Dun gene

The dun gene is a dilution genes that affects both red and black pigments in the coat color of a horse. The dun gene has the ability to affect the appearance of all black, bay, or chestnut ("red")-based horses to some degree by lightening the base body coat and suppressing the underlying base color to the mane, tail, legs and "primitive markings."

The classic Dun is a grayish-gold or tan, characterized by a body color ranging from sandy yellow to reddish-brown. Dun horses always have a dark stripe down the middle of their back, a tail and mane darker than the body coat, and usually darker faces and legs. Other duns may appear a light yellowish shade, or a steel gray, depending on the underlying coat color genetics. Manes, tails, primitive markings and other dark areas are usually the shade of the non-diluted base coat color.

The dun allele is a simple dominant, so that the phenotype of a horse with either one copy or two copies of the gene is dun. It has a stronger effect than the silver dapple gene, which acts only on black-based coats, or the cream gene, an incomplete dominant which must be homozygous to be fully expressed, and is visible when heterozygous only on bay and chestnut coats, and then to a lesser degree.

The dun gene also causes "primitive" markings which are darker than the body color. Primitive markings include:


 * Dorsal stripe (stripe down the center of the back, along the spine)
 * Zebra stripes on the back of forelegs
 * Shoulder blade stripe

Dorsal striping does not guarantee that the horse carries the dun gene. The counter-shading gene can also produce faint dorsal striping in bays and chestnuts, even in breeds such as the Arabian horse or the Thoroughbred, where the dun gene is not carried in the gene pool. However, virtually all duns have the dorsal stripe, most have the leg striping, and the shoulder stripes are often fainter, but usually visible on horses with a short summer hair coat.

Shades of dun
The dun gene has a stronger dilution effect on the body than the mane, tail, legs and primitive markings, and so lightens the body coat more. This explains why points on a dun are a shade darker than the coat, or in the case of a "classic" dun, the mane, tail, and legs are often black or only slightly diluted.
 * Dun, also called Bay dun or "zebra" dun. The most common type of dun, has a tan or gold body with black mane, tail and primitive markings.
 * Red dun horses do not have black points, as there is no black on the horse to be affected. Instead, the points and primitive markings are a darker shade of red than the coat.
 * Grullo (GREW-yo, or Grulla, GREW-ya), also called blue dun or "mouse" dun, have a smoky, bluish, or mouse-brown color and can vary from light to dark. They consistently have black points and they often have a dark or black head, which is an identifying characteristic of the this group. The primitive markings are usually all black. Unlike a roan, there are 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 itself is one solid shade.

Dun vs buckskin
Since the dun gene can closely resemble buckskin when it is on a horse with an underlying bay color (black base + Agouti gene), it is frequently confused with buckskin. The difference between these two genes is that dun is a tan color, somewhat duller than the more cream or gold buckskin, and duns also possess the primitive markings. (Though a few buckskins do show a dorsal stripe.) To further confuse matters, it is possible for a horse to carry both dun and buckskin genes. A horse with golden buckskin coloring and a complete set of primitive markings is referred to as a "buckskin dun."

Breeding and the dun gene

 * Red (Chestnut base + Dun gene= Red Dun (horses do not have black points of the classic Dun).
 * Black base + Dun gene= Blue dun, mouse dun or Grullo/Grulla.
 * Bay (black base + Agouti gene) + Dun gene= Classic dun (sometimes called "Bay dun" or "Zebra dun", though such terminology is less accurate).

The three different dun varieties usually occur in proportion to the occurrence of the corresponding base colors in each particular breed. Other variations result from the interplay of additional genes. For example:
 * Bay + Dun + cream gene = "buckskin dun"