Equine coat color genetics determine a horse's coat color. There are many different coat colors possible, but all colors are produced by the action of only a few genes. The simplest genetic default color of all domesticated horses can be described as either "red" or "non-red", depending on whether a gene known as the "Extension" gene is present . When no other genes are active, a "red" horse is the color popularly known as a chestnut. Black coat color occurs when the Extension gene is present, but no other genes are acting on coat color.The Agouti gene can be recognized only in "non-red" horses; it determines whether black color is uniform, creating a black horse, or limited to the extremities of the body, creating a bay horse.