Genetic researchers are discovering Nature's secrets for producing horses with tobiano, overo and appaloosa horse breed patterns. Horse breeders are putting the recipes to work.
Horsemen and women have long been fascinated by the various coat patterns displayed by the equine species, from the zebra's stripes, to the Appaloosa breed's spots and varnishes, to the Paint breed's bold splashes of color. While it may sometimes seem that such breed patterns appear randomly in nature, the more we learn about genetics, the more we've been able to reproduce these patterns in the horses we breed. Although we can't yet precisely control how these patterns are expressed (sometimes as lots of white, and sometimes as just a little), that knowledge is adding to the popularity of Paints, Pintos and Appaloosas as it reduces the risk of producing "solid-colored" horses.
The fact of the matter is, by selective breeding for dominant traits, we can introduce colorful patterns to virtually any "type" of horse we like… creating tobiano-patterned Saddlebreds, or leopard-spotted warmbloods, if that's what suits our fancy.
There are a lot of myths and old wives tales when it comes to white markings on horses. Some people like the appearance of blazes and white stockings on horses, while others are prejudiced against them. Have you ever heard the saying, "One white foot, buy him. Two white feet, try him. Three white feet, there's some doubt about him. Four white feet, you can do without him." Or: "One white foot, ride him for your life. Two white feet, give him to your wife. Three white feet, give him to your man. Four white feet, sell him if you can."
It is easy to refute such prejudices. Secretariat and Northern Dancer, two of the most outstanding racehorses and sires of this century, both had three white feet, and Northern Dancer's outstanding son, The Minstrel, had four. However, it is true that white lower legs are more likely than colored legs to be affected by scratches and photosensitization.
If you breed Paints, Pintos, Appaloosas and Ponies of the Americas, an attractive pattern can make a significant difference in the economic value of your foal crop.
Color genetics are complicated enough when confined to solid-colored horses. However, the rules governing the inheritance of white markings and patterns are even more complex.
The background color on every horse, with or without white markings or a white pattern, is one of the basic colors: bay, black, chestnut/sorrel, brown, dun, buckskin, palomino, cream, roan and gray. Like a horse's background color, his genes control his distribution of white hair.
A number of different genes determine white markings on the face and legs. The horse's base color apparently influences these genes, since white markings on chestnuts tend to be more extensive than those on bays and white markings on bays are more extensive than those on blacks. Complex relationships between the different genes determine the presence, absence and extent of the white leg and facial markings. Because of this, it is difficult or impossible to predict the white markings to be expected on a foal from any given mating.
Since fewer genes are involved, we can more easily predict the inheritance of white areas on the bodies of horses. Just as with solid colors, a pair of genes, one from the sire and one from the dam, determines any spotting pattern. Every horse of every breed, no matter what his color, has a pair of genes for every possible spotting pattern. However, the pattern is seen only when one of the genes is dominant.
In this article, dominant genes are designated by capital letters (e.g., T = tobiano, O = overo, L = leopard/appaloosa complex). The corresponding recessive genes are designated by small letters (e.g., t, o and l). When the two genes for a given trait differ, the horse is heterozygous for the trait (e.g., Tt). When the two genes are the same, the horse is homozygous for the trait (designated TT or tt, for example).
All white patterns are dominant to nonspotting, so heterozygotes are always patterned. The white areas may be difficult to see on a cream, pale palomino, dun or buckskin, or light gray or roan, but if the horse has a dominant gene for a pattern, it is there. We can be 99% certain that any solid-colored horse with no white areas is homozygous recessive (ttooll).
Predicting White Patterns
The fact that any gene for a white pattern is dominant to the gene for no pattern both simplifies and complicates predicting the pattern to be expected from a given mating. Making a prediction is simplified by the fact that we can be 99% certain that a solid-colored horse is homozygous recessive for all patterns, but it is complicated because we can't tell by looking at a spotted horse whether he is homozygous or heterozygous for the pattern.
When a horse that is homozygous for a pattern is mated with a horse of any color, all offspring will have the pattern. For example, if a homozygous tobiano (TT) stallion is bred to a group of chestnut, black, bay, dun and palomino mares, all of the offspring will be Tt and will have the tobiano pattern.
When two heterozygous horses (Tt) are mated, there are three chances out of four that the resulting foal will have the pattern (1 TT and 2 Tt) and one chance out of four that he will not (tt). When a heterozygous horse (Tt) is mated with a solid-colored horse, the chances of the offspring having the pattern are equal (2 Tt and 2 tt).
If a patterned stallion (or mare) has even one foal without the pattern, you can be certain that the parent horse is heterozygous for the pattern. It doesn't work the other way, however. That is, even if a patterned horse consistently has foals displaying that pattern, you cannot be certain that the horse is homozygous for the pattern. The horse might, by chance, have consistently passed on his dominant gene, even though he also has a recessive gene.
For certain patterns (e.g., tobiano), genetic tests are available to determine whether a horse is homozygous or heterozygous. Undoubtedly, as knowledge of the equine genome increases, tests for genes determining other patterns will be developed.
The hallmark of the tobiano pattern is that the white color crosses the center of the horse's back between the neck and the croup. In addition, all four lower legs are white and the head (although possibly having a star, stripe or blaze on the face) is indistinguishable from that of a solid-colored horse.
Inheritance of the tobiano pattern is straightforward. That is, essentially all TT and Tt horses display the pattern, and there are no adverse effects associated with the genes.
In the overo pattern, the white color never crosses the back between the neck and the croup. In addition, at least one lower leg is colored, and the horse has generally extensive white markings on the head. The overo often has a so-called "bald" face and white markings that often extend onto the lower jaw.
The terms piebald and skewbald as regards overo and tobiano horses sometimes cause confusion. Despite the "bald" in their names, these terms have nothing to do with the amount of white on the head. A piebald horse, whether tobiano, overo or leopard/appaloosa, is black and white. Piebald comes from the same root as magpie, a black and white bird. A piebald bird or animal of any species is black and white. (Killer whales, for example, are piebald.) A skewbald horse has any color besides black in association with his white pattern.
Overos have four distinct patterns: frame, calico, sabino and splashed white.
Frame overos generally have solid-colored hooves and legs (or white leg markings that are no more extensive than those on solid-colored horses), white spots with distinct borders in the middle of their bodies and necks, and extensive white on their heads. The white body spots do not connect with any white on the legs. Some frames have solid-colored bodies with no white spots, but such horses usually have bald faces. Others have nearly all-white heads and extensively white bodies, although the midline of the back and the lower legs and feet are colored.
Calico overos have white body markings that have a scattered, irregular border and often connect with white on one or more of the legs. There can be extensive, irregular white markings on the head, but the hair around the eyes is usually colored. Calicos with more than 75% white on their bodies and irregular white markings on their legs above the knees and hocks are sometimes called "loud calicos."
Sabinos are often confused with calicos and classified with them by some authorities. Dr. Elizabeth Santschi of the University of Wisconsin School of Veterinary Medicine, who has done extensive work on the overo lethal white syndrome, considers sabino to be a distinct pattern that is characterized by extensive mixing of white hairs (roaning) with colored patches that are irregularly shaped and flecked with white that blends with small white patches.
Splashed-white overos, rare in North America, look like they have been dipped in white paint because, although they have extensive white on their heads, the remaining white hairs are usually confined to their legs, chests and lower abdomens.
Inheritance of the overo patterns is complicated by the fact that dominant genes determine all four patterns. So, for example, a horse can be a blend of frame and calico.
Since both of the genes for overo patterns and those for tobiano patterns are dominant, it is possible for a horse to display both patterns (TTOo or TtOo). These horses are called toveros. A tovero may, for example, display all of the features of a frame overo, but have white areas crossing his back.
Two common patterns in toveros are designated medicine hat and war bonnet. The medicine hat tovero is almost all white, with colored hairs limited to his ears, poll and sometimes part of his neck and flanks. The war bonnet tovero is all white with the exception of his ears and poll.
The existence of toveros explains the occasional lethal white foal resulting from the mating of an apparent tobiano with an overo.
The dominant gene for the leopard/appaloosa complex controls the characteristic patterns seen on Appaloosas and Ponies of the Americas (as well as a few other breeds in widely scattered parts of the world). Dr. D. Phillip Sponenberg of the Virginia-Maryland College of Veterinary Medicine, an expert on color genetics of horses, distinguishes the following leopard/appaloosa patterns: blanket, snow cap blanket, leopard, few spot leopard, snowflake, speckled, frost, mottled and varnish roan.
Despite the fact that many of the leopard/appaloosa patterns are visually quite distinct from one another, they are all closely related. An individual horse will often show a combination of two or more of these patterns.
Most leopard/appaloosa complex horses have what breed enthusiasts call "mottled" or "parti-colored" skin, small dots or freckles around the muzzle, eyes, ears and external genitalia. If the skin of these areas is pink, the dots are generally dark. If the skin is dark, the freckling appears pink or white.
Leopard/appaloosa complex horses also frequently have striped hooves, even on solid-colored legs, and white showing around the sclera, the outer rim of the eye. White sclera is common in other breeds as well, even those not carrying a dominant L gene, and generally in conjunction with white face markings, such as Paints and Quarter Horses. Striped hooves with wide dark and light bands can also be found in other horses, usually in conjunction with white or partially white leg markings.
Some minimally marked leopard/appaloosa complex horses can be identified only by their mottled skin, white-rimmed eyes, and striped hooves. But all of these characteristics are occasionally seen in horses that do not carry the L gene.
The various appaloosa complex patterns were once believed to be controlled by different genes. Recent evidence strongly suggests, however, that a single dominant gene controls them. Observations that support the existence of only a single gene include the fact that stallions and mares of any of the patterns can have foals with different patterns.
Efforts made to distinguish leopard/appaloosa complex homozygotes (LL) from heterozygotes (Ll) by appearance have had some, albeit not total, success. In general, homozygotes are lighter in color than heterozygotes, but intermediate shades are difficult to classify. Most snowcap blankets are homozygotes.
Because they have what seems to be the most extreme of the leopard/appaloosa patterns, leopards at first appeared to be homozygous. However, leopard stallions occasionally sire solid-colored foals and thus must be heterozygous.
Just as in the case of the tobiano, overo and leopard/appaloosa genes, the gene for white (W) is dominant to the gene for non-white (w). However, the fact that a gene is dominant doesn't mean that it's common. The vast majority (often 100%) of horses in most breeds are homozygous recessive for white (ww). The gene is common in the American Albino breed, whose members are not in fact albinos, having brown rather than pink eyes.
As homozygous white is lethal, WW foals perish as embryos (unlike the lethal white overo, which is born at full term and dies shortly after birth). Thus, living, all-white horses are always heterozygotes (Ww). Grays, creams, leopards with few spots, and war bonnets are sometimes mistaken for whites.
From what we've said about dominants and recessives, we know that although two spotted parents frequently produce solid-colored foals, the reverse is not supposed to happen. A mating between two solid parents is not supposed to result in an overo foal, but occasionally this happens. Such a foal is called a "crop-out." A crop-out occurs when a dominant gene that is hidden in a parent appears ("crops out") in an offspring.
There are three possible explanations for the overo crop-out: 1) One of the parents really is an overo, but the only clue, which was misinterpreted or missed entirely, is an excessively high white stocking, a very wide blaze, or a tiny white spot on the body. A close look at the recent ancestors of a crop-out overo will usually reveal a leg, head or body marking indicating the presence of the dominant overo gene. 2) Even though one of the parents is carrying a dominant overo gene, for an unknown reason the rule that a dominant gene for spotting will be expressed in a horse's coat is broken. 3) An o gene spontaneously mutates to an O gene.
The results of genetic testing (now available at the University of California at Davis) prove that a dominant overo gene generally can be found in one parent of a crop-out. Because crop-outs are fairly common, the overo gene was for a long time believed to be recessive to genes for solid colors. But progeny testing provides irrefutable evidence that overo spotting is caused by a dominant gene.
Crop-out overos bred to solid-colored horses produce spotted foals at the same rate as other overos do. The parent of a crop-out overo is just as likely as a clearly patterned overo to produce a lethal white foal.
Crop-out white horses are occasionally seen in Arabians, Thoroughbreds and many other breeds. Dr. Sponenberg postulates that these crop-outs may indicate a high rate of spontaneous mutation of w genes to W. Just as the crop-out overo goes on to produce spotted foals, the crop-out whites go on to produce white foals.
As we discover more about these many white spotting patterns, we should be able to more accurately predict what patterns will be produced by any two parents.
Avoiding Lethal White Syndrome
The overo pattern in horses can cause lethal white syndrome, a fatal condition that kills a foal shortly after birth. However, researchers have developed a test so that you can check to see if your stallion or mare carries the lethal factor. If a horse does carry it, breeding to a non-overo horse will almost guarantee that a foal will not be born with lethal white syndrome.
A horse with lethal white syndrome carries the homozygous overo gene (OO). Most homozygous overos are all white and die within a day or two after birth. The overo lethal white syndrome is due to a factor carried on the O gene of many overos that, in the homozygous foal, prevents the proper function of the intestine.
Research has shown that nearly all frame overos, as well as nearly all frame blends, carry this factor on their O gene. So essentially all living frame and frame blends are Oo.
In addition, nearly all loud calicos (which are probably calico-frame blends) carry the lethal white factor. However, most calicos that have more color do not carry the lethal factor on their O genes.
Less than 25% of sabinos and less than 10% of splashed whites carry the lethal factor on their O gene.
When two heterozygous overos carrying the lethal factor are bred to each other, the odds are that out of every four foals, two will be overo (Oo), one will be solid (oo), and one will be lethal white (OO).
Since nearly all frames, frame blends, and loud calicos carry the lethal factor, and since it is often impossible to tell a pure sabino or calico from a frame blend, the safest course of action is not to breed overos to overos. When breeding an overo (Oo) to a solid-colored horse (oo), the odds are that out of every four foals, two will be overo (Oo) and two will be solid (oo). So the odds of obtaining an overo foal from an overo to solid mating are nearly as good as the odds of obtaining an overo foal when mating overo to overo without risking the 25% chance of obtaining a lethal white foal.
· There are more than 20 genes in the horse that influence coat color.
· Every horse has a pair of genes for every color and pattern, even though many are not expressed.
· There are only two basic pigments associated with coat color, black (eumelanin - "E"), and red (phaeomelanin - "A").
· Color and pattern variations in horses are the result of other genes modifying the effects of the "E" and "A" genes.
· Bay is the most common of all horse colors. It occurs when a horse inherits dominant black and red genes, with the red "A" restricting black pigmentation to the legs, mane and tail.
· Palominos, buckskins and crème-colored horses are the result of a dilution gene that expresses "incomplete" dominance.
· The dominant dun (D) gene is responsible for turning blacks, bays and chestnuts into grullas, zebra duns, and red duns.
· Purebred Thoroughbreds and Percherons do not carry any dominant dun genes, only recessives, as part of their genetic code.
· If a horse is homozygous for a dominant color gene, 100% of his offspring will exhibit that color - unless another dominant gene is present to modify it.
· Because white patterns dominate solids, when a horse is homozygous for a pattern such as tobiano or overo, all its offspring will have the pattern.
· In the tobiano pattern, the white color crosses the center of the horse's back between the neck and the croup. A tobiano generally has four white legs, and his face markings are similar to a solid-colored horse.
· In the overo pattern, the white never crosses the center of the horse's back. An overo has as least one colored lower leg and often has extensive white markings on the head.
· The leopard/appaloosa complex gene is now believed to be a single dominant gene that can produce a number of different patterns, such as leopard, snowflake, blanket, roan, etc.
· Although white (W) is a dominant gene, it is an uncommon color. Most horses are homozygous recessive for white (ww).
· Crop-outs are said to occur when two solid-colored horses produce an offspring with an overo pattern. Usually one of the parents is really an overo with very little white.
· A single horse can inherit any number of dominant genes that all express themselves, creating highly unusual color and pattern combinations such as "pintaloosas" (tobiano + appaloosa complex), and tovero dun roans (dun + roan + tobiano + overo), etc.
· No matter what color or pattern a horse begins life as, if it inherits a dominant gray gene, it will turn gray, and eventually appear almost white, with age.