Agouti is the common name of the gene that is responsible for bay and brown horses. It is denoted using the letter 'A'.
This gene can only work on a horse that is genetically black. it takes the black pigment of the coat and turns it red. It does, however, leave the points (the mane, tail, lower legs and ear tips) mainly unaffected, resulting in a horse with a red coat and black points.
Shades of agouti
Agouti comes in many different shades, from a very light mahogany bay, to blood bay and even dark brown. The presence of agouti (in general) can be proven by a laboratory test. It is not yet clear whether the different colour variations of agouti (phenotypes) have a unique genetic base or are created by environmental factors. For that reason it is impossible to test if a horse with agouti is any specific shade.
There are three versions of agouti commonly accepted, though none has ever been proven to have a unique genetic base. Within, or besides, these three versions, more variations and shades can be present.
The best known variation of agouti. Bay horses can have any shade from light to dark, though dark variations may also be influenced by sooty. It is unknown whether the bay phenotypes are genetically distinct, or multiple variations caused by other factors.
This 'black and tan' version was at one point believed to be caused by a mutation of the agouti gene called At. That research has unfortunately never been published and the genetic test is no longer available. For now we can only guess at what causes this shade.
Seal brown , or just brown, is a version of bay where the coat is still mainly black haired. The exceptions are the areas around the muzzle, the eyes and the belly. Those are more yellowy shaded, though in some horses they appear more red.
Wild bay , or light bay, is the least known and rarest version of bay. It distinguishes itself from any other bay horse by its very short black points. In other agouti horses, the black on the lower legs extends to the knee. In wild bay horses, the black does not get any higher than the socks. Sometimes the coat, mane and tail are also lighter.
If a horse is quite young these black lower legs can still move higher up the leg. Dilutions can also lower the black points without any wild bay present.
Variations of agouti where the black does reach the knees, but is patched (i.e. not entirely black but showing bay, which is not the result of young age or a dilution) are sometimes considered as wild bay as well. We cannot tell if they are or not, as no genetic test is present.
There is no genetic test that can separate wild bay from other versions of bay. The exact rules of behaviour are still unclear. It may not even be a genetically distinct version of agouti at all, but just another variation within multiple agouti phenotypes.