The color of a horse’s coat is determined primarily by genetics, but nutrient levels and diet can affect a coat’s appearance. Hair pigment serves as a natural barrier for a horse’s skin against sunlight, and the colors of the hairs reflect the inner health of the animal. Multiple nutrients are needed for healthy hair, including protein, omega-3 fatty acids, copper, iodine and zinc.
Coat color is created by the transfer of pigment to the keratin protein within hair cells by melanocytes (pigment-producing cells) in hair follicles. Hairs with longer medullas, such as mane and tail hairs, generally have more pigment than body hairs. If the medulla is destroyed by stress or disease, it will regenerate new hairs with a white or gray pigment. Hair color can also be affected by light, which can penetrate the medulla and cause the hair to appear lighter or more pale.
The completion of the equine genome opened the door to geneticists who can identify mutations that impact basic coat and spotting patterns, Graves said. A gene change can make a horse more susceptible to certain diseases or environmental factors, and it may alter the way he responds to treatments, such as medications or nutrition regimens.
A horse can be “recessive” for a particular trait if he carries two copies of the dominant allele. He can also be heterozygous for a trait, meaning he has one copy of the dominant allele and one copy of the recessive allele.
Many painted color patterns can be traced to specific genes, Graves said. Sabino, for example, is the result of a double dilution of chestnut/sorrel (the copper-penny-colored part of the coat) with buckskin (a brown or black parent with a hidden cream gene).
Other spotting patterns include splashed white, which creates a bluish eye color; champagne, pearl and leopard complex, which produce blue eyes; and paint, which includes the roan, snowflake, few spot leopard and few spot tiger. All of these spotting types have their own unique set of genetic markers. However, it is important to remember that any white marking on a horse should be tested for overo lethal white syndrome (OLWS). This test can only be performed before the foal is weaned. If OLWS is present, the foal will likely be euthanized shortly after birth. If not, the foal will survive. The KIT gene is involved in the formation of this deadly disease, so all white foals should be tested for it.