Origin
The dun coat and attendant primitive or dun factor markings reflect the wildtype coat and are observed in all Equine species. Cave paintings depict horses as being dun and with the primitive markings. The last remaining true wild horse, Przewalski's Horse, too is dun-colored with primitive markings. So too are horse breeds such as the Konik and the Heck horse, "bred back" to resemble the now-extinct Tarpan, many of which are grullo or mouse dun in color.
While no dun horse is without a dorsal stripe, primitive markings also sometimes occur on non-dun horses, particularly those with sooty characteristics and newborn foals. Primitive markings in horses are an example of atavism: preservation of or reversion to ancestral type. While primitive markings are closely linked with the dun coat colors, the variations of expression and presence in non-dun horses suggest that the markings themselves may be governed by a separate genetic mechanism.
Read more about this topic: Primitive Markings
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