Donkey Dun!

 Leah Patton writes:

"Dun" in donkeys is currently being studied.  The typical coloration of many donkeys is "Gray-dun" or "Slate gray".  Although it resembles grulla (dun on black) in some ways, the genetic action may be different.  Most donkeys, even those who appear "Black" have a dorsal stripe that runs from poll to tail.  Nearly all have a transverse stripe across the withers.  This is called the "Cross" in donkeys and mules.  The dorsal is solid, and unbroken except in the case of spotted donkeys, where the white masks out the underlying base color and cross area.  Some shoulder stripes will be WIDE (bold), some are pencil-thin.  The ends of the cross may taper, fade, or be dashed.  Ear marks, leg bars, inner leg zippers, dark anklet, collar buttons (at the throatlatch), ventral stripes, and sometimes barring extensions from the dorsal (ladder marks) are usually present on most animals.  

In donkeys, therefore, the presence of a stripe does not always mean a dilution, since it occurs on gray-dun, dark brown, sorrel (red), Light red (pink), and even near-black.    It is hoped that studying horse dun, sooty/countershading, and donkey dun in relation to each other may help determine genetic control for all.    

Below are three donkeys.  The first  shows a dark brown/gray-dun foal (sometimes called "Charcoal), with a distinct dorsal.  Bred to dark colors, this foal should produce mainly dark colors.  The second is a very dark brown which will shed off slightly darker, retaining the dark cross.  The third animal is frosted/spotted white.  This is an apparent combination of the Gray-ing type gene with spotted.  Note that the spotting pattern and cross are still visible even though the animal has grayed out. 

Dun and Mules

Mules can come in any color a horse or donkey exhibits, with few exceptions.  The tobiano horse pattern comes through incomplete, and there is no documented cream gene in donkeys, so cremello and perlino appearing mules are usually the result of other factors (such as spotting).  Dun is common in mules, especially from dun mares.  However, the cross and stripe can appear on mules from chestnut mares mated to "black" jacks.  This is leading to further research as to whether the cross is genetically present in even black-appearing donkeys.  Mules often exaggerate all of the dun factors, from wide shoulder stripes to zebra bars down the entire leg.