Equine Coat Color Genetics

Lesson Ten

Miscellaneous

There are several horse colors and patterns which don’t technically fit this course because the genetic mechanism for them is unknown. But it is interesting to take a look at them anyway. Perhaps some day the genes will be found, and we can update this section.

Shade, pangare, sooty, countershading

We looked at these a little bit in the section on base colors. Shade refers to how light or dark the color is, for example a chestnut can be so light as to appear palomino, or a deep rich burgandy red. No genetic difference has been found in such widely varying shades. It is most likely a multi-gene influence, and there can be some amount of environmental influence as well (such as mineral levels in the diet).

Pangare, also called mealy, is the extra-light color on the muzzle and underside (chest, belly, inside hind legs) which is fairly common in many draft and pony breeds, and less common and less extensive but still found in many horse breeds. In the past it was theorized to be caused by a single recessive gene, but no such gene has yet been found, and examples of two parents without pangare producing a foal with it and vice versa, two parents with pangare producing a foal without, would suggest it is not one simple gene.

Sooty (occasionally also called smutty in the past – not much anymore) refers to an overlay of black hairs, even on a red base color (“e/e”) which theoretically should not be able to make black pigment. It can range from a little bit to covering the entire horse, and when extreme it can make a horse appear black even if it started out palomino. It often causes vivid dapples, and it often comes and goes with the seasonal shedding. It is believed to be a single gene, but has not been found, so may be a multi-gene feature. If it is one gene, it has a very wide level of expression just like some of the white spotting genes do.

Countershading is a term that has been applied to both lighter undersides and darker areas over the topline (or both). These type of shadings are very common on foal-coats and in draft and pony breeds. Some people use the term countershading to refer to a dorsal stripe that is not caused by the Dun gene. It is unknown what causes these shadings. (Some non-Dun dorsal stripes are caused by the newly discovered “nd1” allele, but not all.)

Light and Dark body marks

There are various types of spots and markings both light and dark that can appear on any color coat. Some are a “somatic mutation”, that is, a mistake (mutation) in the DNA for just that one part of the body on that one particular horse. It’s not part of their overall DNA and would not be passed on to any offspring. Some white and dark spots are caused by scars. And some white or light spots are caused by a skin fungus. But others appear to have at least some kind of genetic basis, since they are known to run in families.

“Bend Or spots” are dark or black spots, usually round or oval with smooth edges, not generally jagged or lacy, which range in size from about dime-sized to egg-sized, occasionally larger or smaller. Like sooty, they can put black hair on even a red-based color. They are especially striking on a light palomino. The name comes from a Thoroughbred stallion that apparently had them or was known for passing them on. (Side note: you will see his name misspelled in many different ways on the internet!) A horse can have just one spot or so many that it looks almost like a leopard appaloosa. This trait does run in families so may be a single gene that might be found some day.

“Birdcatcher spots” are white spots, usually round, generally from pea-sized to silver dollar-sized, which can occur on any color. Like Bend Or spots, they can be present in various numbers, from one to many, and tend to be scattered irregularly over the body, and can increase in number over the years. They are also named after a Thoroughbred stallion who was known for them. They have popped up in most every breed, and seem considerably more random, so if they are caused by a single gene, it is likely a recessive one, or one with an extremely wide range of expression.

“Tetrarch spots” are another type of body spots named after a Thoroughbred stallion. In this case they are usually oval, various-sized light spots that occur on greys. Therefore, it is a temporary color that will vanish as the grey goes white. (You can look up images of The Tetrarch or his daughter Mumtaz Mahal to see examples of this kind of spotting.) However, it does appear very randomly in any breed, so is most likely not genetic, but just an unusual expression of the greying process in some horses.

“Brindle” is a term referring to dark, narrow vertical stripes all over the body. They are not sharp/straight like a zebra’s stripes, but rather irregular with “wavy” edges. There is no Brindle gene in horses, as is found in dogs and cattle for example, but there have been cases of a pattern that looks the same popping up in horses. In some cases these have been found to be chimeras (a good article explaining this is here: http://www.thehorse.com/articles/25872/one-in-a-million-part-1 ), which would not pass on the color to their offspring. However there was another case which was apparently caused by a skin disorder, and was inheritable. (See: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0081625#pone-0081625-g002 ) There is another brindle type mare who has passed on her coloring (http://brindlehorses.com/battyatty/webped/starfoal.htm ). There have also been cases of horses with the Dun gene showing some areas of brindle-like stripes, but not all over their bodies. It is not known whether this is caused by the Dun gene or something else. There are not enough of them to know if there is a pattern in the inheritance of it.

“White striping” is a term referring to stripes that look very similar to brindle stripes, only white instead of dark. (It’s not uncommon to find these incorrectly labeled brindle.) These are believed to be a somatic mutation. They generally don’t cover the whole body, and they are not passed on. Rabicano patterned horses often have striping much like this over the ribs, but their stripes tend to be more roany than stark white, and more regular and evenly spaced.

“Bloody-shoulder” marks, also sometimes called “blood marks” are dark (base color) spots on a grey horse that stay dark instead of turning grey with the rest of the coat. They become more obvious with age as the rest of the horse turns white. They can be any size, and occur on various parts of the body, although the neck and shoulder are the most common places. It is not believed to be genetic since it pops up randomly on any breed of grey horse, and doesn’t appear to run in families.

“Mismarks” is a term commonly used for any random white or grey or dark patches on non-grey horses, or patches of nondiluted color on a dilute color horse, or dark patches within an area that is “supposed to be” white, as on an appaloosa. These are almost certainly caused by a somatic mutation. The term “Calico”, while not traditionally a horse color term, has been used by some to refer to a pinto horse that is a dilute (usually palomino or buckskin) with patches of darker, nondiluted (red or black) hair. If you google the term “somatic mutation” and “horse” you will see many examples, some more accurate than others, of course, but you can probably tell the difference by now. J

This palomino mare has small Bend Or spots scattered over her body, and also a dark spot on her shoulder that doesn’t sun bleach over the summer like the rest of her coat.

White mismarks are sometimes called “oops spots”.

Odd white markings

White markings on the face and legs are very common in most breeds. Some discourage or even prohibit them, so the incidence has gone down dramatically in those breeds, but not disappeared altogether. As we discussed in the pinto section, there are many different mutations that cause various white spotting patterns, only some of which have been found. “Ordinary” white markings are almost certainly not going to turn out to be caused by just one gene (although if they were, the W20 gene would be a likely one). And it’s possible that the various “oddities” are only different manifestations of the same genes that cause the “ordinary” markings. But some of them are so different from the usual stars, blazes, socks, etc. that it’s easy to think there must be something else going on there. At this time, that is unknown.

“Badger face”, also called “reverse blaze” is a term used for a horse with dark down the center of the face where the white would be on a blaze. It is not known what causes this, but they seem to usually occur on horses that have one or more of the overo patterns.

Examples of “reverse blazes” or “badger faces”

Unusually irregular and/or jagged/lacy/speckled-edged markings: These usually go along with sabino-type patterns of white. Most experts feel that this sort of marking is probably caused by something different, genetically speaking, than the “normal” looking white face and leg markings, but it may be many slightly different genes, or random chance, or even environmental influence. For now it is unknown.

Dark spots within white markings: unusual and interesting, but probably not genetic.

Dark spots along the coronet band within white leg markings are called “ermine spots”. They occur on any color, and are not a sign of a pinto gene, although some breeders believe they are more common on tobianos. There can be one spot or many. There is usually a dark stripe in the hoof going down from each spot (normally hooves are pale under white skin). The spots will be whatever color the hair on the leg would be at that place if there wasn’t a white marking (i.e. red on chestnuts, black on bays).

“Belton” spots

This is a term used in dogs, to refer to small round spots of the base color that appear in the white areas. They are similar to “cat tracks” or “pawprints” that appear in the white areas on most homozygous tobianos, but much more uniform in size and spacing. At least one horse color expert believes that horses have a similar gene. You can read the article here: http://equinetapestry.com/category/belton-patterning/

Eye Color

There have not been any genes found in horses yet that affect only the color of the eyes, not the coat. However, there are plenty of horses out there that have blue eyes without any white spotting gene to explain them, or light colored (amber, hazel, green) eyes without any kind of dilution gene. Usually these pop up randomly, but in some cases they tend to run in families, making it more likely there is a genetic cause.

Black horse with no dilution gene, but light brown eyes

Horse with no white markings, but a partially blue eye.

Unexplained diluted colors

Occasionally we come across a horse that is lighter than usual, but for no apparent reason. There is really no way of knowing whether it’s something genetic, but it’s interesting. There are some horses that test as plain black, with no known dilution genes present, but show a persistent light chocolate color (not just late-summer sunbleaching, which is common in black horses). While not overly common, it has occurred enough that it has come to be called “light black”. This Morgan named Prophet has always been this color from birth. He has tested negative for both Cream and Silver.

There is a family of Shetland ponies that at first were thought to be a somewhat atypical expression of the Silver gene, but they all tested “ee” so cannot be Silver. The name “Mushroom” was coined for their color. There is more info on this color here: http://www.kellas-stud.co.uk/mystery.htm

There is a family of Morgans which have an unexplained dilution which is detailed here: http://www.morgancolors.com/oddities.htm

There is also a website that exists specifically to document and try to explain some of these unknown dilution type colors: http://www.new-dilutions.com/

When this site was first established, the Pearl gene fell into that category. Through research by this group and others, the gene was eventually discovered.

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