HOW COAT COLOR GENETICS WORK IN HORSES ?
Illustration of a tobiano pinto pattern on a bay coat.
The color of a horse is not a matter of chance. It is determined by a small number of genetic mechanisms that control the production of pigments, the distribution of these pigments across the body, and the presence of white or spotted patterns. Even if genetics can seem complex, the foundations are relatively simple: all coat colors originate from a limited set of basic combinations, which can then be modified by additional genes.
1. Two pigments behind all coat colors
A horse’s coat results from the combination of two pigments:
- eumelanin, responsible for black and dark brown shades,
- pheomelanin, responsible for reddish tones.
Each horse produces these pigments in different proportions. Genetics determines whether a pigment can be produced and where it appears on the body.
2. Three basic coats: chestnut, bay, black
The entire system relies mainly on two key genes.
2.1 The gene that determines the presence of black pigment
A first gene controls whether the horse is able to produce eumelanin (black pigment).
- If it cannot, the coat is automatically chestnut.
- If it can, the horse may be black or bay, depending on the action of a second gene.
2.2 The gene that distributes the black pigment
A second gene regulates where the black pigment appears:
- when it restricts black to the extremities, the result is bay,
- when it does not restrict it, the result is black.
These two genes alone explain the foundation of all coats before any further modification occurs.
3. Genes that modify the basic coat
Once the fundamental color (chestnut, bay, or black) is established, other genes can alter it by lightening it, causing it to fade over time, or adding specific patterns.
3.1 The Grey gene
The horse is born with a normal color and gradually lightens as it ages.
The original coat remains genetically present, but it becomes invisible.
3.2 The Cream gene
This gene lightens the coat partially or strongly, depending on whether it is inherited once or twice.
It produces coats such as:
- palomino (chestnut diluted once),
- buckskin (bay diluted once),
- cremello or perlino (double dilution).
3.3 The Dun gene
It lightens the body while preserving primitive markings, such as:
- a dorsal stripe,
- leg barring,
- sometimes shoulder shadows.
It produces characteristic coats such as grullo or red dun.
3.4 Other dilutions
Some breeds carry other recognized dilution genes:
- champagne (lightens both coat and skin),
- silver (lightens black pigment, especially in the mane and tail),
- pearl (mainly visible when inherited twice).
4. White patterns and pinto coats
White markings—whether minimal or extensive—are controlled by additional genes. They do not change the underlying base coat but add unpigmented areas.
4.1 Tobiano
Large, clearly defined white patches, often symmetrical and crossing the topline.
4.2 Sabino
Irregular white edges, frequently with high stockings and excess white on the belly.
4.3 Dominant White
A broad range of patterns, from extended markings to an almost entirely white horse.
4.4 Roan
An even mix of white hairs and colored hairs across the body, leaving the head and legs darker.
4.5 Appaloosa-type spotting
These patterns are caused by a distinct group of genes and produce leopard, blanket, snowflake, and other spotted coats.
5. Coat inheritance: simple guiding principles
Although each mating can lead to several outcomes, some rules consistently apply:
- Two chestnut parents always produce a chestnut foal.
- A grey horse passes the grey gene on approximately half the time, depending on its genetic status.
- A bay may carry “hidden” genes, including chestnut.
- Some dilutions only appear when the gene is inherited twice.
Understanding these basics already allows most combinations to be anticipated without complex calculations.
6. The value of genetic testing
Laboratories now offer tests that can identify:
- the horse’s real underlying coat,
- invisible or hidden dilutions,
- the likelihood of passing on specific patterns,
- combinations that should be avoided in rare cases.
These tests are useful for breeding, preserving certain bloodlines, and interpreting atypical coats.
Even though the variety of horse coats may seem vast, their workings rely on a coherent and relatively small set of genes. The three basic coats—chestnut, bay, and black—form the backbone of the system. Dilutions, white patterns, and additional genetic variations are simply extensions of these foundations.
A clear understanding of these mechanisms makes it possible to identify a coat accurately, understand its origin, and better appreciate the genetic features specific to each breed.
