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.

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.

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 (mouse dun), and Chestnut (red dun). It is more difficult to recognize when combined with other dilution genes or if affected by gray. Shades include:

Another characteristic of the dun gene are primitive markings. Dun traits include the following:

Other variations result from the interplay of additional genes:

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.