Dun is probably one of the oldest dilutions, as many suspect it to be the original horse colour. It dilutes both red and black pigment, but leaves certain areas partially unaffected, resulting in the so called 'primitive markings', including a dorsal stripe, leg barring, shoulder and sometimes neck stripes or shadows, darker ear tips, frosting in mane and tail, a dark face mask, mottling on the upper leg, cobwebs on the forehead and brindling. Not all of these have to be present at the same time. In addition, the coat is diluted to a slightly lighter, more dull colour, increasing the contrast with the darker markings.
Dun has been found on horse chromosome 8 and is often abbreviated to D or DD. It is a dominant gene, which means it always shows and every dun horse must have at least one parent with the same gene. Two other alleles of the same TBX3 gene are non-dun1 (nd1) and non-dun2 (nd2). The former also being responsible for primitive markings, but without the characteristic dilution and the latter being the 'off' allele, formerly called 'd'.
The dilution is present in many breeds, especially among the heavier pony type, including (but not limited to): fjord horse, icelandic horse, highland pony, konik, dülmen horse, przewalski horse, sorraia and the shetland pony. It is virtually non-existent in breeds with a lot of arabian or thoroughbred influence, and it appears only rarely among the heavy draft horses.
Dun dilutes the coat to a dull and lighter version of the original colour. Certain parts are not affected, resulting in the contrasting primitive markings as explained below.
Dorsal stripe - The best known marking is the dorsal stripe, which follows the horse's spine all the way down into the tail. This long stripe is darker than the surrounding hairs, but it depends on the horse's other genetics just how much darker it is. While the dorsal stripe is always present on a dun (except when it is rendered invisible by white patterns), the stripe itself is no prove the horse is an actual dun, as it is often part of what is called 'countershading', once also known as sooty.
Leg barring -
Shoulder stripe - Though often seen as a stripe, it may also appear as a shadow or smudge. In more extreme cases, there is more than one stripe, and they may even appear up along the neckline.
Dark face and ear tips -
Chestnut DunChestnut + dun = chestnut dun
Of the three basic dun colours, chestnut dus is the most easily overlooked as it decreases the contrast between the body and the points, sometimes resulting in a dull red horse.
Chestnut duns with flaxen are sometimes called claybank, or claybank dun. As flaxen is quite rare in the quarter and paint horse, claybank was considered extrmeley rare too.
Black + dun = black dun or grullo/a
Black duns have many descriptive names, including mouse dun, blue dun and grå, but most often the colour is referred to as grullo (for stallions and geldings) or grulla (for mares), coming from the Spanish word grulla, which is a good description of the colour.
Bay DunBay + dun = bay dun
Sometimes also called red dun, bay dun is more similar to buckskin and is often mistaken for it.
Dun in the Norwegian fjord horse
The Norwegian fjord horse is famous for its universal dun colour and its nomenclature for the several colour variations is no longer limited to that breed.
- Brunblakk = bay dun, by far the most common colour in the fjord horse
- Rødblakk = chestnut dun
- Grå = black dun or grullo/a, sometimes with 1 cream allele
- Ulsblakk = bay dun + 1 cream allele, also called dunskin in other breeds
- Gulblakk = chestnut dun + 1 cream allele, also called dunalino in other breeds
- Kvit = any base colour, combined with two cream alleles