Bay is the best known variation of agouti. The agouti gene takes the black pigment and moves it across the body of the horse, resulting in patches where red pigment can 'reappear'. It can therefore only work on black pigment, and as a result needs at least one copy of the E-allele to function.
Agouti is the gene that is responsible for bay or brown horses. 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 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, but more may 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.
Wild bay, or light bay, is the least known and rarest version of bay. It separates 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. Often 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 cannot be a 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.
It is possible wild bay is dominant over any other variations of bay. This is not yet ascertained.
The available test in Horse Reality tests for the presence of agouti in general. If present, it is not ascertained if the horse carries bay, wild bay or seal brown. Just that at least one allele of agouti is present, not to which version it leads.