Equine Coat Color Genetics
Lesson Four
The Dun Dilution Gene
We have studied the cream dilution gene. There are four other genes known as of this
writing that fall into the category of genes that dilute: dun, silver, champagne,
& pearl.
The Dun Gene (D)
Although
there are other theories worth considering, dun is considered by most
authorities to affect coat color based upon the principle of simple
dominance. The
The dun
gene produces a dilution of coat color which used to be confused with that of
the cream gene. The terms "dun" and "buckskin" were
actually used opposite to their current usage only a few decades
decades ago.
However, there are important differences in the effects of dun vs.
cream.
Dun dilutes
both black and red pigment in body hair, like cream, and, like cream, it leaves
most of the hair in the points (mane, tail, lower legs) its original
color. However, dun also leaves behind
undiluted body-hair
markings that can be summed up in one word: stripes.
Red or bay body color is diluted by dun to a paler orange-red, yellow-red or yellow; solid black body color is diluted to a mouse-gray.
Undiluted chestnut
ee (any agouti) dd
Dun on chestnut/sorrel: “red dun”
ee (any agouti) D_
Undiluted (dark) bay
E_ A_ dd
Dun
on bay: "dun", or "bay dun", or sometimes "zebra
dun"
E_ A_ D_
Dun on brown is, as would be expected, usually a color
between grulla and dun. E_ At(At or a) D_.
Black
E_ aa dd
Dun on black: grulla, which is the Spanish word for crane,
and so is pronounced "grew-ya".
E_ aa D_
However, in
addition to pigment dilution, the predominant characteristic of the allele dun
(D) is the presence of a particular pattern which includes darker markings or
striping. Several of these factors are
often referred to as “primitive markings” since they are reminiscent of zebras
or Przwalski's horses.
It is
assumed that the dun gene does not cause these markings, but rather that
it leaves the original color of some hairs undiluted, producing these
patterns. It is further assumed that
this is a form of camouflage, like the stripes or spots on other
creatures.
These
markings can include a dorsal stripe, markings/striping on the legs, lines or shading extending
out from the spine from head to tail, on the backs of the ears, and some facial
markings. Striping on the barrel, chest,
and/or along and out from the central line of the abdomen (ventral) have also
been seen on duns.
Dorsal
stripe: a dark stripe
running from the poll to the dock of the tail (along the horse's spine). Dun horses are sometimes called
"line-backed". Though it may
have barbs extending off of it (also called "fishboning"),
it will have a clear (not blurry) quality to most of it.
It's
sometimes difficult to differentiate a dorsal stripe from another common form
of camouflage: a darker area along the spine, gradually fading into the body
color of the horse all along its length, which is a form of "countershading" or "sootiness" (both
concepts to be discussed later).
The dorsal stripe will be an undiluted color, whatever color that part of the horse would have been without the dun gene. On a bay horse, it may be red or black-looking, depending on the shade and sootiness of the base color. On a brown or black based horse, it will be black. When other dilution genes are present, it will be proportionately lighter.
Photos provided courtesy of Cedar Ridge QH www.grullablue.com
Leg barring:
horizontal to diagonal markings that appear on the legs, usually around the
knees and hocks. They may resemble tiger
or zebra stripes, or the rungs of a ladder.
They are usually, but not always, present on dun horses. Sometimes they are not visible on horses
whose body color was a very light shade to begin with, or those with
other dilution genes also present.
Sometimes they are present when each season's new coat of hair first
grows in, but then gradually fade.
Photos
provided courtesy of Cedar Ridge QH www.grullablue.com
Shoulder stripe or shading, fishboning, etc.: stripes or a dark-shaded streaks coming at right angles off the dorsal stripe, anywhere from the top of the neck to the tail Some are thin, some have multiple stripes, and some are just shadows. These are more pronounced and extensive in some duns than others.
Photo
Credit: Blazer Quarter Horses
Ear barring: striping on the backs of the ears. Look for a light area below the dark tips, followed by a horizontal dark area below, ideally followed by another lighter area below that. See these photos:
Photos provided courtesy of
Cedar Ridge QH www.grullablue.com
NOTE: Dark "rims" around
the ears (viewed from the FRONT) are NOT dun factor
markings. Though dun horses also have them (as part of the hair on the points
being left undiluted) these dark rims are found on bay, buckskin, etc. and are
not considered evidence of dun coloring.
Face mask: Dun horses may have a darker
color (undiluted by the dun gene) "mask" on the lower front of the
face. This is most common in the
black-based dun (grulla).
Photos
provided courtesy of Cedar Ridge QH www.grullablue.com
Mottling: Some duns will show mottling on the back of the arm, gaskin, stifle or shoulder. Mottling appears as dark splotches about the size of peas when viewed up close, or like tiny reverse dapples. Some even have this in the place of leg barring.
Photos
provided courtesy of Cedar Ridge QH www.grullablue.com
Neck striping: Similar to shoulder stripes, neck shadowing (when present) varies from definite striping to sooty-looking shading coming down off the top of the neck.
Photo
provided courtesy of Cedar Ridge QH www.grullablue.com
Mane & tail frosting: frosting of the mane & tail can be non-existent to very pronounced. Frosting is near-white coloring of the mane & tail guard hairs. The center part of the mane & tail are the color they would have been before the dun gene was added, but the hairs that run down the sides are silvery-white or flaxen. (This is different from the white "skunk tail" that is associated with rabicano coloring.) Frosting also occurs with the cream, champagne and silver genes, so it is not an indicator of dun.
Photos
provided courtesy of Cedar Ridge QH www.grullablue.com
Cobwebbing : concentric circles of stripes that resemble a spider's web on the forehead of a dun-factored horse. Can be extremely faint. Sometimes there is a dark spot in the center, like "the spider".
Photos
provided courtesy of Cedar Ridge QH www.grullablue.com
Dark lower
legs:
the lower legs will be darker than the diluted body color (remaining undiluted
from the dun gene).
Dark points on a red dun. Note that the lower legs, mane, tail, and face are not
diluted (lightened) by the dun dilution gene.
Courtesy of Cedar Ridge QH
Barbs (or fishboning) off the dorsal stripe: Some dorsal stripes will have barbs that extend perpendicular to the stripe and head down the rib cage. These are not always present, and vary in length and other characteristics. Note the irregular length and thickness of the “barbs” on the pictures below.
Photos
provided courtesy of Cedar Ridge QH www.grullablue.com
Homozygosity for
dun is considered to manifest identically to heterozygosity.
Foal (non-dun)
striping
This is a form of marking that is lumped together with pangare and counter-shading as a form of camouflage. It is not a true dilution, and is only
visible on very young horses. (You may recall that most newborn foal colors are
already much lighter than their adult colors.)
Some horses will retain some counter-shading for their whole lives, but
it is far less distinct and there is less stark contrast with surrounding
colors than with dun striping.
Photos
provided courtesy of Cedar Ridge QH www.grullablue.com
This is an
example of “foal-striping”, and shows how easily it can be confused with
dun. Even the most practiced eye cannot
tell the difference with absolute certainty.
As this foal got older and shed out the dun-like
markings disappeared.
The dun
gene can be present on any color of horse, or with any combination of other
color genes. There are, for example,
palomino dun (informally called dunalino), buckskin
dun (informally called dunskin) and even roan dun
horses. It is not unheard of to find
horses which carry the cream, dun, champagne and roan genes at the same time,
as well as other color-modifying genes we’ll discuss in future lessons.
Other descriptions of dun
"Claybank dun" was the Old-Timers' description of pale, orange-red dun. Some years ago, a Kiger breeder borrowed the term to describe his horses with extremely light body hair, regardless of their base color. However, this is not the original meaning of "claybank dun", and it turned out that most of those "claybank" Kiger horses were simply turning gray. "Claybank" is not used in professional or scientific circles for any dun coloring today.
Olive dun, lobo dun, lilac dun, silver dun... these are other creative terms for apparent horse colors that are produced by various combinations of genes, but they do not describe specific genetic color combinations. For example, a horse that is "nothing but" grulla (E_aaD_) may appear to have an olive cast, be unusually dark looking, have a lilac or even a pale silver sheen... or a cream or other gene may be causing one or the other effect. One mare discovered by this author was apparently a black-based dun plus champagne, and her husband was certain that she was a purple horse... in years past, perhaps someone would have called her a lilac dun. Most likely, her genetic makeup was actually classic (champagne) grulla. Sadly, she was never tested, her parents were unknown, and she was never bred, so no one will ever know for sure!
This
mare is believed to be champagne + dun on black, in a classic pose used to
check for true champagne coloring... "is the skin
under the tail pink with freckles?"
To read
more about dun colors, see more photos, and learn how some of the above facts
about dun were discovered, see http://www.dungenes.org/
(also by this author).
INSERT