Genetically, all horses start out as either chestnut (technically referred to as “red”) in the absence of a special “extension gene” (“e”), or black in the presence of the extension gene (“EE” or “Ee”). The vast range of all other coat colours is created by additional genes’ action upon red or black.
1. Get Back Black
The bay colour is expressed when a common genetic modifier, the agouti gene, works on black pigment. The agouti gene (there are four alleles with varying levels of dominance) restricts the distribution of black pigment either to a points pattern (mane, tail, lower legs, ear rims) or over the body. While a red/chestnut horse can carry the agouti gene, it is not affected since there is no black pigment to control. The darkest shades of bay are commonly confused with black, even by experienced horse people. However, a dark bay will always show a reddish tint in its coat. Horses with a very dark coat that may appear black, but have tan or reddish hairs around the eyes, muzzle, armpits and stifle are sometimes called seal brown, mahogany bay or black bay.
2. Adding White
There are several genes responsible for white on horses (separate from genes that control markings such as blazes). Solid white horses are affected by dominant white (“W”) gene, which prevents the pigmentation of the hair and usually the skin as well. White spotting patterns on the base coat colour are produced by leopard, sabino, tobiano and overo genes, or as mixed white and coloured hair patterns produced by the grey gene (“G”) (progressive whitening with age) and roan genes. They all work differently and some can be combined. More on these “whitening” genes below.
3. Hi Ho Silver
The rather rare white horse has white hair and fully or largely unpigmented (pink) skin. These horses are born white, with blue or brown eyes, and remain white for life. A truly white horse that lives to adulthood occurs one of two ways: either by inheriting one copy of a dominant white (“W”) gene (“WW” is lethal), of which several have been identified, or they are a particular type of sabino that is homozygous for the “SB-١” gene. There are no “albinos” in the horse world. Albino, defined as animals with a white coat with pink skin and reddish eyes, is created by genetic mechanisms that do not exist in horses.
4. Salt and Pepper
In contrast to white horses, grey horses can be born any colour, and lighten as they age. Most will eventually grey out to either a complete white or to a “fleabitten” coat, which retains speckles of the horse’s original colour. Most “white” horses are actually greys with a fully white hair coat. A grey horse is distinguished from a white horse by dark skin, particularly noticeable around the eyes, muzzle, flanks, and other areas of thin or no hair.
5. Strawberry and Blue
The roan gene creates a colour pattern that causes white hairs to be evenly intermixed within the horse’s body colour. Roans are distinguishable from greys because roans typically do not change colour in their lifetimes, unlike grey that gradually gets lighter as a horse ages. Roans also have heads that are either solid-coloured or much darker than their body hair, and do not lighten. The roan patterns may exist at birth or may not be noticeable until the first foal coat is shed. Roans can be strawberry or red (chestnut base coat), bay (red roan body with black mane and tail) or blue (black base coat).
6. Painted Ponies
Pintos are any colour of horse with large patches of white, often referred to as piebald or skewbald in English riding circles. Various white marking genes exist, each producing somewhat different patterns. These are tobiano, sabino, overo and tovero. Several genes are involved in the production of white spotting patterns known as overo. Among those, the gene responsible for the frame-overo pattern is associated with a highly complex lethal disease of newborn foals called Lethal White Overo foal syndrome.
The group of coat patterns caused by the leopard gene complex spans a wide variation from all but solid colouring with a few white marks to a Dalmatian-like all-over dark spotting on white. The amount of white present is not dosage related; even homozygous horses can have minimal expression of white patterning. The variability in the amount of white on leopard complex patterned horses is controlled by yet other genes. However, even solid individuals will exhibit the gene’s secondary characteristics such as vertically striped hooves and mottled skin around the eyes, lips, and genitalia, plus a white sclera of the eye.
8. Lighten Up
The basic colours can be diluted by at least five rather complex genes: champagne and pearl, which affect red colour, dun and cream, which can affect both red and black and silver, which only affects black. The cream gene has a dosage effect in that a single copy of cream produces palominos, smoky blacks and buckskins. Two doses of cream produce cremellos, perlinos and smoky creams. Pearl is recessive; therefore, two copies of the gene or one copy of pearl and one of cream, are needed to see the dilution effect on the coat color. Champagne, dun and silver do not show a dosage effect. Dun dilutes both black and red pigments on the body but does not dilute it in the points. In addition, a predominant characteristic of dun is the presence of a dorsal stripe (list), shoulder stripe and leg barring.
The silver gene is a dominant dilution gene that only acts upon black hair pigment, it lightens black body hair to a chocolate brown and the mane and tail to silver. The resulting colour is usually referred to as silver dapple. The gene may be carried but will not be visible on horses with a red base coat. Silver dapple horses may also be called Chocolate, Flax or Taffy.
Brindle is one of the rarest colours in horses, possibly linked to chimerism: a single organism composed of cells with distinct genotypes. In animals, this means an individual derived from two or more zygotes. Characteristics are any colour with stripes, but most common is a brown horse with faint yellowish markings. A heritable brindle pattern in a family of American Quarter Horses that has been named Brindle1 was identified and announced in late 2016.