The Genetics of Coat Colours

You don’t have to know the genetics behind coat colours to appreciate their beauty! To date, 10 different genes that affect coat colour have been isolated, each of which can come in many different forms. The specific coat colour of each horse depends on several genes being expressed at the same time.

The foundation of all coat colours is chestnut or black. All horse coat colours depend on the presence of absence of two melanin pigments: pheomelanin (red to yellow shades) or eumelanin (brown or black). The more of either pigment the hair contains, the darker it will be. Hair without pigment is white. If a horse has any black hairs on his body, then he carries at least 1 dominant gene for black. Genes always come in pairs (one from the sire and one from the dam), so a horse with any black hairs on his body either carries one or both dominant genes (EE or Ee). If the horse is recessive (ee), this means he has no dominant genes and he will be chestnut.

So if E is always dominant, why are there so few pure black horses around? This is because the expression of the dominant gene can be affected by the presence of other genes (modifier genes). The most common modifier gene is agouti (A) which inhibits the production of eumelanin, causing the expression of the chestnut colour instead which ultimately results in the horse being bay by causing the black hair to be limited to the mane and tail. A horse that is pure black carries a double recessive agouti gene (aa) meaning that this gene is non-functional allowing for the full expression of eumelanin.

Horse base colours are further changed in three ways: shade, sooty and mealy. Shade refers to how the colour differs from light to dark. Bay and chestnut horses can range from bright red to gold to deep liver. It is likely that the same genes are expressed differently among individuals, or there may be other undiscovered genes at play. Sootiness refers to horses that have black hairs interspersed with their base colours. This usually occurs over the back region of horses that are palamino, dun, bay or chestnut. Some believe that factors in the womb may cause the expression of certain genes that result in sootiness. The mealy effect produces yellow or pale areas on the belly, flanks, inner legs, behind the elbows, muzzle and over the eyes. The source of this colouring is still a mystery.

Some of the more striking coat colours such as palamino and buckskin are dilute versions of bay and chestnut respectively. Horses of these colours carry one of the five known dilution genes which reduces the amount of pigment produced. The cream gene (C) determines the intensity of colour in a horse’s coat, but it affects only chestnut coat colours, lightening the red pigments into yellows and sandy tones. In the case of buckskins, the black hair from the original bay mane and tail are not affected, whereas in palominos, the mane and tail are lightened.
Much about markings and colour is still a mystery. From stockings and blazes to strange colours like roan, tobiano, overo and appaloosa, equine coats sport a tremendous variety of different markings and patterns.

So what colour is your horse? It might be difficult to tell from appearances alone, and genetic tests are available to map the colour profile. However, I think it’s just as satisfying to leave the genetics a mystery and simply enjoy their effects.

Sarah Seitz was born in Zimbabwe and moved to South Africa when she was 10 years old. After attending high school at St Andrew’s in Johannesburg, she worked for her father for several years doing wildlife conservation and breeding endangered species. After that, she was introduced to the illustrious world of thoroughbred breeding and racing when she was employed to manage a thoroughbred stud farm for four years. This stud has produced several notable winners in its time, including a Durban July winner. She found herself really enjoying the veterinary aspect of equine management and breeding, so decided to come the USA to study Veterinary Medicine. Far from home and lots of paperwork later, she completed her undergraduate BSc degree at the University of Kentucky in Lexington, and she is currently in her final year of Veterinary Medicine at Washington State University in Pullman, Washington. Sarah’s primary veterinary interest is large animal medicine and theriogenology. Sarah also enjoys volunteer work and has traveled to several countries including India, Nicaragua and Ecuador to provide free veterinary services to underdeveloped communities.