A bay colored horse will have a reddish-brown basecoat with black points (mane, tail, legs, muzzle, ears and often around the eye). They can have white markings on their legs and face and still be called bay. If mixed with a pattern gene (such as Roan, Appaloosa or Pinto) they can have white in various places as directed by the above genes, and as long as they have black points (genetically if you can't tell visably) they are still called bay. The colour is often seen in all breeds, and one breed (Cleveland Bay horses) are exclusively bay.
There are different terms to describe the shades of bay, however these vary depending on where in the world they origionate, the most common are as follows:
- Mahogany Bay
- Black Bay
- Dark Bay
- Blood Bay
- Red Bay
- Golden Bay
- Light Bay
However the shade is generally based on wether the horses base coat is more of a blackish, reddish or yellowish tone.
There are many proposed theories for a bay horses coat, but most agree that the basic genes are
EE or Ee (which makes the horse black) and the dominant gene AA or Aa - Agouti (which makes the black restricted to the points). Agouti is stronger than the black gene, and as such even one dominant gene (Aa) Will make the black stay to the points.