Dun gene

Another characteristic of the dun gene are primitive markings.^[5] Dun traits include the following:

• Dorsal stripe down the center of the back, along the spine, seen almost universally on duns
• Horizontal striping on the back of forelegs, common on most duns, although at times, rather faint
• Facial mask, a darker area around the nasal bone and forehead, sometimes making the head close to the undiluted color, some patterns colloquially called "Cobwebbing."^[1]
• Transverse stripe, a crosswise stripe along the shoulders perpendicular to the dorsal stripe, very common in donkeys, less common in horses, and if present, often faint, usually only visible on a short summer coat, if at all.^[1]
• Frosting: light hairs found on either side of the mane and on both sides of the dock of the tail.

Other variations result from the interplay of additional genes:

• Chestnut + dun + cream gene (single copy) = "dunalino" or "palomino dun"
• Bay + dun + cream gene (single copy) = "dunskin" or "buckskin dun"

A single copy of the

Dorsal striping alone does not guarantee the horse carries the dun gene. There two types of non-dun, called non-dun1 and non-dun2. Non-dun1 horses have no dun color dilution but may keep primitive markings, while non-dun2 horses have neither the dun color dilution nor primitive markings.[9]

The Fjord horse breed, which is predominantly dun, uses unique Norwegian-based terminology to distinguish between the different shades of dun horses. "Brown dun", or brunnblakk is a zebra dun, rødblakk is a red dun, grå - literally "gray" - is a grullo, buckskin duns are called ulsblakk or white dun, and a dunalino (dun + palomino) is called a "yellow dun" or gulblakk. A cremello, perlino or smoky cream is called "white" or kvit.^[10]

Dun mimics

Historically, before modern genetic studies distinguished between alleles, diluted colors were sometimes lumped together and simply called "dun".^[11]

The dun gene, when on a "bay dun" horse, can closely resemble buckskin, in that both colors feature a light-colored coat with a dark mane and tail. In particular, buckskins with non-dun 1 primitive markings can easily be confused with dun.^[11]: 32 Genetically, a bay dun is a bay horse with the dun gene. A buckskin is bay horse with the addition of the cream gene, causing the coat color to be diluted from red to gold, usually without primitive markings. Visually, a bay dun is a tan-gold color, somewhat darker and less vivid than the more cream or gold buckskin, and duns always possess primitive markings.^[12] Today, pedigree analysis, DNA testing, studying possible offspring, and the vividness of primitive markings are used to determine whether a horse is a dun.

A red dun may also be confused with a

Grullos are sometimes confused with roans or grays. However, unlike blue roan, dun has no intermingled black and white hairs, and unlike a true gray, which also intermingles light and dark hairs, the color does not change to a lighter shade as the horse ages. With a dun, the hair color is one solid shade and remains so for life.

To further confuse matters, it is possible for a horse to carry both dun and cream dilution genes; such horses with golden buckskin coloring and a complete set of primitive markings are referred to as a "buckskin dun" or a "dunskin". On such horses, the light-shaded primitive markings are most noticeable during the summer months, when the winter hair sheds. A palomino that also carries dun, showing primitive dorsal striping or leg bars indicative of a red dun may be called a "dunalino."

Countershading such as light dorsal stripes resulting from the presence of the gene nd1 (see section below) may be difficult to detect on light-colored horses.

Genetics

There are three known

The dun dilution effect is caused by pigment only being placed in a part of each hair. Specifically, hairs from diluted areas only have pigment along one side of them, while hairs from darker parts such as the dorsal stripe have pigment all the way around.[4]

Genetic analysis and DNA sequencing results published in 2015 link dun color to the T-box 3 (TBX3) transcription factor. When functional, it creates dun coloring, including the primitive markings, and when recessive, a horse is not dun. In humans and lab mice, TBX3 is critical to development. Abnormalities are linked to a collection of developmental defects called ulnar–mammary syndrome, and the null allele (being unable to produce any TBX3 at all) is thought to be embryonic lethal.^[14] In non-dun horses, the TBX3 protein is still functional, and is still produced in most cells, but not expressed in the hair cortex. Where the coat is diluted, the color is not uniform throughout each hair, but rather is more intense on the outward-facing side of the hair shaft and lighter underneath. In the darker areas, where the primitive markings occur, the hair shaft is of uniform color. One of the researchers involved in the study said it could be called a "microscopic spotting pattern".^[11]: 31 This phenomenon is new to science and has not been observed in rodents, primates, or carnivores.^[15]

The location of TBX3 expression may also determine the striping pattern of zebras.^[15]

• 
  Dorsal stripe and light guard hairs on a dun horse
• 
  Transverse shoulder stripe

Non-dun alleles

There are two forms of non-dun color, non-dun1 and non-dun2, caused by different mutations. Non-dun1 horses have some primitive markings, while non-dun2 horses do not.

Prior to domestication of the horse, dun, non-dun1, and the leopard complex are thought to have been the wild type modifiers of the base colors bay and black for wild horses. It is thought that the non-dun2 genetic mutation (as well as the development of chestnut base color) occurred after domestication. Ancient DNA from a horse that lived about 43,000 years ago, long before horses were domesticated, carried both dun and non-dun1 genes.^[1]

The non-dun mutations appear to "disrupt the function of a