OMIA 001972-9796 : Coat colour, dun in Equus caballus

Mendelian trait/disorder: yes

Mode of inheritance: Autosomal Dominant

Considered a defect: no

Key variant known: yes

Year key variant first reported: 2016

Species-specific description: "Dun is a wild-type coat color in horses characterized by pigment dilution with a striking pattern of dark areas termed primitive markings." (Imsland et al., 2016). These primitive markings were discussed at some length by Darwin (1859) (chap 5, pp. 163-167).

Inheritance: By studying the results of matings in Icelandic toelter horses, Adalsteinsson (1978) concluded that "a dominant dilution gene, D, converts bay to yellow dun with dark mane and tail, chestnut to yellow dun and dun mane and tail, and black to blue dun (mouse, grullo". In contrast, "The palomino gene, c cr . . . is hypostatic to black and blue dun. In heterozygous form, c cr converts bay to buckskin, and chestnut and sorrel to palomino, and results in blue-eyed white when homozygous."

Mapping: Imsland et al. (2016) "mapped the Dun locus to a region on horse chromosome 8 (chr. 8: 18,061,745–18,482,196) using microsatellite markers and then fine-mapped the locus with a 27-SNP panel to a 200-kb region containing only one gene, TBX3"

Molecular basis: Imsland et al. (2016) showed that "pigment dilution in Dun horses is due to radially asymmetric deposition of pigment in the growing hair caused by localized expression of the T-box 3 (TBX3) transcription factor in hair follicles, which in turn determines the distribution of hair follicle melanocytes." In contrast, "Most domestic horses are non-dun, a more intensely pigmented phenotype caused by regulatory mutations impairing TBX3 expression in the hair follicle, resulting in a more circumferential distribution of melanocytes and pigment granules in individual hairs" (Imsland et al., 2016). These same authors "identified two different alleles (non-dun1 and non-dun2) causing non-dun color. non-dun2 is a recently derived allele, whereas the Dun and non-dun1 alleles are found in ancient horse DNA, demonstrating that this polymorphism predates horse domestication" and concluded "These findings uncover a new developmental role for T-box genes and new aspects of hair follicle biology and pigmentation".

Associated gene:

Symbol Description Species Chr Location OMIA gene details page Other Links
TBX3 T-box 3 Equus caballus 8 NC_009151.3 (20646967..20661149) TBX3 Homologene, Ensembl, NCBI gene


Note: the references are listed in reverse chronological order (from the most recent year to the earliest year), and alphabetically by first author within a year.
2016 Imsland, F., McGowan, K., Rubin, C.J., 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, M.C., Barsh, G.S., Andersson, L. :
Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses. Nat Genet 48:152-8, 2016. Pubmed reference: 26691985. DOI: 10.1038/ng.3475.
1978 Adalsteinsson, S. :
Inheritance of yellow dun and blue dun in the Icelandic toelter horse. Journal of Heredity 69:146-148, 1978. Pubmed reference: 731005.
1859 Darwin, C.R. :
On the Origin of Species by Means of Natural Selection John Murray, London :, 1859.

Edit History

  • Created by Frank Nicholas on 22 Dec 2015
  • Changed by Frank Nicholas on 22 Dec 2015
  • Changed by Frank Nicholas on 06 Aug 2016