Dun is a dominant dilution gene of equines characterized by lightening of the body color, leaving the head, lower legs, mane and tail undiluted. Dun is also typically characterized by “primitive markings” consisting of a dark dorsal stripe and sometimes leg barring, shoulder stripes and concentric marks on the forehead (spiderwebbing, cobwebbing). Dun with primitive markings is considered the “wild-type state” and is found in wild equids such as Przewalski horses, zebras and wild asses. The expression of the primitive markings (with or without dun) in the domestic horse is variable, with the dark dorsal stripe being the most consistent and visible feature. Dun dilutes both red and black pigment, and the resulting colors range from apricot, golden, dark gray, olive and many more subtle variations. Dun is present in many breeds of horses including (but not limited to) Appaloosa, Bashkir Curly, Iberian horse breeds (rare, except in Sorraias), Icelandic Horse, Mustang, Norwegian Fjord, Paint, Paso Fino, Peruvian Paso, Quarter Horse and several of the pony breeds. The names assigned to the various dun shades vary by breed.
A team of researchers has recently shown that Dun dilution results from radially asymmetric deposition of pigment in the growing hair controlled by localized expression of the TBX3 gene in hair follicles. The absence of Dun dilution (more circumferential distribution of pigment in the hair) results from a 1,617 bp deletion of DNA that impairs TBX3 expression in hair follicle. An additional SNP change was shown to govern the presence or absence of primitive markings. Three variants in DNA sequence explain phenotypes related to Dun dilution – D (presence of dun dilution and primitive markings), nd1 (not Dun-diluted; primitive markings are present but expression is variable), nd2 (1,617 bp deletion, not Dun-diluted, primitive markings absent). With respect to variant interactions, D is dominant over nd1 and nd2; nd1 is dominant over nd2.
The VGL offers a DNA test that will provide information for both dun dilution and the primitive markings. Below is a table with nomenclature for the variants and interpretation of results.
Dun dilution results are reported as:
|D/D||2 copies of Dun dilution detected.|
|D/nd1||1 copy of Dun dilution and 1 copy of nd1.|
|D/nd2||1 copy of Dun dilution and 1 copy of nd2.|
|nd1/nd1||Horse is not Dun dilute but may have primitive markings.|
|nd1/nd2||Horse is not Dun dilute but may have primitive markings.|
|nd2/nd2||Horse is not Dun dilute. Primitive markings are absent.|
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 [Epub ahead of print]
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