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.

I’m Looking for Photos

Color and markings are about right for Token, though of course this is not a mare.

I’m looking for photographs I can use to illustrate some of the horses in Tourist Trap. All of the horses have the kind of spotting seen in the Appaloosa breed in the United States, and in several other breeds around the world. The ones I need would be of riding horses, animals useful on a long, cross-country trek covering 30 or so miles a day: modern Appaloosas or mustangs.

The ones I have managed to photograph could almost stand in for Flame’s chestnut leopard mare, Token, who is intended to be about 16 hands, and

This boy could stand in for Splash if he were bay rather than chestnut.

Amber’s Splash, a bay varnish roan with a moderate blanket and spots over his hips, more like 14:3. The other three are between these in size: Freckles, a bay leopard, Dusty, a buckskin leopard, and Raindrop, a grulla with a large spotted body blanket. Gail Lord’s foal photo could be Raindrop as far as markings are concerned, but I’d really like to find photos that could stand in for all of them.

Roi has never seen a horse with the leopard gene, and this is his first view of the horses the party will be riding across plains occupied by mammoths, saber-tooth cats, and other Pleistocene animals:

If this foal’s blanket enlarges with maturity, it could grow up looking like Raindrop. Photo credit Gail Lord.

Roi blinked hard when he saw the horses, thinking for a moment he must have hit his head on something, after all. They were polka dotted. Flame’s mount was white sprinkled with round copper spots. Amber’s, a little bay roan with curious dark lines on its nose, looked less exotic until it turned as she halted it. Then it became apparent that it had a large white area, punctuated by dark bay spots, over its hips. One of the two led horses had a black-spotted white body, but its neck, legs and chest were a dark mouse gray, set off by a black head and mane and a black and white tail. The other had markings resembling those of Flame’s mount, but its spots were a lighter, sandy color with a good deal of black mixed with the white of its mane and tail. Then Flame and Amber jumped down from their horses and slammed into Roi, and he had to concentrate on keeping upright.

If you have photos you’d be willing to let me use, contact me at sbowling at mosquitonet dot com.

The Genetics of White on Horses

Blazed face on chestnut

We have now looked at how the basic colors black, seal brown, bay and chestnut, are produced by the interaction of the agouti locus (which determines where black and red pigment will be produced) and the extension locus, which determines whether black or red pigment can be produced, noting along the way that at the agouti locus more red is dominant to more black, while at the extension locus the opposite is the case.

Blaze on grey. It’s obvious near the nose, where the change in skin color shows, but only the lack of fleabitten spots show it higher on the face.

We have also looked at a number of loci with alleles that modify the appearance of black and/or red pigment: dun, cream, champagne, silver dapple, mushroom and Arab dilute. Two types of dilution widespread in mammals do not seem to be present in horses: the type of brown seen in chocolate Labrador retrievers, and the type of “blue” (really a gray shade) seen in blue Great Danes and Dobernmans. In both cases, the genes modify black pigment and can be found by genetic testing, but have not been found in horses. We do, however, have blue and gray horses, but the cause is quite different.

Separated star, white on nasal bone, snip and chin spot.

In addition to the base colors and their modifications, horses frequently have white hairs. Even the palest of cream horses will have pigment granules in the hairs. True white hairs, however, do not have pigment granules. The granules are placed in the growing hair by melanosomes, which originally form as part of the neural crest in the fetus, and then migrate to their final position in the skin. If they fail to reach some part of the skin, that part will be white, and usually the skin will lack pigment as well. Even the eyes are likely to be blue if surrounded by pink skin and white hair. White on the legs often is paired with light or striped hooves.

Stars, snips, and other white facial markings are very common in horses, as are white feet and ankles. The terminology for these markings varies, based on how extensive they are and exactly where they occur. These

Rear leg makings on the same horse shown at the right.

minor markings depend on a number of genes as well as chance–genetic clones may have different markings, suggesting that even the environment within the mare may have an effect.  Generally chestnuts have more white markings than bays, which in turn have more than blacks, so the other color genes may affect melanocyte migration. It has even been demonstrated that bays with one chestnut gene at the extension locus have more white than those with two wild-type genes.

Usually these white markings on face and legs run in parallel — a horse with a wide blaze will generally have white on the legs as well and vice versa, if only the minor marking genes are involved. A horse with a wide blaze and no white on the legs, or one with high stockings and a plain face, often has the minimal version of one of the white body-spotting genes. Further, face markings are generally confined to the midline of the face, and may be continuous or broken, giving stars, snips, narrow lines down the nasal bone or chin spots.

Star on rabicano chestnut. This horse will be shown again when I discuss the rabicano roaning pattern.

Aside from these minor markings and scars, white hair on horses falls into two categories. Grey, roan and some near-roan variants fall into a group in which individual white hairs are interspersed through the coat. In greys, this is progressive with age–greys are born colored and become progressively lighter with age, though the fist change may be from a lighter shade such as chestnut to black. Roans are born with white hairs in the coat and stay the same or darken with age. A second group includes a number of types of white spotting: tobiano, frame, splash, dominant white, two types of sabino, manchado and the leopard complex.

Over the next few weeks I’ll be covering the genes that produce these white areas on the body and interspersed white hairs.

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.

Champagne: Another Dilution Gene in Horses

Champagne Tennessee Walking Horse

The champagne allele, at the champagne locus, (Ch) was for many years ignored or confused with palomino, buckskin or dun. In fact the first edition of Sponenberg, published in 1983, has some of the same photographs of hoses, identified as “pink-skinned palomino,” or “lilac dun.” used in the second edition (1996) to illustrate the champagne group of colors. It was a relatively rare allele until recently, but is now known to occur in a number of breeds of horses, and is becoming more common due to selection for the champagne colors.

The champagne locus has two alleles, champagne (Ch^C) and wild-type (Ch⁺). Champagne is dominant to wild-type, though a horse with two champagne alleles may be slightly lighter than a horse with one champagne and one wild-type. In general, however, one cannot look at a horse in the champagne color group and say whether one or two copies of champagne allele are present. Certainly the difference is not nearly as marked as between buckskin and perlino.

Another champagne, I think a classic champagne, with lighter points.

The diluting effect on red is similar to a single cream allele, but champagne has far more effect on black pigment than does cream. Since the pigment in the skin and eyes of all horses is black (eumelanin,) eyes and skin color are affected by the champagne gene. Skin color is paler than normal, often with some mottling. Most champagne foals are born with blue eyes which darken to amber, and the foal coat is often darker than the adult coat.

Champagne also tends to produce a very shiny look to the body coat, as well as reverse dappling. In the case of champagne imposed on clear chestnut, the result is an almost metallic gold color. These gold champagnes are easily confused with palominos, and in fact may be closer to the “new-minted gold coin” body color than the commoner dark-skinned palominos. Genetically, however, they are quite different, though both are dilutions of chestnut base color.

If the base color is black, the result is what is called a classic champagne. The color is not uniform. The body is diluted to a sort of pinkish beige, but the mane, tail and to some extent the lower legs are a darker shade of brown.  Why this contrast occurs when the base color is uniform black is not clear—microscopic examination of the pigment granules is needed in champagnes, as in another color group, the silvers.

Champagne imposed on bay gives amber champagne. Horses of this color have a shiny yellow to tan body color with chocolate points. Actually looking at a horse, it is surprisingly hard to distinguish between classic champagne and amber champagne. I think both photos are of amber champagnes, but I am not absolutely certain.

Both bay and chestnut vary a great deal in the depth of red color and the amount of interspersed black hair in the body coat, and the addition of the champagne gene does not change this. Thus there is a wide variation in the exact shade of color in the various types of champagne, and I suspect that many champagnes are still not correctly identified. The champagne gene has now been identified with a mutation in the gene SLC36A1, and a commercial test for the mutant allele is now available. Champagne is not known to be associated with any health problems.

(First published 12/5/2010 at which time I had neither photographs nor the DNA information.)

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.

Pearl: a Palomino Complication

Sorry, I do not have a single photo of a pearl–just haven’t been able to find one. Sponenberg doesn’t, either. You’ll have to go with the descriptions.

Last week we discussed palomino as if there were just two forms of the gene associated with palomino color: cream and non-cream. The whole story is a little more complex, but I’ll have to introduce some genetic terminology to explain it, even though I’ve used the terminology, without explaining it properly, several times already.

The three new words we’ll be using are locus, allele and wild–type.

Locus means place in Latin, and it originally meant a place on a chromosome. Since genes code for proteins and are now known to be a little more complex than just the place on the chromosome, it now means the particular protein coded for.

There can be slightly different forms of a protein having the same function, and the different stretches of DNA (genes) that code for these slightly different forms are called alleles. Using this terminology, every horse has two alleles, one from each parent, at each locus (plural loci.) Last week we discussed two alleles, cream and non-cream, at the cream (C) locus.

The complications come from the fact that there are in fact three rather than just two alleles at the cream locus. Each individual horse can have any two of these three alleles. To avoid confusion, I am also going to introduce the term wild-type for the gene assumed to be the “normal” allele at a locus in the wild ancestor of a domesticated animal. What we called “non-cream” last week is in fact the wild-type gene that gives normal full color.

(Note that the wild ancestor of the horse is not the “wild” horse of the West—these are in fact feral, descended from domesticated stock. The only true wild horse alive today is Przewalski’s horse in Asia. The Tarpan in Europe was also wild, but became extinct in the 19^th century.)

Using our new terminology, the cream locus has three alleles: wild-type, cream, and pearl. Pearl was recognized quite recently, and it has a very low frequency except in a few Spanish and Portugese breeds and their derivatives. It could be considered a weaker allele than cream, as it has less diluting effect on the coat.

A horse with one wild-type allele and one pearl allele will look very much like a wild-type horse—chestnut, bay or black depending on what genes are present at other loci. A very close look will show skin slightly lighter than normal, or with small pale spots.

A horse with two pearl alleles will have the red pigment diluted only slightly more than would be expected for a horse with one cream allele and one wild-type allele. Black pigment, however, will be diluted much more than is the usual case for a horse with one cream and one wild-type allele. Thus a bay with two pearl alleles at the cream locus dilutes to tan or gold on the body with chocolate mane, tail and lower legs. A chestnut becomes virtually identical to a pumpkin-skinned palomino (technically gold champagne.) And a black becomes a grayish tan with chocolate mane, tail and lower legs. All of these colors appear very similar to those produced by a single dose of the champagne gene, which is a completely different gene at a different locus, but give very different breeding results. Luckily there is a DNA test for pearl.

If a horse has one pearl allele and one cream allele, the resulting color will be cream, usually slightly darker than the cream resulting from two cream alleles (cremillo, perlino or smoky cream.) In particular the eyes are generally blue or amber, and darker than those of cream horses with two cream alleles.

As I mentioned before, there are a number of different ways of diluting horse color, and when two or more at different loci are combined some very odd colors can result and it may not even be possible to tell what genes are present—or what colors can be produced—without DNA testing.

Next week I’ll consider linebacked dun—one of the few horse genes where the wild-type allele is rare today in many breeds.

(If anyone has photos I could use to illustrate some of these horse coat colors, I would really appreciate them.)

Palomino (Cream) Genetics

Sometimes scientists get it wrong. With time other scientists generally catch and correct the errors, but the initial efforts to explain the palomino color were wrong on two counts: first, the assignment of palomino dilution to the albino locus C (for color,now known to be the gene that codes for the enzyme tyrosinase) and second, the assumption that all dilute colors were palomino. We now know both are false, but the early investigators did explain why palomino does not and cannot breed true.

Palomino. A bay and another palomino are in the background.

A palomino is, ideally, a horse the color of a new-minted gold coin with a white mane and tail. At one time, breeders tried to get them to breed true, and there are still breed registries based on palomino color. But two dark-skinned palominos, mated together, will produce only about half palomino foals, and many of them will not be the pure gold with white manes and tails wanted. Why?

Palomino is an example of what is sometimes called over-dominance or partial dominance. The color is due to a dilution gene, cream or cremillo, acting on a chestnut background. The locus is still called C, with primary alleles C⁺ and C^Cr. A single dose of cream will dilute red pigment to golden yellow, while having very little effect on black pigment—thus the dark skin. A double dose will further dilute the red to a pale cream hard to tell from white, and black to a shade that varies from a slightly dirty white to pale gray.

Palomino with a Bend Or spot on the neck, far more conspicuous on a palomino than it would be on a chestnut.

All horses, in fact all mammals, have two copies of each gene, one from the father and the other from the mother. If the basic color of the horse is chestnut and the horse has a cream gene from one parent and a non-cream gene from the other, the result will be a palomino. If one parent is a cremillo (the result of a double dose of cream acting on chestnut) and the other is chestnut all of their foals will be palomino. But if both parents are palominos, about a quarter of their foals will get the non-cream gene from both parents and will be chestnut, a quarter will get the cream gene from both parents and will be cremillos, and half will get one of each kind of gene and be palominos.

Cremillos are popular with some horse owners today, but at one time they were considered very undesirable by palomino breeders. They have pink skins and blue eyes, and they may be more subject to sunburn than horses with dark skin and eyes. They are not, however, albinos or due to any form of the albino gene. The cream gene has been found and sequenced, and a DNA test for cream is available.

Palominos don’t necessarily have a clear gold body color, or white manes and tails. Remember chestnuts have varying amounts of black hair sprinkled through the coat, and these black hairs will remain and become even more conspicuous if the red of the coat is lightened to gold. Some chestnuts even have what are called Bend Or spots, areas much darker than the body, or even black. These will be much more conspicuous with  the C⁺  C^Cr combination..

Further, chestnuts often have manes that are self-colored or even darker than their bodies. These characteristics will carry over into the dilute animals, and it is not unusual to find palominos with considerable black shading or dappling, and black hair mixed into their manes and tails.

What happens if the cream gene is combined with a base color other than chestnut?

The effect of a single does of cream dilution on a bay, giving buckskin. There is considerable confusion between buckskin and dun, but this horse has the palomino or cream dilution.

One dose of cream on bay gives a buckskin, with a yellow body and black mane, tail, and lower legs. A double dose of cream gives a perlino, a cream horse with mane, tail and lower legs very slightly darker than the body, blue eyes and pink skin.

A single dose of cream on black may be missed entirely, and the horse just called black. Some blacks with a single dose of cream are slightly lighter than normal, and are called smoky. With a double dose of the cream gene, a black becomes a smoky cream, again with blue eyes and pink skin.

Although the darkest variants of cremillo, perlino and smoky cream can be distinguished from each other, the lighter variants are very difficult to tell apart. Often they are just called cream, the distinction becoming important only if they are bred.

A base color of brown or very deeply black-tipped bay? I saw one once in winter coat, and at first glance he looked like a blue roan. Looking closely, however, he did not have a mixture of black and white hairs; rather each hair had a cream base and a black tip. I was able to recognize the same horse in summer coat only because a stable employee pointed him out. In summer coat he was a typical seal brown.

I emphasized palominos with black skin because it turns out that gold horses with lighter skin (sometimes called pumpkin skin) are due to a completely different gene, champagne. I’ll talk about this later.

If you want to read some very basic information about genetics, especially genetics of coat color, have a look at http://bowlingsite.mcf.com/Genetics/Genetics.html

Types of Color Genes in Horses

The basic coat colors of horses already described can be modified by a large number of other genes. These genes may dilute the phaeomelanin or eumelanin pigments. They may make some of the hairs on the body white. They may organize white or black hairs as to where on the body they occur. They may cause areas of the body, face or legs to be white, usually underlain by pink skin. They may affect the mane and tail more than the body, or leave certain areas unchanged when the rest of the coat is lightened. Finally, there are genes that darken the apparent color by adding black hair to the coat.

Not all of the genetic basis for these colors are understood, but I will try to explain the ones we know something about.

The first group is dilution genes.

At one time, the assumption was that dilute was palomino. Talk about a gross oversimplification! There are at least six different dilution loci in horses, three of which were initially thought to be palomino. These are:

Cream (C), which probably has two alleles in addition to wild-type and is responsible for dark-skinned palomino, buckskin, smoky, cremillo, perlino, and pearl, among others.

Dun (Dn), which produces a dorsal stripe as well as dilution, and which sometimes leaves the head and lower legs dark. Dun, red dun and grullo are all dun colors. Dun on bay is sometimes called buckskin, but it is genetically a completely different color from cream on bay.

Champagne (Ch), a dilution gene which on a chestnut base produces what was once called pumpkin-skinned palomino, but which on other colors can only be called champagne.

Silver Dapple (Z), called by different names in different breeds and sometimes called taffy. The allele producing dilution affects black more than red, and mane and tail more than body.

Mushroom (Mu) is a rare color which at first sight looks like silver dalpple, but is quite different genetically.

Finally, there is a form of dilution in Arabians which appears to be genetic.

The second group involve interspersed white hairs.

Grey (G) is the commoner form especially in Thoroughbred and Arabian breeds, and produces white hairs showing first on the head and increasing with time. Aged greys are not infrequently pure white, but they normally retain black skin. In fact, black-skinned whites are really greys.

Classic Roan (Rn, tends to darken with age, and white hairs may not occur or be vary sparse in head, legs, mane and tail. Scars on roans are often of the base color, lacking white hair.

Other types of roan occur, but in most cases the genetics are not well understood

The third group is made up of spotting genes.

At one time, these were limited to face and body markings, two types of pinto, and Appaloosa. We now know of a bewildering variety.

Face and leg markings are widespread and appear to be quantitative in inheritance, but their inheritance is poorly understood. Even nomenclature varies.

Tobiano (To) is a type of vertically oriented spotting in which the head normally remains plain or conservatively marked but the legs are white.

Sabino is still a catch-all term for paints not known to be due to a specific gene, though sabino-1 is often treated as a locus. Minimal sabinos often have both blazes and high white on the legs. Roaning can be part of this pattern.

Frame (Fr) is a type of horizontally oriented white spotting. Minimal Frames generally have very wide blazes or bald faces but pigmented legs.

Splashed white (Spl) has white spreading up from below and often nearly all-white heads.

Manchado and Brindle are rare and not well understood.

White (Wh) produces white with pink skin and dark eyes, but this phenotype may also be produced as the white extreme of several of the spotting genes.

Leopard (Lp) is often called appaloosa in North America, but the gene is found worldwide in breeds from ponies to draft horses. This gene produces a wide variety of patterns, but at least one Lp^Lp allele seems to be necessary for all of them.

Finally, there are at least two mechanisms that darken the coat.

The genetics of sooty and shading are still uncertain.

Note that a horse may have any combination of these genes. In the trilogy I’m working on now Roi has a horse, Buttermilk, who is a palomino classic roan with Lp^Lp giving her white splashes over her hips: E^eE^e C⁺C^Cr Rn^RnRn⁺ Lp^LpLp⁺.

I have old posts on all of these (see index, at the top of the page) which I will be freshening up with new photos and re-issuing over the next few months.