What do you mean by A2/A2? I have seen you mention it before.........
Most milk in the US comes from Holsteins... most Holsteins (if not all) produce milk which is genetically classified as A1/A1.
This "pair" of genes determines the type of protein found in the milk.
So, dairy animals can be A1/A1, A2/A1, or A2/A2.
There is research that has determined that many dairy allergies and potentially many health problems from ADD to autism have been caused by the wrong kind of protein in milk. It is believe by some that this was the result of a mutation years ago and affects the proteins in dairy cattle world wide.
There is a good read called "The Devil in the Milk" which explains all of this in great detail.
It has become commonplace to test dairy cattle now to determine their classification.
The breed of dairy animal with the highest percentage of homozygous A2 (A2/A2) is Guernsey at close to 90%, jersey is about 60% I believe, Holsteins it is rare.
I am not certain what the percentage is in dexters, but do know that many dexter owners now test.
It is required that dexter bulls be genotyped to be registered anyway, so many folks during this same test will test for other factors such as polled, chondro, and/or color, in addition to A2 status. The homozygous A2 animals are in high demand right now and bring a premium.
Basic genetics is in play here... it's not a complicated punit square like color can be.... it's simply 2 pairs.
For instance an A2/A2 cow, bred to an A2/A1 bull will produce A2/A2 offspring 50% of the time and A2/A1 offspring the other 50% of the time.
So... rather a long answer to your question, but... all sheep are A2/A2... and all of my cows are A2/A2.
I do have one bull I use who is heterozygous A2 (A2/A1)... he is a great bull and wonderful in every other way.
When he has offspring that are heterozygous A2 I simply sell them to someone who isn't planning on milking and only uses their cattle for beef.
Here's a link for an overview for those who are curious and want more information...
http://articles.mercola.com/sites/articles/archive/2009/07/09/the-devil-in-the-milk.aspx