Wild horses come in a variety of coat colors, but through domestication most have lost their original wild Dun coats of pale hair and zebra-like dark stripes. To better understand this Dun pattern researchers from Uppsala University, Sweden, partnered with the Huntsville Institute of Biotechnology, Alabama, and Texas A&M University to analyze the distribution of pigment in individual hairs. 

"Dun is clearly one of the most interesting coat color variants in domestic animals because it does not just change the color but the color pattern," Leif Andersson, one of study researchers from Uppsala University, explained in a news release. "We were really curious to understand the underlying molecular mechanism why Dun pigment dilution did not affect all parts of the body."

"Unlike the hair of most well-studied mammals, the diluted colored hairs of Dun horses are not evenly pigmented the whole way around. They have a section of intense pigmentation along the length of the hair, on the side that faces out from the body of the horse, whilst the rest of the hair has more or less no pigment," Freyja Imsland, lead author and Ph.D. student, said in Uppsala University's news release. "The hairs from the dark areas of Dun horses are in contrast intensely pigmented all around each individual hair. In spite of scientists having studied hair pigmentation in detail for a very long time, this kind of pigmentation is novel to science, and quite unlike that seen in rodents, primates and carnivores." 

Based on a genetic analysis and DNA sequencing, researchers found a single gene that codes for the T-box 3 (TBX3) transcription factor that determines Dun versus non-Dun coloring. Horses that have lost their Dun color have a TBX3 mutation.

Furthermore, researchers discovered two forms of dark, non-Dun color – non-Dun1 and non-Dun2 – caused by different mutations. While the non-Dun2 variant occurred recently, most likely after domestication, the Dun and non-Dun1 variants predate domestication, researchers say. 

Researchers believe their findings could explain zebra stripe patterns where TBX3 is expressed across their entire bodies. 

The study was recently published in the journal Nature Genetics. 

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