I did quick reading and was mentioned that they used Percherons to create the breed. Arabs were used to create Percheron, to add stamina. I bet that's where it comes from.
You got me reading as well.
They tested grey and non greys from a number of horses which included TB, Scottish, English and Scandinavian native breeds and found that the greys had a particular mutation which showed that they had a common ancestor. They all had Arabs in their background indicating that the original grey mutation was from an Arab horse.
Given how widely Arabs have been used to refine other breeds, it’s going to be very common. I think there was a lot of Arab blood bred into the Highland pony, including Percherons but also one of the Highland Chieftains brought them north in the 16thc to add lightness.
Equally, you'd think that Spanish Arabs would've influenced the horses of the country, especially as their breeding was in the hands of the Royals and the government. I'm surprised that it's so rare.
It's not a subject that I know very well. My first mare, Sasha, a Welsh, was a grey but i gave her colour very little thought. If fleabitten’s are Gg heterozygous and those that go white are GG, then Sasha would’ve been the later and Stookie the former. Stookie’s shoulder mark seems to have been a deactivated area where normal pigmentation continued.
They say that they don’t know what causes the flea-bitten marks. Some have it as a characteristic of greys while others suggest that it may be unrelated and dark coloured horses have them, the fleabites only showing due to the greying.
it's an interesting subject.