This post covers agouti, the gene that makes a black horse into a bay. Agouti is dominant, meaning you only need one copy for it to express HOWEVER the horse also needs an "E" or black gene. If the horse is red based (ee) then the agouti gene will not express. So, in order for a horse to be bay based his genetic composition has to be either EeAa, EEAa, EeAA, or EEAA. A horse who is ee Aa, ee aa, or ee AA, will always be chestnut based. Here are a few examples of horses with at least one copy of agouti, and at least one copy of black. They are E?A?
This is Abra Khadabra, an Arabian mare from Epic Arabians. She is EEAa.
This is Galea MHF. She is a Pura Raza Espanola from WATCHMAN P.R.E. She is also EEAa.
This is Magic, a thoroughbred mare from Swan Creek Andalusians . She is ee Aa, and notice how her "A" has no effect on her chestnut color.
As a last little note, if researching for this post has taught me one thing, its that if you are planning to breed to a homozygous bay stallion because you only want a bay foal, make sure you ASK SPECIFICALLY what he is homozygous for. Some stallion owners call their bays that are homozygous black(EE), but only heterozygous for agouti(Aa) homozygous bay, but you still have a chance for a black foal there. Also they sometimes say homozygous for bay, but the horse is only EeAA, so you can end up with a chestnut foal.
Which is part of the reason I want to do this blog. IMO Homozygous bay should mean the horse is EEAA. If he is EE?? he is homozygous black. If he is ??AA he is homozygous agouti. The only thing that should be homozygous bay IS a horse who will only throw bays, and that is EEAA.
If you have any questions in regards to this post, feel free to leave them in the comments!