Red dun horses have coloring resulting from the action of two different genes. One gene is recessive and prevents production of black pigmentation but allows red pigmentation. The other dominant gene dilutes the basic coat color to a lighter red with primitive markings in a darker shade.
Primitive markings include a dorsal stripe, horizontal stripes on the backs of the forelegs and a shoulder blade stripe. The dorsal stripe is seen on virtually all duns and the leg striping is common. The shoulder stripe is less common and is usually fainter than the other markings when present. The mane, tail and lower legs are not usually affected by the dun gene and show the base coat color.