The dun gene is a dilution gene that affects both red and black coat color pigments. The gene is associated with "primitive marking" and 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.
The dark stripe down the middle of the animal's back is the most recognizable marking associated with dun horses. Other markings include a darker tail and mane, as well as a darker face and legs. The classic dun is a gray-gold or tan color, characterized by a sandy yellow to a reddish-brown body color. Depending on other underlying genetic coat color factors, a dun horse may appear as a light yellowish shade or a steel gray. Manes, tails, primitive markings and other dark areas are usually the shade of the non-diluted base coat color.
The Dun allele is dominant, meaning that a horse that carries either a single copy (heterozygous – D/d) or two copies of the gene (homozygous – D/D) will develop the dun colorations. Dun is unlike the silver dapple gene, which affects only black-based coats; or the cream gene, an incomplete dominant gene that requires two copies in order to be fully expressed. The dun gene affects both black and red-based horses.
The dun dilute gene is characterized by markings which are darker than the body color. These markings include:
- Dorsal stripe (a stripe down the center of the back, along the spine); these are seen almost universally on all duns
- Horizontal striping on the back of forelegs; these common on most duns, though at times rather faint
- Shoulder blade stripes; these are the least commonly seen of the primitive markings.
|Dun: Horse has two copies of the Dun mutation.
|Dun: Horse has one copy of the Dun allele and one copy of the nd1 allele associated with primitive markings.
|Dun: Horse has one copy of the Dun allele and one copy of the nd2 negative allele.
|Non-Dun: Horse has two copies of the nd1 allele associated with primitive markings.
|Non-Dun: Horse has one copy of the nd1 allele associated with primitive markings, and one copy of the nd2 negative allele.
Imsland F, McGowan K, Rubin CJ, Henegar C, Sundström E, Berglund J, Schwochow D, Gustafson U, Imsland P, Lindblad-Toh K, Lindgren G, Mikko S, Millon L, Wade C, Schubert M, Orlando L, Penedo MC, Barsh GS, Andersson L. Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses. Nature Genetics 2015 Dec 21. doi: 10.1038/ng.3475