Horse Color Genetics: Final Summary

If you are looking for the A to Z post for today (F) scroll down or click on the button to the left.

Last week we reviewed the base color and dilution loci. Today we will do a final review of the interspersed white hair and white marking genes, along with the darkening genes. Although the blog series will end today, links will be put in the index to all  posts in this series.

There are two main loci responsible for interspersed white hair. These are Grey (born dark with white hairs becoming more numerous with age) and Roan (born roan with white hairs constant or decreasing with age.)

The Grey locus is the syntaxin-17 (STYX17) locus on equine chromosome 25. It causes an initial increase in melanocytes  followed by their depletion. There are two alleles at this locus: grey and wild-type, with gray being incompletely dominant. (Horses with two copies of the grey allele lighten faster than horses with one grey and one wild-type allele, are less likely to develop a fleabitten appearance, and are more likely to develop melanomas with age.) At this time the progression of graying (dark vs. light mane and tail) and the color of dark hair (usually black, but some individuals become rose grey, with the dark hair remaining red) are not known to be subject to genetic control. In any case the final result is a mostly white horse.

The Roan locus is close enough to the Extension locus that there is significant linkage. It is considered part of the KIT linkage group on equine chromosome 3. There are two alleles: roan (dominant) and wild-type. At one time possession of two roan alleles was thought to be lethal, but this has now been shown not to be true. Classic roan causes interspersed white hairs on the body, but the legs, mane and tail normally remain dark. The frosty pattern, in which the mane and tail are also affected, may be a variant of roan, but the genetic mechanism is at present unknown. Scars commonly lack white hair, causing dark corn marks.

Spotting loci are far more numerous, and some produce roaning as well as white areas.

Blazed face on chestnut

Minor spotting genes may be responsible for white facial and leg markings. These genes are present in most breeds, and facial and leg white tend to increase in tandem. Animals with wide blazes and no white on the legs, or with high stockings and plain faces are very often minimally marked animals with one of the other spotting genes.

The Tobiano locus is closely associated with the KIT locus, and hence on equine chromosome 3. There are two known alleles, tobiano and wild-type, with tobiano being an incomplete dominant. Generally tobianos are crisply marked, with white crossing the topline. Legs are normally white and the face is plain or has minor markings. Minimal tobianos may have high stockings with plain faces; in the maximal pattern only the head may be colored. Roan or colored spots known as paw prints may occur in white areas on animals with two tobiano alleles. There is a dominant modifier which in the presence of both tobiano and cream produces what is called a calico pattern—the yellow of the buckskin or palomino is broken up, with some areas being red.

The Frame locus is on equine chromosome 17, and is at the locus that controls endothelin receptor b (EDNRB.) The alleles are frame and wild-type. The frame allele is lethal in double dose, producing the so-called lethal white foal syndrome, so all frame horses should have one frame and one wild-type allele. The minimal expression of frame is extensive white on the head with colored legs. The maximal extent may have color confined to the topline and legs. The fact that the frame allele still seems sometimes to come out of nowhere need further clarification—a masking gene may also exist.

The sabino pattern is a combination of spotting and roaning, and extremely variable in expression. It may also have more than one genetic explanation. The Sabino-1 locus is part of the KIT complex (equine chromosome 3) and has two alleles, sabino and wild-type. The sabino allele is incompletely dominant over wild-type, as horses with two sabino alleles generally have more white (even to being almost completely white) than horses with one sabino and one wild-type allele. There are other mutations near the KIT locus that cause white spotting, some of which appear to be lethal in double dose.

The Splashed White locus is yet another that seems to be near the KIT locus, though not at it. The locus probably has two alleles, splashed white and wild-type, with splashed white behaving as an incomplete dominant. The minimal effect of splashed white may not be detectable, or the horse may be more extensively marked with white legs, possibly white underbody and generally white on the head, sometimes to the extent that the whole head is white. Think of a horse trotting through a puddle of white paint with its head lowered. Splashed white is also associated with deafness.

Manchado is a relatively rare type of spotting found in several breeds in Argentina, though that may be because of the Argentine fascination with coat color. Parts of the body, often including the top of the neck (and mane) are white, often with round colored spots. The genetic basis is unknown.

White with pink skin and dark eyes may be a separate gene, possibly lethal in horses with two white alleles. At the moment, this is somewhat up in the air.

The Leopard locus is the Transient Receptor Potential Cation Channel, Subfamily M, Member 1(TRPM1) locus. It has two alleles, leopard and wild-type, but an enormous array of patterns. Leopard is incompletely dominant over wild-type—horses with two leopard alleles generally have fewer leopard spots than those with one leopard and one wild-type gene, and have a high incidence of night-blindness.

Finally, darkening due to black hair in the coat may occur in at least three forms. Black hair may be scattered throughout the otherwise red parts of the coat, producing a sooty effect. Black tipping on otherwise red hairs appears to be associated with the agouti locus, and produces shaded effects where the back appears darker than the rest of the horse. Actual black striping of the coat, brindle, is rare but documented. Some types of roan, especially sabino, may produce a type of brindle with white stripes. The genetics are unclear in all of these cases.

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.

The Leopard Gene in Horses 4

This is an update with photos of an article originally posted April 17, 2011.

Not all horses with the leopard gene have blankets of any size, and not all have spots. The gene can also produce two specific types of roaning, called frost and snowflake.

Chestnut varnish roan. This horse has almost no white markings, and a full mane and tail.

These roan patterns are quite separate from that produced by the roan gene, which becomes less prominent with age and leaves head, legs, mane and tail dark. The leopard gene produces horses which are normally colored or at most have a few white hairs over the rump at birth, but develop roaning (frost) or scattered white spots (snowflake) as they age. In contrast to grey, the pattern eventually stabilizes rather than producing a pure white horse.

In frost, the roaning tends to be most prominent over the hips. So-called varnish marks are common — areas where the bones are close to the surface, such as the hipbones and nasal bones, retain pigment while the rest of the coat is roaned. An aged varnish roan may be almost white except for these varnish marks.

Another chestnut varnish roan, this one with a blaze and three white stockings. This one has the white organized into a near blanket over the hips, with spots on the blanket.

Snowflakes are small white spots scattered randomly over the body, but often most numerous and prominent on the front part of the horse. They tend to become larger and more numerous with age, until in extreme cases the horse appears white with colored specks. This gives what is often called a speckled pattern, not to be confused with flea-bitten grey. Note that not all of the photos shown at the link are true snowflakes — the term is used very loosely.

Both types of roan may be combined with any of the blanket or spotting patterns, or may occur alone or together. Figure 8.140 in Sponenberg is a beautiful example of a combination of snowflake, varnish roan, blanket and leopard spotting all on the same horse. (Put Figure 8.140 on the search inside field.) Since the leopard gene can produce any of these effects, alone or in combination, breeding leopard-pattern horses can lead to some interesting results.

The remaining named horse in Tourist Trap, Amber’s mount Splash, is a bay varnish roan with a small spotted blanket, in color rather like the horse on the left side, but with black points and no blaze. He’s a gelding, about 14.2 hands – just enough smaller than the other four to have problems with fords. Roi has seen only solid colored horses on Central, and his first look at Splash gives this impression:

“Amber’s [horse], 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.”

I will summarize the equine color loci and alleles next week with links back to where they are mentioned, but I have covered most of the known color genes in horses. That doesn’t mean more won’t be found!

A last comment: Horse Power, with the Leopard horse on the cover, is FREE today on Amazon.com.

The Leopard Gene in Horses Part 3

This is a repeat with some updating and additional photos of an article originally posted April 10, 2011

Not all horses with white markings produced by the Leopard gene are leopards. The white markings are generally symmetrical and present at birth, but they vary a great deal from horse to horse and may even be absent entirely. The minimal expression is white over the top of the rump, and the broad term for the pattern is blanket. Note that I am speaking only of white produced by the leopard gene. Leg and face white are generally independent of the leopard gene.

Edges of the white blanket may be crisp, flecked or roaned.

Sponenberger divides the white patterns by percent of white at birth. The modification I am using in Tourist Trap is:

10% or less	white spots over hips
10% to 20%	lace blanket
20% to 40%	hip blanket
40% to 60%	body blanket
60% to 80%	near leopard
90% to 100%	leopard

Note that “leopard” in this table includes both leopard and few-spot leopard, and that the size of the blanket has nothing to do with whether spots are present. If one copy of the leopard allele and one of the wild-type allele are present, whatever white areas are on the horse will normally have spots of the base color. If two copies of the leopard allele are present, the white markings will have few or no spots, and the pattern is often called snowcap or few-spot.

The Pattern-1 gene is heavily implicated in the amount of white, but it is almost certainly not the only modifier.

Spots will normally be of the base color, but may show a concentration or dilution of color. Thus they may appear darker or lighter than the base color. The horse on the book cover on the right sidebar shows spots on the neck, suggesting that at least some of the spots are darker than the body color. (That horse, by the way, is a stand-in for Raindrop’s granddaughter.)

The description of Roi’s horse, Raindrop, in Tourist Trap is that of a body-blanketed grulla approaching a near-leopard. She has white coronets and spots significantly darker than most of her body, which is already dark and somewhat bluish for a grulla. Roi’s first sight of her gives the following description:

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

“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.” Raindrop is later referred to as having a sparse mane (black) and being the color of polished slate. The dark dorsal stripe typical of duns would have been in the white-blanketed area, and hence invisible.

Genetically, she would have had two recessive black alleles at the Agouti locus, at least one wild-type allele at the Extension locus, at least one dun allele at the Dun locus, and one leopard and one wild-type allele at the TRPM1 locus.

Next week I’ll talk about the roan, flecked and snowflake patterns produced by the Leopard gene. Again, these patterns are often called Appaloosa in the United States, but they occur in horses worldwide.

The Leopard Gene in Horses (continued)

The pattern most people first think of in Appaloosa horses is the one that gave the gene its name—leopard. This pattern gives a white horse with round or oval spots of base color. There may be shading of the genetic base color on the flanks, behind the elbows or on the head.

Most people would call this horse a chestnut leopard. In fact he combines a white rump, extreme roaning or snowflake, and clearly defined spots. Note the haloes on several of the spots.

Genetically, a leopard must have at least one Pattern-1 allele in order to have most or all of the body white. In addition, it must have one leopard allele and one wild-type allele at the TRPM1 locus. Two leopard alleles will lead to a few-spot leopard, with only a few colored spots. Other factors leading to the leopard pattern undoubtedly exist, but are still unknown.

The mane and tail may be mixed in color if some of the mane and tail hair grow from colored spots. The spots may have roan edges, called haloes, which normally develop after birth. Blacks tend to have more and larger leopard spots than do chestnuts, with bay being intermediate. Also, horses with black mixed in the coat (sooty) will sometimes have the black and red colors form separate spots.

Three of the horses in Tourist Trap have leopard markings.

Another view of the same horse. Note the white “lightning strike” markings on the forelegs.

Token is the mare ridden by Flame. She is fairly tall—around 16 hands. She is a chestnut leopard, white with copper spots. Genetically, she is homozygous for the most recessive of the extension alleles, has two copies of the Pattern-1 allele and one of the leopard allele. She is wild-type at all dilution, pinto spotting, grey and roan loci. She could have genes for minor white marking on face or feet, but they cannot be seen.

Dusty is the gelding ridden by Timi, who would just as soon not be riding. He is the calmest and laziest of the group, and the easiest for a novice rider to handle. He is also the least responsive to leg pressure. Dusty is a buckskin leopard, around 15 hands tall. He has wild-type extension genes, bay alleles at the agouti locus, and one cream and one wild-type gene at the cream locus. His pattern-1 and leopard alleles are the same as Token’s. He has quite a lot of white in his mane and tail, so they are not noticeably sparse.

Penny is the guide and her horse, Freckles, is a bay leopard gelding. Freckles is a little keener than the horses assigned to Penny’s clients, but he’s a bit younger and the cross-country trip is part of his training. Freckles’s underlying bay color is a little sooty, so he has both red and black spots. Genetically he is the same as Dusty but with sooty and without the cream allele.

The other two horses have the leopard allele but are not leopards, and I’ll talk about them next time.

This was first posted, without photographs, April 2 2011.

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.

White Horses

A few horses are all white, with dark eyes and pink skin. These are not to be confused with aged greys, which may have a pure white coat but retain dark or at least mottled skin, or few-spot leopards, which generally have mottled skin. This type of white can occur from the spotting genes we have discussed as producing pintos, especially if more than one type of spotting gene is present. There is also a type of dominant white which is lethal if two copies of the allele are present but which if one white and one wild-type allele are present produces a healthy white horse.

Remember also that many of the dilution genes we have discussed can produce a very pale cream color often mistaken for white, though most of these horses have light eyes.

All white marking genes on horses, from a conservative white star to a white horse with colored ears, seem to work by preventing the pigment-producing cells from getting to parts of the horse’s body. They do not affect or replace other genes for color. Thus no matter how extensive the white markings on a horse, it will still carry alleles at all of the color loci we have discussed. Further, it will pass those alleles on to its foals.

The white spotting genes grouped as “pinto,” “paint” or “parti-color” may occur in any combination consistent with the survival of the foal. (Two copies of the frame allele at the frame locus, for instance, results in white foal syndrome and early death regardless of what else is present.) A horse could easily have one frame allele, together with two each for sabino-1, tobiano and splash. Because the white areas from these alleles tend to affect different parts of the horse, the result could be a white horse. When bred to plain mates, the offspring would probably be spotted.

Horses with spotting due to a single locus can also be white or nearly white if they are close to the extreme version of that pattern. Several of the spotting patterns converge to a “medicine hat” or “war bonnet” pattern with maximal white. This is probably most common in so-called tovero horses—those that combine tobiano spotting alleles with any of the non-tobiano alleles at other spotting loci. In general the ears are the last areas to lose pigment.

There is one type of pink-skinned white with dark eyes that does not appear to produce spotting in the offspring. White to white breedings of this type, however, always produce some colored foals. Examination of the numbers of white and colored foals suggest that two white alleles at this locus are a prenatal lethal—the foal never develops or is aborted so early that the breeder assumes the mare has missed. This type of white is believed to be due to a dominant gene.

Publicity shot of the Lone Ranger with Silver. Note the dark eyes and pink skin.

The white allele seems to have a surprisingly high mutation rate. Thus whites have been produced from colored parents in several breeds, and then reproduced as if they were dominant whites. I do not know whether DNA proof of parentage was available in these cases, however.

I do not believe a gene test has been developed for this type of white. Gene tests at other loci could be very useful, however, in determining what other color alleles the horse carries and could pass on to its foals.

White is a spectacular color and for that reason was popular in the age of horse power for flashy coach or cavalry horses. At least two western heroes — the Lone Ranger and Hopalong Cassidy – rode dominant whites, Silver and Topper. The downside? Keeping a white horse clean may be a problem, and the pink skin may be subject to sunburn.

White hoses are not albinos, although the initial name of the “breed” registry was the American Albino Horse registry. More online information can be found at The American White and Cream Horse and the Camarillo White Horse Association, though there is some question as to whether the two are genetically the same.

Horse Colors: Manchado and Brindle

This time I’m discussing a pair of markings that may or may not be genetic: manchado and brindle. Sorry, I don’t have any photos, but scroll down the White Horse page and look at Figure 8.120 in Sponenberg.

Manchado is in the pinto group if it is genetic, but it has hardly been investigated at all. Sponenberg says it is primarily found in Argentina, but it is found there in several breeds. This is taken by some as indicating an environmental cause, and by others as indicating that Argentineans pay more attention to horse color than do people in other parts of the world.

There is no question that manchado is different from other spotting genes. At first glance, it is a combination of pinto and leopard (Appaloosa) traits, but manchado horses do not have known leopard or pinto genes. The minimal expression is white on the top of the neck, giving a partially white mane, like tobiano. The head and legs normally remain dark. The white areas are crisp-edged, but these white areas normally have round or oval colored spots within them.

There are a few photos on the web, most notably one showing a Throughbred stallion and an Arabian mare, both breeds which are rarely spotted. Sponenberg shows a photo of a Welsh Pony with the Manchado pattern, but does not state whether is particular individual was from Argentina.

Brindle is a little better understood, but not by much. There are three types of bindles, one involving black stripes, one involving white stripes, and one in which the horse is actually a chimera. In the first two cases, much more common genetic mechanisms appear to be necessary.

For black stripes, the horse must have black interspersed hairs, a condition called sooty by geneticists. In most horses, the interspersed hairs are uniformly mixed into the coat or more numerous toward the back of the horse. In a few horses, the black hairs are organized into vertical stripes.

If white hairs are present, as in roans, they may also occasionally be organized into vertical stripes. This is also referred to as brindle, though it is not known whether this type of brindle has any relationship to the type with black stripes. The rabicano  pattern is an example of this type of brindling.

Finally, it is possible that two fertilized eggs are merged in early gestation. The chimera that results actually has tissues with two different DNA sets, and these tend to be arranged in vertical stripes. A brindle of this type could actually combine any two colors found in horses.

A website from White Horse Productions has excellent photos of these and other rare modifiers in horses. Scroll through the entire page.

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 Frame Gene in Horses

Chestnut paint. This particular horse appears to have the frame allele rather than sabino, judging from the face and lower legs, in spite of the ragged appearance of the spots.

Frame is another type of spotting gene in horses, formerly lumped into overo and sometimes called frame overo. It has nothing to do with the KIT locus, unlike tobiano and sabino-1.

Frame involves patterns of white which do not usually include roaning, though frame may occur in conjunction with other genes that cause roaning. In frame, the white areas tend to be arranged horizontally on the sides of the horse, and almost never cross the back. Frame is also almost the only pinto pattern in which the legs remain pigmented, though normal leg markings may occur.

Like all spotted horses, frame horses may vary from mostly colored to predominantly white. A frame horse will almost always have a wide blaze or bald face, and an apparently unspotted horse with a bald face but no white leg markings is likely to be a minimally marked frame. As the white expands the head may become mostly white and the white areas on the sides may expand to cover most of the horse, with the spine and legs being the last areas to lose color.

Another Frame. Note minimal leg markings with wide blaze and white spots on sides.

Frame is due to a single allele, frame (Fr), at a locus called endothelin receptor b (EDNRB) on equine chromosome 17. The locus has two known alleles, frame (Fr^Fr) and wild-type (Fr⁺). Frame horses, some of which are so minimally marked as to look solid, have one frame and one wild-type allele.

Breeding two frame horses together may produce lethal white foals, with two frame alleles. Such foals are born white, and the part of their nervous systems that controls the lower intestinal tract does not develop properly. They normally die within 72 hours of birth, though most are euthanized as soon as they are recognized. Most breeders avoid mating two frame horses together in order to avoid the production of such foals.

The frame allele can be tested for. Such testing has demonstrated that some genetically frame horses appear to be solid colored. Whether this is due to a suppressor gene or genes or is simply the extreme end of random variation of amount of white is unknown.

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