There is now a genetic test for the Dun gene. It is available from U.C. Davis:
The Dun Gene
Dun is a dominant gene in the dilution category. (Remember, dominant means you only need one gene to get a visible effect, and dilution type genes are those that cause the color to be lightened in some way.) It is expressed on any base color, and there is no visual difference between a horse that is heterozygous or homozygous for dun. Because it is dominant, it cannot skip generations; a dun horse must have at least one dun parent. But there are occasions when the dun may not be visible. A horse that is also grey, for instance, will eventually look white, leaving no clue about its base color. Some pintos have so much white that the base color is impossible to determine. And some cases of multiple dilution genes make it hard to tell (i.e. a cremello or perlino may not show any visible signs of dun if they have it). In those cases, or if a horse could be homozygous for Dun, the test will be very handy.
The Dun gene is one that affects both red and black pigment equally, lightening red hair to a peachy or pale-red shade and black hair to a mousy-grey or almost blue-grey tone. The skin and eye color are not diluted. The dilution effect is thought to be caused by the pigment being restricted to one side of the hair shaft, rather than an actual lightening of the pigment itself, but this theory is not proved yet. Dun is unlike any of the other dilution type genes in that it is "location-specific"; that is, it dilutes the hair on the body, but leaves the points (lower legs, mane, tail) undiluted. It also is unique in that it causes various markings that are collectively referred to as "dun-factor" or "dun-markings". These include a dorsal stripe, striping on the upper legs, "frosting" on the outer edges of the mane and tail, "cobwebbing" (concentric circles of darker hair) on the forehead, white eartips, "zippers" (thin stripes of lighter colored hair running vertically down the back of the lower legs), and dark shading or striping over the withers and/or lower neck. There is often a darker "masking" on the face. All duns will have the dorsal stripe and leg barring at least, but the other markings are variable from one individual to another. Not all duns will have all of the markings, and non-duns can have some of them. Any color can have a dorsal stripe (believed to be caused by sooty or countershading) but a non-dun dorsal stripe has a different "look" and "character" to it than a dun dorsal stripe. It certainly can take some experience to tell then apart, though, and not all horses "follow the rules", so it's nice that there is now a test to tell dun from non-dun in those confusing cases.
Duns of all shades do not generally get dapples like the kind that appear on other colors. It is quite rare to find a dun with dapples (and most of those that do have them are both cream and dun). Duns, like champagnes, sometimes get "reverse dapples" -- darker spots rather than lighter ones, but these are not overly common either. Duns also, like champagnes, do not show "sooty" tones which are common on cream-dilutes and nondilutes. The theory is that since the gene dilutes both black and red pigments, the black "sootiness", if present, would be diluted enough that it would not stand out visually. Although dun shades, like any color, can range from light to dark, the dilution cannot be "hidden" by black hair the way a cream-dilute can.
Dun on Chestnut/Sorrel
Dun on a chestnut base color is called Red Dun. They can range from a distinctive light peachy or apricot tone to a darker shade that could pass for a sunbleached chestnut at first glance. The points are left undiluted, so are whatever shade of red the horse would have been if it didn't have the Dun gene. The dun-factor markings are also red.
Dun on Bay
Dun on a bay base color is variously called Dun, Bay Dun, Zebra Dun, Yellow Dun, and sometimes less common terms like "peanut-butter dun" (a pretty accurate description of the typical body color) and "buckskin dun" (meaning a dun that looks like a buckskin; not a genetically correct term -- if taken literally, that term would mean a horse with both a cream gene and a dun gene). They look similar to a buckskin, with yellowish bodies and black points. But the body tone generally tends to be more "flat" or "earth-toned" and less golden than a buckskin. Since both colors can have a wide range of shades, the best way to tell them apart is to look for the dun-markings. Although a buckskin may have a dorsal stripe, strong leg barring is diagnostic of dun. (In cases where there is still some doubt, genetic testing can now sort out the buckskins, duns, and dunskins.) The dun-factor markings on a bay dun will be whatever color the hair would have been in that place without any dilution -- typically a dark red for the dorsal stripe and any wither/neck stripes/shadows, and leg barring that is dark red higher up the leg and black closer to the knees/hocks.
Dun on a black base color is called Grulla (sometimes spelled grullo), or in some places, Black Dun. They have a greyish body color, but unlike a grey, which when examined closely is a mixture of dark and white hairs, a grulla's hairs are all the same greyish color. It can tend toward a tannish shade in some horses but is usually a "cool" tone tending more toward bluish. A sunbleached smoky black can do a very good grulla imitation, but they are usually more yellowish in tone. The grulla's dun-factor markings will be black.
Dun on Brown
Dun on a seal brown base color does not have a specific official name (and is not recognized as a separate color by any registry), but is generally called Brown Dun by those who have one. The ones on a darker base shade would look like a grulla with the tan muzzle of a seal brown, but would most likely be registered as grulla. The ones on a lighter base shade would look like a darker shade of bay dun, and these have been called "Lobo Dun" in some places. In the past these would have been hard to identify, but now that there is a test for Seal Brown, they can be distinguished from other duns.
Dun combined with other dilution type genes
When a dun also happens to have a Cream gene, the colors are called Dunalino (on chestnut), Dunskin (on bay), and Smoky Grulla (on black). (Most registries, though, do not recognize or understand these colors, and will simply call them palomino, dun, etc.) Typically these are lighter in shade than a horse with only one or the other gene, but not always. Some dunalinos are the same shade as an average palomino, with only the dun-factor markings to show they have a Dun gene as well. While the average dunskin is somewhat lighter than the average buckskin, since both have a wide range of shades from light to dark, many dunskins simply look like a buckskin with dun-markings. There is usually no visual difference between a grulla and a smoky grulla. Horses with two Cream genes and at least one Dun gene don't look much different than their counterparts without a Dun gene. They will have the dun-factor markings, but they can be so light that it is very difficult (or even impossible) to see them. Some perlino duns have a noticeable dorsal stripe and leg barring in a slightly darker shade than the body color, but some lighter ones, and most cremello duns, have very faint striping.
When a dun horse also happens to have a Champagne gene, the colors are called Gold Dun (on chestnut), Amber Dun (on bay), Classic Dun (on black), and Sable Dun (on brown). Typically these horses look like lighter versions of the champagne color they would have been without the dun. (See the Champagne article for more detail.)
There have not been enough documented examples of dun horses
who also have a Silver gene to say for sure what they would look like (although
the Silver would have no visual effect on a red dun). Likewise with Pearl; there aren't enough of them identified
yet to know for sure, although it is thought that one Pearl gene would have
little or no effect and two genes would probably react similarly with dun as a
Cream gene would.
Breeds that have Dun
Not all breeds have the Dun gene. Some of the most common breeds do not have dun, most notably
the Thoroughbred and Arabian, and most Warmbloods.
Even in breeds where it occurs, it is not a very common color.
Many of the American breeds do carry dun, including the QH and derivative
breeds (Paint, Appy, POA) and most of the gaited breeds (ASB, TWH, MFT, Paso)
although it is very rare in those. Note
that in many breeds it is quite common to call buckskins "dun" and
therefore you should never presume a horse registered as "dun" is
actually dun, especially in British breeds (i.e. Connemara, Welsh).
There is a very good educational website about the Dun gene
and the various dun-dilute colors at: http://www.duncentralstation.com/
which is highly recommended to anyone wanting to learn more about the
color. You can see pictures of many
horses showing a variety of colors and shades.
There are also closeups of all the dun characteristics, which is helpful
in identifying them.
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