Understanding the genes behind your horse’s colouring

From the deep richness of a liver chestnut to the shimmering gold of a palomino or the striking contrast of a tobiano, a horse’s coat colour is often one of the first things we notice—and it’s all thanks to genetics. Here we break down how a horse’s coat colour is determined, what genes are involved, and why two seemingly similar horses can produce a very unexpected foal.

Definitions

Before we dive in here are some terms you need to be familiar with:

Gene

A gene is a segment of DNA that carries the instructions for making a specific protein or performing a particular function in the body. In horses, certain genes control things like coat colour, eye colour, height, and even behaviour.

Think of a gene as a recipe in a cookbook — it tells the body how to make something, like pigment for coat colour.

Allele

An allele is a version of a gene. Most genes come in pairs, with one copy inherited from the sire and one from the dam. These copies can be the same or different.

For example, the MC1R gene has alleles E (allows black pigment) and e (only red pigment). The combination of alleles (like E/E, E/e, or e/e) inherited from the sire and dam determines what pigment the horse can produce.

So if a gene is the recipe, alleles are the flavour variations of that recipe — changing the final result, like whether the horse is black, bay, or chestnut.

Types of allele

• Dominant allele = Only one copy needed for the horse to express the trait (e.g. Cr)
• Recessive allele = Two copies needed for a trait to be expressed in the horse (e.g. e/e)

The base coat colours

All horse colours start from just three basic coat colours: chestnut, bay, and black. These are determined by the interaction of two key genes:

• Melanocortin 1 Receptor (MC1R) (often called the extension or red factor gene): This gene controls whether a horse can produce black pigment. The dominant form (E) allows black pigment production, while the two recessive forms (e and ea) prevent it. Horses with two copies of a recessive allele (e/e or ea/ea) can only produce red pigment and are therefore chestnut.
• Agouti Signalling Protein (ASIP) (also called Agouti): This gene determines where black pigment appears on the horse. The dominant A allele restricts black to the points of the horse (mane, tail, lower legs, ear rims), creating a bay coat with dark points. The recessive a allele allows black pigment to spread uniformly, producing a black coat—if the horse has at least one E from the MC1R gene.

While the base colours are limited, shades vary widely. Chestnuts, for example, can range from pale gold to dark liver chestnut. Although hundreds of genes are involved in mammalian coat colour, we still don’t fully understand what causes all the subtle differences in shade in horses. The genetics behind this are something we still need to learn a lot about.

Dilution genes

Some horses appear lighter than their base colour due to dilution genes, which reduce the amount of pigment produced or transferred to the hair. Some dilutions affect red or black pigment specifically, while others affect both (red and black). They can also vary in whether they lighten the whole coat or just certain parts of it.

Six well-known dilution genes include:

• Cream: A dominant gene with a dosage effect in that one copy of the Cream gene (N/Cr) produces palominos (chestnut base coat) and buckskins (bay base coat), but two copies (Cr/Cr) result in very light colours like cremello (chestnut base), perlino (bay base), or smoky cream (black base).
• Pearl: Found at the same location as the Cream gene, but it is recessive. This means the dilution effect only appears when the horse has two copies of the Pearl allele (Prl/Prl) or one Pearl and one Cream (Prl/Cr).
• Champagne, Dun, and Silver: All dominant so only one copy of the dilution-causing allele is needed to produce an effect. Champagne gives a distinctive gold coat with pinkish skin and light eyes. Dun lightens the body but usually leaves the points darker, adding primitive markings such as dorsal stripes. Silver mainly affects black pigment of the points and is most noticeable on black or bay horses. Chestnuts can carry the Silver gene without showing it. Notably, horses with the Silver gene may have an eye condition called MCOA (multiple congenital ocular anomaly), especially if they carry two copies.
• Mushroom: A recessive dilution affecting red pigment. Chestnuts with two copies of the mushroom gene have a sepia-toned coat. On a bay base, the result is a lighter red body colour with darker points, suggesting that it increases black pigment production, having the opposite effect on black pigment as it does on red.

White spotting patterns

White markings and patterns are controlled by a variety of genes, and they can appear on any base colour or dilution. These patterns fall into two broad categories:

• Distributed white: Where white hairs are mixed into the coat. This includes Roan (white hairs throughout the body but not the head or legs) and Grey (progressive loss of pigment over time). Both are dominant traits. Grey horses are also more prone to melanoma.
• Patchy white: These include patterns like Appaloosa, Dominant White, Sabino 1, Splashed White, Tobiano, and Overo. Each has a distinctive pattern and location. For example, Appaloosas typically have symmetrical white over the hips, while Tobianos usually have white legs and a dark head. All these patterns are caused by dominant genes.

Some of these spotting genes come with additional effects on other body systems. For example:

• Horses with two copies of the frame-overo gene (O/O) suffer from Lethal White Overo Syndrome, a fatal condition in foals.
• Horses homozygous for the leopard complex (LP/LP) in Appaloosas can have congenital stationary night blindness, meaning they can’t see in low-light conditions.

Why genetic testing matters

In some cases, a horse’s colour can be determined just by looking. But appearances can be deceiving. For example, a chestnut horse might carry genes that could produce a black foal—something you can’t tell just by sight.

That’s where genetic testing comes in. It helps clarify confusing or subtle colour phenotypes, predict the outcome of breeding pairings, and flag any associated health concerns. Knowing your horse’s genetic makeup is more than just a matter of curiosity—it can prove a valuable tool for breeders and veterinarians alike.

In summary

Horse coat colours may look like art, but they’re built on science. With a few key genes laying the groundwork and many others adding nuance, the spectrum of equine colours and patterns is as complex as it is beautiful.