Coat Colour Introduction Horses

Introduction to equine coat colours

In horses, coat colour is a complex trait influenced by a variety of genes interacting with each other. Mutations in these genes (or loci*) can interact in intricate ways, leading to a diverse range of coat colours and patterns.

The original coat colour of the wild horse is called “dun”: a diluted tan body with a darker muzzle, a dark stripe down the back, and faint dark striping on the legs. Many modern horses, however, differ dramatically in colour, ranging from black to gold to covered in white spots. These colours are all produced by two types of pigment: eumelanin (dark pigment; appears dark brown or black) and phaeomelanin (light pigment; appears as brownish/red to yellow). A lack of any produced pigment results in white hairs instead.

These coat colour mutations can interact with each other, for example as seen when a horse carries the Cr-locus dilution mutation while being affected by the E-locus chestnut mutation, resulting in a diluted red/yellow coat known as palomino. However, certain mutations can counteract each other’s effects. For instance, a horse with the recessive red E-locus mutation will not display the effects of the A -ocus black mutation, since it lacks the ability to produce black hairs in the first place.

Perhaps the most challenging aspect of coat colour genetics in horses is the dominant white phenotype. Many domestic horses display some degree of white colouration on their coat, ranging from a small white blaze to a piebald pattern to even an entirely white coat. Most of the mutations responsible for white colouration inherit in a dominant manner, making it interesting for breeders to know if their white-spotted horse carries one or two copies of the responsible mutation. However, researchers have to date identified over forty different mutations capable of causing some degree of white coat patterning, and more are regularly discovered. This makes it highly impractical to test a spotted horse for all possible variants. As most of these mutations are specific to one or several particular breeds, it is important to keep a horse’s breed in consideration when testing for white or spotting mutations.

* Many coat colour genes are known as “loci” (singular: locus), and are designated with a particular letter that stands for the effect caused by the mutant allele. Particularly in breeding communities, the names of these loci are often far more well-known than the names of the genes themselves.

Tested colour and pattern loci

• A-Locus (Agouti) – Turns the base coat entirely black.
• Dominant White – A range of mutations that result in large white spots, up to an entirely white coat.
• Splashed White (SW) – A range of mutations that result in white spots “splashed” over the base coat, associated with defects to hearing.
• Frame Overo Factor – Results in splashed white over the base coat, but is lethal to foals when homozygous.
• Appaloosa Patterns – Cause the leopard-spotted patterns distinctive of the Appaloosa, associated with defects to vision.
• E-Locus (Extension, Chestnut) – Coat cannot produce dark hair, resulting in a chestnut base coat.
• Roan – Body gains a scattered mixture of dark and light hairs, while the legs, mane and tail remain solid.
• Cr-Locus (Cream Dilution) – A range of mutations that dilute both dark and light pigment, resulting in colours like buckskin, palomino and cream.
• Champagne – Turns red hair to gold, and black hair to brown, resulting in colours like gold champagne and amber champagne.
• G-Locus (Grey) – Gradually turns the base coat to speckled grey or white as the horse ages.
• Dun Dilution – Causes the dun colouration of wild horses.
• Silver – Dilutes dark pigment, resulting in a dappled, silvery-grey coat colour, associated with defects to the eyes.
• Brindle – Alters the coat texture and colouration, adding uneven vertical stripes.