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.
A bay dun, also called a "classic" or "zebra" dun
A “blue” dun, or grullo
A red dun
Cave painting at Lascaux: Dun is thought to be a wild type coloration
A dilution gene is any one of a number of genes that act to create a lighter coat color in living creatures. There are many examples of such genes:
Two rats: left: agouti, right: coat lightened to blue by a dilution gene
Great Danes lightened from black to blue by the dilute gene.
Doberman: black with tan in the back, blue with tan in the front. The light brown areas were hardly lightened at all
Blue-light brown brindle dog