Dun gene


The dun gene is responsible for the coat color seen in wild equines, which was lost in most domestic horses. A dun horse has a lightened color over most of the body while leaving the points darker. Both red and black pigment are affected. A dun horse always has a dark dorsal stripe down the middle of its back, usually has a darker face and legs, and may have transverse striping across the shoulders or horizontal striping on the back of the forelegs. Body color depends on the underlying coat color genetics. A classic "bay dun" is a gray-gold or tan, characterized by a body color ranging from sandy yellow to reddish brown. Duns with a chestnut base may appear a light tan shade, and those with black base coloration are a smoky gray. The mane, tail, legs, and primitive markings are usually the same color seen on non-dun horses. Interactions with other coat color genes can result in a wide variety of possible colors.

Taxonomic distribution

Dun is believed to be the ancestral or wild type color of horses. Many equines appearing in prehistoric cave paintings such as in Chauvet Cave are dun, and several closely related species in the genus Equus show dun characteristics. These include the Przewalski's horse, onager, kiang, African wild ass, an extinct subspecies of plains zebra, the quagga, and an extinct subspecies of horse, the tarpan. Zebras can also be considered a variant of dun where the dilution is so extreme it turns the hair nearly white, and the primitive markings extend across the entire body.
Neither the non-dun1 nor the non-dun2 mutations were found in any other equids.
File:Lascaux2.jpg|thumb|center|Cave painting at Lascaux: Dun is thought to be a wild type coloration

Color traits

The dun gene has a dilution effect, lightening the body coat, but has less of an effect on the primitive markings and on the point coloration of the mane, tail, ears, and legs.
Dun visibly affects all the three base colors, bay, black, and Chestnut. It is more difficult to recognize when combined with other dilution genes or if affected by gray. Shades include:
  • Dun, also called bay dun, classic dun, or zebra dun is the most common type of dun, and has a tan or gold body with black mane, tail, and primitive markings. Genetically, the horse has an underlying bay coat color, acted upon by the dun gene.
  • Red dun, also called claybank, is a light tan coat with reddish instead of black points and primitive markings. Genetically, the horse has an underlying chestnut coat color, acted upon by the dun gene. Thus, as there is no black on the horse to be affected, the undiluted underlying color is red.
  • Grullo, mouse dun or blue dun, is a smoky, blue-gray to mouse-brown color and can vary from light to dark. They consistently have black points and they often have a dark or black head. The primitive markings are usually all black. Genetically, the horse has an underlying black coat color, acted upon by the dun gene.
Another characteristic of the dun gene are primitive markings. 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."
  • 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.
  • 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 = "dunalino" or "palomino dun"
  • Bay + dun + cream gene = "dunskin" or "buckskin dun"
A single copy of the cream gene on a black base coat does not significantly lighten black hair, though it may have a subtle effect, and thus a single copy generally has no visible effect on a grullo, either. Conversely, double copies of the cream gene create very light-colored horses. Thus, if a horse with two cream dilution alleles also carries the dun gene, it also will be cream-colored, with primitive markings not visible to any significant degree.
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.
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" is called a "yellow dun" or gulblakk. A cremello, perlino or smoky cream is called "white" or ''kvit.''

Dun mimics

Historically, before modern genetic studies distinguished between alleles, diluted colors were sometimes lumped together and simply called "dun".
The dun gene, when on a "bay dun" horse, can 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. 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. 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 perlino, which is genetically a bay horse with two copies of the cream gene, which creates a horse with a cream-colored body but a reddish mane and tail. However, perlinos usually are significantly lighter than red dun and have blue eyes.
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 may be difficult to detect on light-colored horses.

Genetics

There are three known alleles of the dun gene: dun, produces dilution and primitive markings. Non-dun1 horses do not have dun dilution but may exhibit some primitive markings. Non-dun2 horses have neither dilution nor primitive markings. Dun is a dominant gene; however, at least one study found a statistically significant variation in the shade of dilution depending on whether one or two copies of the dun gene are present. Two non-dun parents cannot produce a dun foal. Horses that are non-dun1 d1/d1 or d1/d2 may have some asymmetry in pigment distribution, producing primitive markings, but to a lesser degree than dun horses. Homozygous non-dun1/non-dun1 horses typically have clearer primitive markings than heterozygous d1/d2 horses. The primitive markings from non-dun1 are more visible on a bay or chestnut horse; they blend in on a black. A horse with two copies of non-dun2 lacks primitive markings.
Dun has a stronger effect than other dilution genes in that it acts on any coat color. In contrast, the silver dapple gene acts only on black-based coats, and the cream gene is an incomplete dominant which must be homozygous to be fully expressed, and when heterozygous is only visible on bay and chestnut coats, and then to a lesser degree.
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. Freya Imsland, lead author on the paper which first described dun at a molecular level, writes in her thesis "The precise nature of the reduced pigment intensity in Dun is highly unusual; it can in essence be described as a microscopic spotting pattern, whereby the pigment distribution within each hair is radially asymmetric."
Genetic analysis and DNA sequencing results published in 2015 link dun color to the T-box 3 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 is thought to be embryonic lethal. 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". This phenomenon is new to science and has not been observed in rodents, primates, or carnivores.
The location of TBX3 expression may also determine the striping pattern of zebras.