http://www.designsforequines.com/files/Download/dungene.pdf
This site has an excellent description of the dun gene and it's effects on the coat color, as well as photos of the primitive markings. Two important points to remember (1) the dun gene is dominant, either the sire or the dam must be a dun in order for a foal to be a dun and (2) there must be a prominent dorsal stripe.
Some horses exhibit a fake dorsal stripe called countershading - information on countershading can be found here: http://www.duncentralstation.com/LookALikeColors.html
Cedar Ridge Quarter Horses has extensive information on dun factor and countershading.
There is currently a dun zygosity test available through UC Davis, but they are still working to isolate the actual dun gene.
. Please visit