The Dun Factory
  Dun-factored Quarters and Paints
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Dun factor is a dilution gene.  It dilutes the coat color and adds primitive markings including a dorsal stripe and leg barring.

Dun Factor affects on different base coat colors:

Color of horse                            resulting color with dun factor added

BayDun with black points
Sorrel, chestnut          Red Dun
Black                                          Grullo
Palomino                                    Dunalino (dun factor will appear faded)
Buckskin                                     Dunskin or Yellow Dun (dun factor may appear faded)
Smokey Black                            Smokey Grullo (dun factor may appear faded)
Amber Champagne                    Amber Champagne Dun
Sable Champagne      Sable Champagne Dun
Classic Champagne    Classic Champagne Dun
Gold ChampagneGold Champagne Dun

Dorsal stripe and front leg barring are always* apparent in dun-factored horses.  Horses also carrying a cream gene may have a faded appearance to their dun factor markings.  Other dun factor markings that may be present are:
Cobwebbing on forehead
Bars over the eyes
Ear tipping or barring
Withers barring
Leg barring on hocks or inside hocks
ventral (belly) stripe

*Some grullo horses can be so dark that they appear black and it is very hard to see their dun factor. 
A dun cannot be produced unless at least one parent is a dun.

How to tell countershading from dun factor on a foal: A dun-factored foal will have a dark, prominent stripe down the back that has clear defined edges and runs from mane into the tail.  He will also have definite leg barring.
Foals with countershading do not have leg bars and the stripe down the back does not have clear defined edges and does not run from tail to mane (usually stops short of the withers).
Also the dorsal stripe should appear to "run" into the tail with lighter hairs the same color of the coat to either side of the stripe.
a division of Bluveld