due to us cross breeding these cows instead of using imported cows as such, the breeding centres have been able to at least maintain A1A2 breeds instead of the actually desired A2A2 breeds and thus the issue in India might not be that huge as in countries with milk from A1A1 cattle like US.
I am in the researching phase of setting up my dairy farm and came across this information. It has nothing to do with nationalist propaganda as politicians may lead some to believe. I mean the revolution was started in New Zealand, for crying out loud. But people advocating buying milk of Indian Cows are termed as Hindu Zealots, so thought to clarify that. A normal dairy farmer may never know the difference unless awareness is increased. I have vowed to make sure all my cows, even of the Holstein breed will be of the A2A2 variety but to weed out this mutated gene from a present herd can take a decade of careful pruning. And until the govt becomes serious about this, A1A2 milk will still continue to pollute India.
I would like to point out that even in US and Europe it is small scale dairy farmers who have taken it upon themselves to weed out the A1 gene. Big corporations with vast herds don't pay any heed to these findings and also take it upon themselves to discredit scientists with this viewpoint.