The Dun Gene is easily the most misunderstood of all the color genes. Buckskin and Dun are not the same thing. A buckskin with a dorsal stripe does not automatically become a dun; they are two separate genes, though a horse can have both, and horses without the dun gene can be linebacked. Like cream, dun is dominant: a horse must have a dun parent in order to be dun itself.
Dun is considered a “primitive” color, and many wild and feral horse populations are exclusively or almost all dun: Sorraias, Tarpans, and Przewalski’s horses to name a few. Norwegian Fjords are all dun, though with different base coats.