Differing breeds of cattle, and where they originated.
Interesting. Milk here is 99% Holstein, with some Jersey added to increase milk fat content.
Otherwise there is no way of knowing what breed the milk came from.
Shorthorn and Ayershire cattle are not common here for dairy operations. Though my neighbour did have a few Short horns hanging around when he was milking, more for showing than anything else.
Money talks in dairy and milk quota are very expensive, Holstein cattle are the top milk producers.
Edit:
Well now I stand corrected, seems there Ionia a dairy here that caters to A2 specialty and they do a good job of explaining the genetic component.
https://walkerfarms.ca/a2-milk/#:~:text=Regular milk in Canada comes,cows produce A2 Protein Milk.
Wow you learnt a new thing every day! Thanks!