What is the difference between A1 and A2 milk and which breeds produce A2?

A1 and A2 refer to two variants of the beta-casein protein in milk. Some people report easier digestion of A2-only milk. Most modern dairy cows (Holstein, Friesian) produce a mix; some heritage breeds (Jersey can be either, Guernsey often A2, Brown Swiss often A2) produce predominantly A2. Breed and individual cow vary.