Dun Horse Genetics: Red Dun, Bay Dun and Grullo
By Breedio team
In short
A dun horse carries one dominant dilution gene (TBX3) that lightens the body and draws primitive marks such as a dorsal stripe and leg barring. Here is how to recognise red dun, bay dun and grullo, and how to tell them from a buckskin.
What makes a horse dun?
A dun horse carries a dominant dilution gene that lightens the body coat and leaves dark primitive marks: a dorsal stripe, leg barring and often shoulder bars. The head, lower legs, mane and tail stay darker than the body. One copy of the dun allele is enough, so a dun foal always has at least one dun parent.
This article is part of our horse coat colours guide. Dun is often confused with buckskin and palomino, which come from a different gene (cream). We separate the two below, because the difference matters when you plan a mating.
How does the dun gene work?
Dun is a variant at the TBX3 locus. The University of Kentucky Gluck Equine Research Center describes it as dominant with no dosage effect: a horse with two copies looks the same as a horse with one. The Non-Dun2 allele is a large TBX3 deletion with no primitive markings and no dilution.
Three practical consequences for breeders:
- A dun horse cannot appear from two non-dun parents. If a foal is dun, one parent is dun.
- A heterozygous dun (one copy) passes the gene to about half of its foals. A homozygous dun (two copies) passes it to every foal.
- You cannot tell one copy from two by eye. Only a DNA test shows it.
Dun works on top of the base coat. It does not replace Extension or Agouti: a chestnut (e/e) stays red-based, a bay keeps its black points, and a black (a/a) stays black-based. Dun then dilutes the body and adds the marks.
What are the primitive marks of a dun horse?
Primitive marks are the signature of dun. Look for them in this order:
- Dorsal stripe: a dark line from the mane to the tail, often darker and wider than any shading on a non-dun horse.
- Leg barring: horizontal "zebra" stripes on the forearms and gaskins.
- Shoulder bars: a faint transverse stripe across the withers or shoulder, sometimes doubled.
- Cobwebbing: fine dark lines radiating over the forehead, best seen on a young horse in good light.
- Dark ear edges and a dark mane and tail, sometimes with lighter hairs along the edge of the mane.
Marks can be bold or faint. A foal's marks are often easiest to read in the first coat. They can fade or be harder to see on a heavily dappled or very pale horse, and on a grey horse they disappear as the coat whitens.
There is also a related allele, Non-Dun1, which gives primitive marks without the body dilution. A horse with only a stripe and no lightening is not necessarily dun.
Which dun colours exist?
Dun has a name for each base coat. In France, "souris" is the common word for the black-based dun, and "isabelle" is used for buckskin, so do not mix the two.
| Base coat | Dun name FR | Dun name EN | Key marks |
|---|---|---|---|
| Chestnut (alezan, e/e) | Alezan dun | Red dun | Reddish body, darker red dorsal stripe and leg barring, mane and tail often flaxen or red |
| Bay (bai) | Bai dun | Bay dun | Tan or yellowish body, dark brown or black dorsal stripe, dark lower legs with barring |
| Black (noir, a/a) | Souris | Grullo | Slate, mouse or smoky grey-brown body, black head, legs, mane and tail, black dorsal stripe |
| Palomino | Palomino dun | Dunalino | Cream body, faint stripe, light mane and tail |
| Buckskin (isabelle) | Isabelle dun | Dunskin | Pale gold body, dorsal stripe and barring visible on a lightened coat |
A grullo is not a grey. A grey foal is born coloured and whitens over the years. A grullo keeps its slate shade for life and carries primitive marks from birth.
In which breeds is dun common?
Dun is part of the wild-type pattern in several primitive and northern breeds, and it is a selected colour in others.
- Fjord horse: dun shades are the standard colours of the breed, with an accepted range of dun tones and a dark dorsal stripe.
- Konik: the Polish primitive breed is dun (mouse-dun) with a strong dorsal stripe and leg barring.
- Highland pony: dun is one of the typical colours of the breed, with stripe and zebra marks common.
- Connemara: dun and grullo are found in the breed, in some lines more than others.
In comparison, a purebred Arabian never carries dilution genes, so dun, cremello, palomino and buckskin are not purebred Arabian colours. A dun-coloured "Arabian" is a cross.
In France, the coat recorded on the papers is registered by the SIRE/IFCE. If you are unsure whether a horse is dun, a DNA test confirms it and avoids a wrong coat on the papers.
How do you tell a bay dun from a buckskin?
Both have a yellow-tan body and black points, so they are easy to mix up. The difference is genetic: buckskin is a bay with one cream allele, and bay dun is a bay with one dun allele. A quick check:
| Feature | Bay dun | Buckskin |
|---|---|---|
| Gene | Dun (TBX3) | Cream (one copy) |
| Dorsal stripe | Present, dark, from mane to tail | Absent, or only faint shading |
| Leg barring and shoulder bars | Usually present | Absent in most horses |
| Cobwebbing on the forehead | Often present | Absent |
| Skin and eyes | Normal pigment | Normal pigment (pink skin and blue eyes need two cream copies) |
| Foal colour | Born dun with marks | Often born paler, darkens to gold |
A stripe alone does not prove dun. Many bay and chestnut horses show a countershaded dark line down the spine, from the pigment pattern of the coat, and Non-Dun1 horses carry primitive marks without dilution. Look at the whole picture: stripe plus leg barring plus shoulder bars plus a diluted body, and ideally a dun parent. When the marks are faint, a DNA test settles the question.
What happens when dun meets cream?
Dun and cream are two independent genes, so a horse can carry both. One cream copy on a dun horse gives:
- Dunalino (palomino plus dun): a cream body with a faint stripe.
- Dunskin (buckskin plus dun): a pale gold body with the dorsal stripe and barring still visible.
- Smoky black plus dun: a dark horse that can look like a grullo or a black.
Two cream copies give cremello, perlino or smoky cream, with pink skin and blue eyes. On such a pale coat, primitive marks are very hard to see even when the dun gene is present. Our cremello and perlino article covers that case. Because the marks are diluted by cream, a pale horse of unclear colour is the best candidate for a test.
How do you plan a mating with dun?
Because dun is dominant, planning is simple:
- Dun mare x non-dun stallion: if the mare has one dun copy, about half of the foals are dun on average.
- Dun x dun: foals can be dun, or non-dun if both parents have one copy. If either parent is homozygous, every foal is dun.
- Non-dun x non-dun: no dun foal.
Check other genes too. A dun on a grey background will whiten, and the marks will not show once the horse has greyed. Grey is dominant, so a grey foal must have a grey parent. Our grey horse genetics article explains the timeline. If you also work with pinto breeding, read the Tobiano, frame overo and OLWS article: two frame carriers have a 1 in 4 risk per foal of lethal white foal syndrome, so test both parents.
A coat does not show every gene, so any calculation gives a range. The coats of the grandparents narrow it, and a DNA test confirms the genotype.
Try the foal coat colour calculator
Enter the coats of the mare and the stallion, optionally their parents' coats and DNA test results, and the foal coat colour calculator gives you the possible adult coats with their odds. It covers Dun, Cream, Grey and the other main genes, and warns about the lethal white (OLWS) risk. It is free and runs in your browser. See also all Breedio tools, including the Gestation Tracker to follow your mare through the ~340 days until the dun foal arrives.
