The dun gene is a dilution gene that affects both red and black pigments in the coat color of a horse. The dun gene lightens most of the body while leaving the mane, tail, legs, and primitive markings the shade of the undiluted base coat color. 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. Manes, tails, primitive markings, and other dark areas are usually the shade of the undiluted base coat color. The dun gene may interact with all other coat color alleles.
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 (like the striped leg barring) extend across the entire body.
Neither the non-dun1 nor the non-dun2 mutations were found in any other equids.
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 (bay, classic, or zebra dun), black (mouse dun or grullo), and Chestnut (red dun). 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 or grulla, also called blue dun or mouse 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 (single copy) = "dunalino" or "palomino dun" Bay + dun + cream gene (single copy) = "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 (cremello, perlino, and smoky cream). 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.