The dun gene is a dilution gene 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-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 gray-gold or tan, characterized by a body color ranging from sandy yellow to reddish-brown. A dun horse always has a dark stripe down the middle of its back, a tail and mane darker than the body coat, and usually darker face 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 undiluted base coat color.
The dun allele is a simple dominant, so the phenotype of a horse with either one copy or two copies of the gene is dun. It has a stronger effect than other dilution genes, such as 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 when heterozygous is only visible on bay and chestnut coats, and then to a lesser degree.
The dun gene also is characterized by primitive markings, which are darker than the body color. Primitive markings include:
- Dorsal stripe (stripe down the center of the back, along the spine), seen almost universally on all duns
- Horizontal striping on the back of forelegs, common on most duns, although at times, rather faint
- Shoulder blade stripe, the least commonly seen of the primitive markings
Dorsal striping does not guarantee the horse carries the dun gene. A countershading gene can also produce faint dorsal striping, even in breeds such as the Arabian horse or the Thoroughbred, where the dun gene is not known to be carried in the gene pool. A primary characteristic of the dun gene is the dorsal stripe, and most duns also have visual leg striping. The shoulder stripes are less common and often fainter, but usually visible on horses with a short summer coat.