Horse Color Summary 1

As a final summary of horse color genetics, let’s go over the loci, what they do, and the alleles at each locus. My primary reference is Sponenberg.

The Agouti locus is widespread in mammals, and is involved with whether and where an animal produces eumelanin (black) or phaeomelanin (red) pigment. The alleles known in horses, listed with the most dominant first, are Wild Bay (Wild-type), Bay, Seal Brown and black. Agouti is hypostatic to Extension, meaning that the effects of the agouti alleles can be seen only if the extension gene allows the animal to produce both eumelanin and phaeomelanin. Note that at this locus, the redder the color, the more dominant.

The Extension locus is the same as the melanocortin receptor one locus, or MC1R. Like agouti, it influences whether eumalin or phaeomelanin gets into the coat and occurs in most mammals. The alleles are dominant black (still not confirmed), wild-type, and chestnut. This locus may also have genetic control over the depth of black tipping. Only wild-type and tipping allow the agouti genes to show. In this series, more black is dominant over more red. Extension is epistatic to agouti.

Agouti and extension determine the base color of the horse—bay, brown, black or chestnut.

The various dilution genes generally affect phaeomelanin and eumelanin differently, mane and tail hair and body hair hair differently, and not uncommonly are associated with patterns of dilution.

The Dun locus has two alleles. Wild-type is dun and is dominant over non-dun, but the wild type is rare in many breeds. When present, dun dilutes both black and red pigment on the body, but the degree of dilution varies a great deal. Head, legs, mane and tail are generally much less affected than is the central body, and dorsal stripes almost always occur. “Zebra stripe” markings often occur on legs and the shoulder region. The dorsal stripe may continue down the center of the mane and tail, with the edges diluted.

Gus

The Cream locus is also known as the membrane-associated transporter protein (MATP) locus. It probably has three alleles: Wild-type, pearl, and cream. The dominance hierarchy here is complex. A horse with two wild-type alleles is normal color. A horse with one wild-type and one pearl allele looks normal color except for slightly lighter skin. A horse with two pearl alleles will have red lightened to gold and black lightened to beige. A horse with one cream allele and one wild-type allele will have red lightened to gold and black lightened only very slightly. A horse with one cream and one pearl allele will have red lightened to pale cream or ivory and black lightened to beige. Finally, a horse with two cream alleles will be a very pale color, as red lightens to cream and black to a slightly dirty white.

The Champagne locus is the SLC36A1 locus. It has two alleles: Champagne (dominant) and wild-type. Champagne dilutes red to gold and black to brown or tan. The mane and tail are generally diluted less than is the body.

The Silver Dapple locus is the pre-melanosomal protein 17 (PMEL17) locus. It has two alleles, silver (dominant) and wild-type. The silver allele dilutes black strongly but has little or no effect on red. The allele also produces very strong dilution in mane, tail and lower legs, at times producing horses that appear black with white manes and tails. Far commoner are horses with a blue to chocolate body, often heavily dappled, with distinctly lighter manes and tails. At one time common primarily in ponies.

The Mushroom locus has not yet been located. Two alleles are suspected, wild-type (dominant) and mushroom (recessive.) Mushroom horses resemble silver dapples, but lack dappling and have tested chestnut at the extension locus.

Arab dilution is another possible locus. This is believed to be a recessive allele with a strong lightening effect on black but little or no effect on red. Both Mushroom and Arab dilution are very rare.

I will summarize patterns of white, including grey and roan, next week.

Jarn’s Journal Year 5 Day 85

Year 5, Day 85

This year it is much clearer why the People do not stay here permanently.

Last year the final group hunt was highly successful, and I had to modify the heat pump to keep some of the excess meat frozen. As a result, the fish from the lake and Giraffe’s hunting with Patches were more than enough to keep us fed.

This year the final hunt brought in almost nothing, and the group dispersed early.

I followed one of their hunts, mostly flying over them, and for the first time realized that their upright stance, together with their ability to sweat freely, actually helps them hunt. Not just ability to wield spears, not just being able to see farther, but endurance.

Ever watch a four-legged animal run? They contract and stretch their bodies, and pump their lungs in the process. Breathing speed is tied to running speed. That’s not true for two-legged runners, and while two legs are not as fast as four, they can keep going a lot longer. I wonder if my own race evolved an upright stance for the same reason?

The People need a group with at least one expert tracker to keep after a single animal until it is tired, which is why Giraffe by himself cannot keep us fed. With Patches, he is able to keep track of a single animal and wear it down, and Patches is also good at picking the weakest of a group to follow. But it is Giraffe’s ability to run for hours, carrying water to avoid dehydrating himself, that allows him to chase an animal to exhaustion.

All of this, of course, assumes that there is an animal to chase down, and right now there isn’t. Luckily I can teleport to areas where game is plentiful, find a pride of lions hunting (usually at night) and teleport a quarter of zebra or wildebeest away from them once they’ve made a kill. With the modified heat pump, I can freeze the meat and only have to “hunt” about once each two fivedays. I still have too much empathy for the prey animal to make a kill myself, but we are eating quite well – well enough that I think I can resume my mapping.

Jarn’s Journal is a fictional journal of a fictional human-like alien stranded on Earth about 125,000 years ago. The entire Journal to date can be found at my Author site.

The Leopard Gene in Horses

This information was initially blogged, without photographs, on March 29, 2011.

All genes for white markings produce a wide range of amounts of white. The leopard (Appaloosa) gene produces not only a wide array of amounts of white, but also of patterns. Unlike other spotting patterns, it is often progressive with age.

Because the patterns produced by the leopard gene vary so much, I will spend more than one week on them. This week, I will focus on breaking the patterns down into components, following Sponenberg, and commenting on their distribution and genetics.

In the United States, the leopard gene and the patterns it produces tend to be associated with specific breeds, notably the Appaloosa and Pony of the Americas breeds. The Colorado Ranger and the mustang often exhibit the leopard complex colors, as well.

Worldwide, however, the leopard complex patterns are very widely distributed throughout Europe and Asia as well as the Americas. Further, most breeds which have any of these patterns have all of them—a further indication that a single gene is necessary. The only exception at the current time is that a second gene locus, Pattern-1, may be needed to produce the full leopard pattern. A number of other modifiers probably exist, but they are not known. None of these modifiers, however, seems able to do anything without the presence of at least one Leopard allele.

Genetically, the Leopard allele is one of two possible alleles (the other is wild-type) at the Transient Receptor Potential Cation Channel, Subfamily M, Member 1 locus, thankfully abbreviated to TRPM1. This locus is on equine chromosome 1. Leopard is incompletely dominant over wild-type. The locus is called Lp, and the alleles are Lp^Lp (Leopard) and Lp⁺ (wild-type.)

Pattern-1 has not been located exactly, but it may be linked to the Extension locus (determines chestnut) on equine chromosome 3. Pattern-1 increases the amount of white in the coat and is necessary for full expression of the leopard pattern (not to be confused with the Leopard gene.) Yes, the terminology is confusing!

The white sclera and mottled skin show clearly on this POA, which also displays varnish marks.

The first set of characteristics produced by the Leopard allele includes mottled skin, striped hooves, and a white sclera in the eye. White ear tips can also occur. These characteristics are not definitive, as other color genes may cause them, but almost all horses with the Leopard gene show at least one of them.

Horses with the Leopard gene may show other white markings, including the normal face and leg markings. If the leg markings are not present, white may still show on the cannon bones in what are generally called lightning marks or lightning stripes.

Another thing the Leopard allele may do is to introduce interspersed white hairs in either of two patterns. Frost gives a fairly uniform distribution of white hairs over the body, most prominent over the hips and in minimal cases only over the hips. Unlike classic roan, the roaning develops after birth and increases with age up to a point. The dark head and legs of classic roan are generally not visible in this pattern. Unlike grey, the horse eventually reaches a relatively stable color.

Snowflake has a similar developmental pattern, but is most prominent on the foreparts and the white hairs are concentrated into small white spots.

Extreme frosty or snowflake patterns may develop into a speckled appearance, white with small colored areas. All leopard-complex roans may also have varnish marks, with areas over bony prominences (notably the nasal bones and hips) retaining dark pigment.

The Leopard allele may also produce larger but symmetrical white markings, generally starting with a few small white areas over the hips and working forward and downward until the whole horse is white, with the flanks and throat being the last areas to lose color. This is the pattern most strongly influenced by the pattern-1 gene. If the pattern-1 allele is present, white is more extensive than if it is not present. Full white is only possible with the pattern-1 allele. These symmetrical white markings are usually present at birth, though they may increase with age.

The Leopard allele can produce colored round or oval spots over the body. In most cases, these are visible only against a roan or white background, but occasionally they can be seen against pigmented areas of the coat. The spots may be darker or lighter than the base coat color.

Surprisingly, these spots are more likely and more numerous if the horse has one Leopard allele and one wild-type allele. If the horse is homozygous for Leopard (has two Leopard alleles) the spots are more likely to be absent or sparse.

Finally, two doubtful or deleterious aspects of the Leopard allele may be noted. First, leopard interacts with black-pigmented hair to make it brittle. The result is the sparse manes and rat tails often seen on leopard-complex horses whose base color is black or bay and who retain dark color in their manes and tails.

Second, homozygotes for the Leopard allele are generally night blind. This is rarely a problem with modern usage of horses, but should be kept in mind if riding a homozygous  Leopard over unfamiliar ground in darkness.

The named horses in Tourist Trap all have the Leopard allele. I’ll describe Raindrop, Token, Splash, Freckles and Dusty as we get to the combinations of leopard markings that each represents. In fact, I’ll give the full color genotypes I’ve given each. The horse on the cover of Horse Power, near the top of the right sidebar, also has the leopard gene., as does Dottie in the story. In fact, Dottie is supposed to be a granddaughter of Raindrop, and inherited both the Leopard and Dun alleles from her.

The Splashed White Gene in Horses

Splashed white is another spotting gene in horses. It resembles tobiano in that the pattern is usually crisp-edged, and there is no tendency for the kind of uneven roaning often seen in sabino. Splashed white is more common in Europe than in North America, but is becoming common in Paints.

The best description of splashed white is that the horse looks as if it had been dipped feet-first in white paint with its head lowered. Minimal white markings may not be recognized as due to a spotting gene. The next stage includes a blaze that widens toward the muzzle and may extend up the sides of the head, white extending above the knees and hocks, and possibly a belly spot. With stronger grades of spotting the entire head is often white, as well as the entire underbody, and eventually only the ears may retain pigment. Eyes are usually blue or have blue chips. Splashed white can be confused with very crisp sabino markings without roaning, but sabino-1, at least, can be identified through genetic testing.

I am sorry I have no photographs of this pattern, but it is rare in North America. Even Sponenberg’s photos are of Icelandic horses. Splashed white can be very difficult to tell from a crisply marked sabino without roaning. The amount of head white would be unusual for a tobiano. In general tobiano markings look as if white paint was dripped over the horse from the top, while the white in splashed white gives more the appearance of coming up from the bottom. The pattern occurs and is being selected for in Paints, and is known in Icelandic horses,  Welsh Ponies, and Finnish Draft Horses. It also is known in the Appaloosa.

Splashed white appears to be associated with deafness in horses, though many splashed whites have normal hearing.

Splashed white is believed to be due to a dominant or incompletely dominant gene, though the wide range of patterns produced by this gene makes genetic studies difficult. There is evidence of at least one white horse being homozygous for splashed white. At the present time, a DNA test for this gene is not available. There is conflicting evidence as to whether this pattern is associated in any way with the KIT locus.

The Tobiano Gene in Horses

Tobiano was the first type of white body spotting in horses recognized as being genetically distinct. Like other white markings, it varies widely in extent, with tobiano horses ranging from white with a colored head to normally colored with white hooves and lower legs, and perhaps a white area in the mane or tail. A few tobianos have blue eyes which are apparently produced by the tobiano gene.

The tobiano pattern has relatively crisp-edged white spots that cross the topline. The arrangement tends to be vertical, though not to the extent of a striped pattern. The head normally remains dark, though the white markings seen on non-spotted horses may be present. At times the dark skin extends under the edges of the white patches, giving a “halo” effect. Portions of the mane and tail growing from white areas are normally white, and in fact this may be the only obvious expression of tobiano in a minimally marked horse.

The pattern is due to a dominant allele, tobiano, at the tobiano locus, To. This locus is near but not at the KIT locus on chromosome 3, and a marker test is available. A horse with two copies of the tobiano allele is perfectly viable and not usually whiter than one with one tobiano and one wild-type allele. It is, however, more likely to show “paw prints” or “bear paws”–roan or spotted areas within the white patches.

Tobiano can occur on any base color: intense, dilute, or with interspersed white hairs. It does occasionally have an odd effect in the presence of one copy of the cream gene. The colored part of the coat “breaks up” into patches of dilute and non-dilute hair. This variation of the pattern is called calico. Calico is thought to be due to a dominant gene at a third locus which can only be detected if both tobiano and cream alleles are present. Theoretically, smoky calico should occur with areas of smoky, black, and white, but I cannot find any reference to this color in Sponenberg.

Although tobiano is dominant, tobiano foals are now and then produced by parents that appear non-spotted. On close examination one of these parents is generally a minimally marked tobiano, with extensive leg white and vary little face white.

The tobiano in the video is a good example of the pattern. Note the way the white markings on the neck are carried into the mane, and the way the white patches cross the topline.

The Roan Gene in Horses

Roan, like grey, is a pattern gene which sprinkles white hair over an otherwise normally pigmented animal. However, the pattern of white hair, the progression with age and the response to scarring are quite different from grey.

It should be pointed out that horsemen use the word “roan” quite loosely. In Thoroughbreds, for instance, it is used as a synonym for grey, particularly rose grey. There are several forms of roan covered by this loose usage, but the one discussed here is classic roan, which is due to the dominant roan gene. Frosty roan, varnish roan, roaned, rabicano and the roaning caused by some white spotting patterns will be discussed separately.

In classic roan the head, legs, mane and tail remain fully pigmented but there is an admixture of white hairs on the body of the horse. Foals are born roan or shed their foal coat to roan, and beyond that point the roan pattern is not progressive with age. In fact, roans may darken with age. They may also change appearance with season, appearing lightest when the coat is shortest and darker in winter coat.

Corn marks (flecks of the base color) are common on roans, and scars often lack roaning. Photographs of wild horses often show this to an extreme, as dominance battles frequently leave extensive scars.

Roan is due to a dominant gene. At one time, the gene was thought to be a lethal when two roan alleles were present at the roan locus, but more recent work has shown this not to be true. The gene itself has not been found, but it is known to be near, if not part of, the KIT locus on equine chromosome 3. There is clear linkage with chestnut at the extension locus, and tobiano is also linked. As an example of this, if a bay roan is bred to a chestnut, most of the foals will be bay roans or chestnuts, with only a few being chestnut roan or bay. Linked genes do not follow the rules of totally independent inheritance. A linkage test for roan is available if you want to know if a roan is homozygous.

Roan is quite variable in its intensity. Now and then a roan foal comes from two parents thought not to be roans, but close examination of the parents generally shows one to be a roan with very little roaning.

Roan may occur on any base color with any combination of diluting genes and marking genes. Black roans are often referred to as blue roans, bay roans as red roans, and chestnut roans as strawberry roans, but there are also references to purple roans, lilac roans, and honey roans. Further, a “red roan” could have either bay or chestnut as the underlying color, while some dark bay roans were called blue roan or purple roan. The modern practice is to put the base color first, followed by “roan.”

Roi (in Homecoming) will someday get a palomino roan mare with leopard (Appaloosa) markings—a horse overlooked by others because of her color but in fact quite a good horse. She is also a good example of the way different color genes can combine.

(The 3 photos on the left were taken at my cousin’s horse farm in Alabama.)

The Grey Gene in Horses

Grey is frequently considered one of the basic colors of horses, but it is more correct to think of it as a pattern of white hairs. Further, it is the only pattern that changes systematically and predictably with age, and one of the few patterns which can hide most other color genes.

The grey locus is well documented, with two alleles. Grey is dominant to wild-type, and is due to a “4.6 kilobase duplication into intron six of the STYX17 (syntaxis 17) locus, on chromosome 25.” The practical meaning of this is that the grey gene can be tested for, and carriers of wild-type identified.

The horse nearer the camera is a grey that has turned pure white, but the dak eyes and muzzle identify it as a grey.

Gray is a pattern of interspersed white hairs that increase in a fairly predictable fashion with age. I say fairly predictable, because there are several patterns of greying, and any genetic controls for which pattern will occur have not yet been found. The speed at which greying occurs is also quite variable, though in most cases a horse is light grey or white by ten years of age. In all cases, however, the greying begins first on the head. This is in sharp contrast to roan, where the horse is born roan and the head remains dark.

Greys can be born almost any color, but when the foal coat is shed, the horse

This photo clearly show the white rear fetlock. With increasing age, this marking will probably remain visible only in the skin color.

can usually be identified as a grey. Other changes are more variable. The foal may be born with red body pigment, and remain red as the white hairs begin to appear, leading to a rose grey—often miscalled a roan. A red foal coat may shed to black, which then greys as the fraction of white hairs steadily increases. Or the foal may be born black, regardless of the genetic color, and then grey from the black.

Some greys develop a white mane and tail early. These horses generally become pure white with age, though their skin normally remains dark.

A famous grey, General Robert E. Lee’s Traveller. Good example of mane and tail remaining dark.

Others retain a dark mane and tail as the body lightens. These individuals may retain some dark shading on the legs and even body for a long time, and some never become entirely white.

Some grays are dappled at the intermediate stages—the body is covered with circular areas of lighter hair surrounded by darker circles. Others are more uniform—iron greys. Many, as they grow older, develop reddish flecks and are called flea-bitten greys. So-called blood marks—larger areas of red coat—may also develop.

Fleabitten grey with blood mark. Note that the fleabitten stage may come after the grey has become pure white.

One down side of grey is that greys are particularly prone to developing melanomas. Usually these are benign, but not in all cases.

It is worth pointing out that all “white” horses with dark skin are actually grey. All other genetic mechanisms for a white coat in horses also produce pink skin.

Greys can have any of the dilution or white marking patterns in addition to the grey pattern. I had a grey and white frame (paint) myself at one point, and while he looked white with slightly darker mane

Grey aged to white

and tail, the frame markings stood out sharply when I bathed him—the skin under the grey areas was black, while that under all of his white markings was pink. He eventually developed a flea-bitten pattern only over the dark skin.

Two greys are mentioned in Homecoming. The first is Derik’s grey, probably a dappled grey. Coryn took the paralyzed Roi for a ride on the second, Cotton, a horse aged to pure white. The novel I’m currently working on, Rescue Operation, will have two greys, an iron grey called Shadow and a dappled grey called Silver. Both are descended from Arabian stock allowed to run wild on a plateau for a couple of hundred years.

Two Rare Dilutions and a Summary of Dilution Genes in Horses

This is an updated repost; the original version was posted December 19, 2010. I have added photos and changed several of the links.

Two dilution genes are so rare that their effect on all base colors is not even well understood.

Mushroom has been found in only a few breeds: Shetland Pony, Haflinger and possibly the American Quarter horse. At first glance, it looks like silver dapple acting on a black background. The body color is a flat beige or sepia, and the mane and tail are lighter than the body. But mushroom horses, unlike silver dapple, are very rarely dappled. Further, their eyelashes normally remain dark.

DNA tests show conclusively that these horses do not carry silver. Even more surprising, gene tests indicate the underlying color is not black, but chestnut.

Mushroom has been shown to be due to a recessive gene, tentatively identified as the mushroom allele at the mushroom locus. The effect on base colors other than chestnut is at the present time unknown.

It is difficult to know how common the mushroom allele is, partly because most mushroom horses are misidentified as silver dapples.

The other rare dilution as been found in two closely related Arabian horses. Their pedigrees suggest a recessive gene, and their appearance suggest that the effect of the double recessive is similar to that of a single champagne gene, though there is less effect on red pigment or skin color.  Eye color is lighter than normal. However, this is based on only two horses.

A single horse can have any two alleles at each locus, but there are six different loci that have at least one allele that causes dilution. Thus a horse can easily be a palomino and a dun (linebacked palomino) or a dun and a silver dapple on a black background (silver grullo.) Telling which genes are actually present based on the appearance of the horse, however, can be a major problem without DNA testing. In many cases, a horse with multiple dilution genes will just look cream, or even white. I mentioned this before, when I posted a photo of a horse the owner thought was palomino and I thought was buckskin silver dapple. Here’s another photo of the same horse.

We can summarize the loci and the alleles we have discussed previously, with links to the previous posts, as follows:

Liver chestnut (note the lightening toward the hooves), bay, and red chestnut.The one almost hidden is palomino.

Agouti locus: This has been shown to be the agouti signalling protein (ASIP) locus. The exact number of alleles is uncertain, but probably include wild-type bay (some red on lower legs), bay, seal brown (black with some red shading) and non-agouti (black.) More red is dominant to more black in this series. Most blacks, and particularly most intense blacks, are due to non-agouti.

Extension locus: This has been shown to be the melanocortin one receptor (MS1R) locus. There are three alleles. The most dominant first, they are dominant black, wild-type, and recessive red (chestnut). This locus determines whether black pigment can be produced. Two copies of the recessive red allele or one of the dominant black allele completely hide whatever is present at the agouti locus. Dominant black is relatively rare and still subject to some controversy.

Because agouti and extension interact, I covered both in separate posts, here and here. Because these base colors can be modified, I also described the classes of modifying genes.

Cream locus: This has been shown to be the membrane-associated transport protein (MATP) locus. The alleles are (in order of dominance) cream, wild-type, and pearl. Red pigment is affected far more than black, especially if the horse has one wild-type gene. Palomino, buckskin and smoky black are the result of a single cream allele with the other being wild-type. Two cream alleles give cremillo, perlino, or smoky cream, which cannot always be told apart. The pearl allele is a relatively recent discovery, but it appears to be at the cream locus.

 

Red Dun

Dun locus: As of 2009 the locus had not been found, so no direct DNA test was available. There is, however, a test for a linked gene. The alleles are dun (wild-type) which is dominant to non-dun. Both red and black pigment are affected, and in addition dun produces a dorsal stripe and other variable striping effects. There is a dun, specifically a grulla, in my novel, Tourist Trap.

Champagne locus: This has been reported to be a mutation in Exon 2 of SLC36A1, and a gene test is available. The alleles are Champagne (dominant) and wild-type, and it does not matter whether one or two doses of Champagne are present. The effect is to dilute both red and black, but a single dose of champagne causes more dilution of black on the body than does a single dose of cream. Eye and skin color are also affected.

Silver Dapple locus: This has been shown to be the pre-melanosomal protein 17 (PMEL17) locus. The alleles are silver (dominant) and wild-type. This gene dilutes black to a variable extent, but has little or no effect on red, and appears to dilute the coarsest hairs most strongly. Like champagne, it is a simple dominant.

There are three more groups of loci which produce white areas or hairs on the horse: those that produce white markings on head and legs only, those that produce interspersed white hair, and those that produce white areas on the body.  I’ll repost these with updated pictures later, but you can find the original posts by using the index.

Silver Dapple—Another Dilution Gene in Horses

Silver on brown. Without silver, this horse would be mostly black, with black mane and tail.

Silver is another dilution gene in horses, quite distinct from cream/pearl, dun, or champagne. The silver dapple color is quite common in ponies, especially Shetlands. Body color on these ponies ranges from chocolate to blue, often with quite pronounced dappling and light mane and tail. But the gene occurs as well in many other types of horses, especially the gaited breeds in America and a number of breeds in Europe.

Different breeds and areas have different nomenclatures for horses with the silver gene. In Australia, silver is called taffy. In the Rocky Mountain Horse, it is referred to as chocolate.

Typical color for silver on black. Note the light eyelashes.

In all cases the gene occurs at the silver (Z) locus, and the alleles are silver (Z^Z) which is dominant, and wild-type (Z⁺) which is recessive. It is possible to test for the presence of the silver allele, which is at the PMEL17 locus.

The dilution genes we have discussed so far all have the same effect on black and red pigment, or somewhat more effect on red. Silver appears to have no effect on red pigment, and a highly variable effect on black. Interestingly, the coarsest hairs, whiskers and eyelashes, are most affected, often appearing nearly white. Manes and tails, also coarse, are generally affected more than the body coat.

Silver Dapple; Rocky Mountain Horse.

A genetically black horse may have the body color lightened so little it still looks black. On the other hand, the body may appear blue, chocolate or dead-grass color, but without the reddish cast typical of a chestnut. The mane and tail are generally lighter than the body, and the lower legs may be a little paler near the hoof. The contrast between mane and body color may vary—at one extreme the horse may have a mane only a little lighter than the body; at the other a black horse with a white mane and tail is quite possible. A chocolate silver with light mane and tail may be mistaken for a flaxen-maned liver chestnut.

Silver dapple on a bay background.

A genetically bay horse may show little effect of the silver gene aside from the light eyelashes and whiskers, or may have a variable amount of white hair in the mane and tail and a lightening of the black lower legs toward the hoof. The body color stays red, being unaffected by the silver gene. At the light extreme, a silver bay (called a red silver) may be very difficult to distinguish from a flaxen-maned chestnut. Usually the lower legs darken to near-black before lightening again near the hoof, but it may take a gene test to be sure.

Although I have not seen a red silver in person, I have seen what I suspect to be a buckskin silver. Such a horse could easily be the result of at least two types of breeding expected to produce palomino: a red silver misidentified as a chestnut to a palomino, or a chestnut carrying silver invisibly to a buckskin.

The owner identified this horse as a palomino, but I strongly suspect it is buckskin (cream on bay) with the silver dapple gene. Palominos not uncommonly have black hair in the mane and tail, but very rarely on the lower legs. This illustrates the difficulty of identifying horses with multiple dilution genes.

Clear chestnut completely hides the presence of the silver gene, though in theory a chestnut with a large amount of interspersed black hair or black eyelashes or whiskers would have that black replaced by interspersed blue or chocolate an the body and white eyelashes and whiskers. Without a magnifying glass and a very careful, hair-by-hair examination, however, this would likely go undetected. Since skin color is mostly due to black pigment, that also could be affected, though the silver dapples I have seen have normal skin color.

Silver dapple has been a rare color in North American horses other than ponies, but this is changing as breeders select for rare and unusual colors.

Some silver dapples, especially those with two copies of the silver allele, do have an ocular abnormality, though it is rare that vision is actually affected. This may be due to a linked gene, rather than the silver allele itself, but it is probably safest to have the eyes of silver animals intended for breeding checked.

Upper photos courtesy of Safyre Sporthorses.

Dun: a Wild-type Dilution Gene in Horses

Red Dun. Notice how flat the color looks compared with chestnut.

The colors of all wild animals are a tradeoff between camouflage, which hides the animal from its predators or hides the predator from its prey, and display, which involves making the animal more attractive to members of the opposite sex or more threatening to rivals of the same sex. In equines, camouflage may involve blending into the herd (as in zebras) or blending with the background (often dry grass.) Bay, black and chestnut are not very good camouflage colors, but flatter, duller shades of these colors are.

Dorsal stripe on the same red dun as the first picture.

The dun gene flattens and dulls the coat color over most of the body, sometimes leaving head, lower legs, and manes and tails darker than the body. Both red and black pigments are affected. It also produces a highly variable degree of striping of the coat. In general a dun horse will have a dark stripe running from the mane to the base of the tail, which in some cases continues down the center of the mane (dark mane center with light edges as in the Fjord horse) and tail. (Dorsal stripes do occur on other colors, but they are rarely unbroken from mane to tail.) In addition duns often have zebra-like stripes on the legs (especially near the knees and hocks.)

Dun Fjord horse. Note that the dorsal stripe continues up the middle of the mane. This horse also has tiger striping (faint) on the hocks.

Less commonly, they will have spiderweb-like markings on the forehead, or a cross stripe over the wither area—a marking common in donkeys. All of these markings are grouped as primitive marks.

One early study of dun suggested that the dulling is due to a crowding of the pigment granules to one side of the hair. My own observations tentatively support this, but I am aware of no published studies—looking at individual hairs under a microscope doesn’t seem to be popular today.

Dun is thought to be the wild-type gene for horses, and it is definitely dominant to non-dun. Why do we think it is the wild-type gene?

Dun Fjord horse. Note that the dorsal stripe runs into the tail, and the faint zebra markings on the hocks.

First, cave paintings.  Almost all show the darker head typical of dun, and some also show other primitive marks. Cave artists were limited by the available pigments, but their renditions are certainly compatible with the various types of dun.

Second, the wild horses that survived long enough to have their color recorded. These include the living Przewalski’s horse of Asia and the now extinct Tarpan of Europe, both duns.

Dun, though a dominant gene, is not that common in most horse breeds today. Why? During domestication, an occasional mutation to non-dun must have occurred. Human beings are attracted to what is different, and the earliest domesticators of the horse probably prized these intensely colored variants—to such a degree that in many horse breeds of today dun is either non-existent or very rare.

A darker shade of dun on bay. This horse had the dorsal stripe (clearer in another photo) and a clear shoulder stripe.

The words dun and buckskin are rather loosely used, and often treated as synonyms. Genetically, however, it is better to reserve buckskin for a bay with one cream gene at the cream locus, and dun for the whole suite of colors produced by one or two doses of the dun gene. The colors include red dun (dun on a chestnut background) various shades of tan with black mane, tail and lower legs known as  zebra dun, (dun on a bay background) and various shades of dark slate gray to tan to silver with dark points known as grullo (dun on a black background.)

In my  science fiction book, Tourist Trap, I have both wild horses assumed to be descended from some transplanted from Earth during the Pleistocene, described as striped duns, and a domestic mare, Raindrop, whose base color is grulla (feminine form of grullo.) Those striped duns are assumed to be duns of various base colors with very strong primitive marks. I might add that Raindrop’s color and markings correspond almost exactly with those of the foal in the last picture.