Horse Coat Color Simulator

How Horse Coat Color Genetics Work

Horse coat color inheritance is controlled by multiple genes that work together to determine a horse’s base color, white markings, and dilution effects. Think of genes like instructions passed down from both parents. Each parent gives the foal one copy of every gene, and the combination of those genes determines the final color.

The two most important “base color” genes are the Extension (E) gene and the Agouti (A) gene.

• Extension (E) gene: controls whether the horse can produce black pigment at all.
  • E = black pigment can be produced
  • e = black pigment cannot be produced
  A horse with two recessive copies (ee) will always be chestnut/sorrel because it cannot make black pigment.
• Agouti (A) gene: controls where black pigment appears on the body.
  • A = black pigment is restricted to the mane, tail, legs, and ear edges (bay horse)
  • aa = black pigment covers the whole body (black horse)
  Agouti only affects horses that can already produce black pigment.

This means:

• ee = chestnut/sorrel
• E_ A_ = bay
• E_ aa = black

Other genes then modify these base colors:

• Cream lightens color (palomino, buckskin, cremello)
• Dun creates primitive markings like dorsal stripes
• Gray causes the horse to slowly turn white with age
• Tobiano, Overo, Sabino, Splash White create white spotting patterns

Some genes are dominant, meaning only one copy is needed to show the trait. Others are recessive, meaning two copies are required. Some genes are incompletely dominant, where one copy creates a milder effect and two copies create a stronger effect.

How to Predict Foal Colors

Foal coat color depends on which gene copies the foal inherits from each parent. Every foal receives:

• One copy of each gene from the sire (father)
• One copy of each gene from the dam (mother)

The important thing to understand is that horses can carry hidden genes that do not visibly show in their own coat color.

For example:

• Two black horses can produce a chestnut foal if both parents carry the recessive chestnut gene (e).
• A bay horse may carry black or chestnut genes that can appear in offspring.
• A horse may visually look solid-colored but still carry white spotting genes.

Here’s a simple example:

• Black horse genotype: Ee aa
• Black horse genotype: Ee aa

Because both parents carry a hidden chestnut allele (e), there is:

• 25% chance of chestnut foal (ee)
• 75% chance of black-based foal

Genetic testing helps breeders identify hidden genes so they can better predict foal colors and avoid dangerous genetic combinations.

Tobiano Genetics Explained

Tobiano is one of the most common white spotting patterns in horses. It is caused by the dominant TO gene.

Because Tobiano is dominant:

• A foal inheriting one copy (TO/n) will reliably show the tobiano pattern.
• Two copies (TO/TO) usually produce a more extensive amount of white coverage.

Typical Tobiano traits include:

• Large smooth white patches
• White crossing over the topline/back
• White legs
• Darker head with normal facial markings

Breeding outcomes:

• TO/n × n/n = about 50% Tobiano foals
• TO/TO × n/n = 100% Tobiano foals

Tobiano itself is not linked to lethal health problems, which makes it safer to breed than some other white pattern genes.

Frame Overo and Lethal White Syndrome

Frame Overo is a white spotting pattern caused by a mutation in the EDNRB gene.

Horses with one copy of the gene (O/n) usually have:

• Jagged horizontal white patches
• Dark toplines
• Bold face markings

The major danger comes when a foal inherits two copies of the Frame gene (O/O).

This causes Overo Lethal White Syndrome (OLWS).

Foals with OLWS are usually:

• Almost completely white
• Born with an improperly developed intestinal tract
• Unable to digest food normally

Sadly, these foals cannot survive.

If two Frame carriers are bred together:

• 25% chance of normal foal
• 50% chance of carrier foal
• 25% chance of lethal white foal

Because of this risk, responsible breeders test horses before breeding and avoid crossing two Frame carriers together.

Appaloosa Leopard Complex (LP) Genetics

Appaloosa patterns are mainly controlled by the Leopard Complex (LP) gene.

LP affects both coat color and skin characteristics.

Horses with one LP copy (LP/lp) often show:

• Mottled pink-and-dark skin
• Striped hooves
• Visible white sclera around the eye
• Some spotting or roaning

Horses with two LP copies (LP/LP) often show stronger Appaloosa characteristics and more white patterning.

Horses with two LP copies have a higher risk of Congenital Stationary Night Blindness (CSNB), which affects vision in dim light.

Cream Dilution in Horses

The Cream gene lightens coat color. It is called an incomplete dominant dilution gene because one copy and two copies produce different results.

One Cream copy (Cr/n) partially dilutes color:

• Chestnut → Palomino
• Bay → Buckskin
• Black → Smoky Black

Two Cream copies (Cr/Cr) create much lighter “double dilute” horses:

• Chestnut → Cremello
• Bay → Perlino
• Black → Smoky Cream

Double dilute horses usually have:

• Blue eyes
• Pink skin
• Very pale cream-colored coats

Example breeding:

• Palomino × Palomino

Possible outcomes:

• 25% chestnut (no Cream gene)
• 50% palomino (one Cream gene)
• 25% cremello (two Cream genes)

Common Horse Coat Color Outcomes

Some coat color pairings commonly produce predictable results:

• Chestnut × Chestnut = always chestnut because both parents can only pass recessive e genes
• Black × Chestnut = black or chestnut depending on whether the black parent carries recessive chestnut
• Bay × Chestnut = bay, black, or chestnut possible depending on hidden genes
• Palomino × Chestnut = about 50% palomino and 50% chestnut
• Buckskin × Bay = bay, buckskin, black, smoky black, or chestnut-based colors depending on hidden genes
• Tobiano × Solid = about 50% Tobiano if the Tobiano parent has one copy of the gene
• Gray × Non-Gray = about 50% gray if the gray parent carries only one Gray gene

One important thing to remember: visible color does not always reveal the horse’s full genetic makeup.

That is why DNA testing is now widely used by breeders. It helps predict foal colors more accurately, identify hidden recessive genes, avoid dangerous genetic combinations, and confirm homozygous or heterozygous status.