Dun

Dun is the original coat color of all horses (wild-type). Over time, two mutations (nd1 and nd2) caused the now wide-spread undiluted phenotypes most modern horses display. Horses with one or two copies of this variant (D/D, D/nd1, D/nd2) are Dun and show the diluted coat color and the primitive markings (zebra stripes, dorsal stripe) on their legs and back. A dun horse must have at least one dun parent.
Nd1 (non-dun1) leaves the gene partially functional and allows a non-diluted coat color but may keep the primitive markings. Nd2 (non-dun2) switches the gene off completely and the horse displays a non-diluted coat color without primitive markings - this is the most common phenotype in modern breeds.