A Horse of a Different Color

What is your favorite color of horse? Is it the pitch black of Walter Farley's Black Stallion? The whiteness of the Lone Ranger's Silver? The sunshine gold of Roy Rogers' Trigger? How does a breeder capitalize on the fancy colors that bring big bucks? Unfortunately, it's not always as simple as breeding a black stallion to a black mare to get a black foal. The late Ann Bowling, PhD, author of Horse Genetics; and Phillip Sponenberg, DVM, PhD, of the Virginia-Maryland Regional College of Veterinary Medicine, have both done extensive research into the specifics of coat color genetics and their expertise will help you understand how horses pass color from one generation to the next.

Remember Genetics Class?

The genes of the horse determine everything about that animal, and they are passed from generation to generation in chromosomes. The location of a gene on a chromosome is called the locus, and there are two alleles, or alternate states of a particular gene, at each locus. A dominant allele is one that masks the traits of the recessive (unexpressed) allele, and the way the two are paired indicate which allele is expressed. If a horse carries two dominant or recessive alleles for a gene, he is said to be homozygous for that trait. If he carries one dominant and one recessive allele, then he is heterozygous for the trait.

Two main pigments in the hair account for all colors in mammals. The first is eumelanin, which is responsible for black or slate blue and, although very rare in horses, brown. The second is pheomelanin, which produces colors ranging from reddish brown to yellow. Many horses have a combination of both. In contrast, white hair is basically hair without color and results from a lack of pigment granules.

Sponenberg puts color into two categories--horses with black points (mane, tail, and legs) and horses with non-black points.

Black, Bay, and Chestnut

Black and chestnut are controlled by the Extension locus. The dominant and recessive alleles for Extension are represented by E and e, respectively. (Keep in mind that nomenclature, or the naming system, has not been standardized for coat color genes, therefore there might be different abbreviations used in various publications. For example, the dominant and recessive alleles for overo have been called O/o, O/N, OO/oo, OV/ov, and Ov/ov. However, the accompanying text will always provide an accurate definition. Here we adopt the most commonly used conventions.) Black, brown, and bay colors have EE or Ee genotypes (genetic code, as opposed to phenotype, or physical characteristics), where the black pigment is present in the hair and skin. Chestnut (ee) has black pigment only in the skin. According to Bowling's book, "The presence of black pigment is inherited as a trait dominant to its absence, so matings between two chestnut (ee) horses should not produce any black/brown/bay offspring." In other words, any combination with an E allele will yield a black or bay horse.

The Agouti locus, named for a South American rodent with black-banded hairs, controls the distribution pattern of eumelanin, so it acts only in the presence of an E allele. The dominant agouti allele (A) allows the color distribution to occur only in points, such as the mane, tail, and lower legs. For example, since Cleveland Bays only have black points, they must have at least one copy of the A allele (AA or Aa genotype). The recessive form (a) does not restrict black hair distribution. Thus, with the genetic combinations EEaa or Eeaa, you will get a uniformly black horse. Since Friesians are black, it is suspected that they are homozygous for the a allele (aa).

Palomino, Buckskin, and Dun

There are at least three color dilution genes responsible for lightening bay and chestnut horses. Palominos and buckskins are both heterozygous for the cream allele, which dilutes the pheomelanin to yellow. Palominos, with their golden coats and flaxen manes and tails, are diluted chestnuts. Buckskins, golden horses with black legs, manes, and tails, are diluted bays. Both of these color types have dark skin and eyes.

Cremellos are homozygous for the cream allele and dilute both the black and red pigments to pale ivory. Thus, cremellos have ivory hair, pink skin, and blue eyes. Perlinos are very similar to cremellos except that their manes and tails are darker. Researchers have not yet determined why palominos' manes and tails are light and what causes the genotypic differences between cremello and perlino coat colors. For more information on the cream allele, see "Cremello Gene Found" on page 24.

Unfortunately, palominos and buckskins don't breed true because the color is produced by heterozygosity. "If a breeder attempts to duplicate the color of a favorite palomino, say by breeding that palomino to another palomino, the predicted colors and their frequencies among the offspring will be 50% palomino, 25% red, and 25% cremello," said Bowling's book (see "Dominant Trait Inheritance" on page 60). For breeds in which cremellos/perlinos are not accepted for registration, "the better mating choice would be palomino crossed with a red (ee or chestnut)--the expected proportion of palominos is the same as from a palomino crossed with palomino mating (50%), but no cremellos would be anticipated," the book stated.

A dun-colored horse is basically a red horse with darker red points, a shoulder stripe, dorsal stripe, and leg bars. The dun trait dilutes the eumelanin and pheomelanin of the body hair, but not the points. In what would normally be a bay horse, the horse's color is instead yellow-red with black points, and in an otherwise black horse the color would be mouse-gray with black points. However, the genes that control the stripe patterns are currently unknown.

The final dilution that occurs in bay/ black/red horses is called silver dapple, which is not necessarily an accurate term since dapples might not be present. This color is most often seen in ponies; the black color changes to a dark, black-chocolate, or chocolate with a silver gray or flaxen mane and tail. An otherwise bay horse's coat is diluted to a silver-maned chestnut.

White, Gray, and Roan

These colors are produced by the action of at least three genes. The rare, truly white horse will lack pigment in the skin and hair from birth, but his eyes are dark. The allele for dominant white (W) is very rare in contrast to the gene for non-white (w). All non-white horses are (ww). Since the homozygous white (WW) is reported to be lethal, we assume that all horses with dominant white are heterozygotes (Ww). Occasionally you will see a white horse produced from dark parents; this can result from a genetic mutation producing the W allele or as a consequence of as yet unidentified genes for recessive white color in horses. White or almost white horses can produce the combined effect of multiple spotting alleles, such as roan, sabino, tobiano, Appaloosa, and overo. It is important to know the coat colors of the ancestors, as this is an indication of their genotypes.

A gray horse is normally born with color, but has an ever-increasing number of white hairs because of a progressive graying allele (G). Solid-colored horses have two copies of the recessive non-gray allele (gg). The gray color is inherited as a dominant trait, so a gray foal must have at least one gray parent (GG or Gg). These horses can be a clear gray, or have colored flecks (known as "flea-bitten"). The flecked pattern shows the base color that is obscured by the white hairs. Lipizzan horses have a high frequency of the gray allele, and gray horses are almost always used in their performances.

Roans might superficially look like grays, but they have white and colored hairs mixed together mostly on the body, and their heads and legs are typically darker. Unlike gray horses, the number of white hairs in the body does not increase with age. Inheritance of the roan color follows a dominance pattern (Rn for the roan allele, rn for the non-roan allele). Sometimes, people will add a color name to the roan. A "blue roan" is associated with a black, bay, or brown base coat and "red roan" is typically a chestnut/sorrel. A study of roan occurring in Belgian horses suggested that homozygosity for roan (RnRn) resulted in an embryonic lethal condition and resorption of the fetus (vanVleck and students then at Cornell). If this is the case, then all roan horses would be heterozygotes (Rnrn) and never true-breeding. However, scientists in Japan described true-breeding roans among Japanese horse breeds, demonstrating that homozygous roan is not necessarily an embryonic lethal.

Not to be confused with the roan color, "roaning" can occur in solid-colored horses. In this instance, white hairs are interspersed irregularly. You will see the majority of white hairs in the flanks and barrel rather than all over the body as in a true roan. It can be seen with two solid parents, but the inheritance pattern is still unknown.

Tobiano Pattern

This is the most common spotted pattern in the United States. The overall impression of a tobiano is a white horse with large colored patches usually on the head, chest, and flanks. All four legs are often white, and the head is normally colored. Facial markings are conservative, and tobianos rarely have blue eyes. While the white pattern usually has a distinct edge, sometimes white and colored hairs mix at the edge. "Ink spots," or small, round, colored spots in the white areas, can also occur. The tail might be a combination of white with black hairs or white with red hairs, and the topline is typically crossed somewhere with white.

Occasionally, horses with the tobiano gene exhibit minimal white color. The reason for minimal white is unknown and might be due to other, as yet undefined, genetic interactions.

Like grays and roans, this spotting pattern is a dominant trait, so a tobiano foal must have had a tobiano parent. Because it is absent from several breeds, such as Thoroughbreds, Standardbreds, and Arabians, many researchers believe that it has a single historical origin, but where and how it began is unknown.

The genotype for homozygous tobiano is ToTo, and the heterozygous tobiano is Toto. If one parent is homozygous for tobiano, all its offspring will be tobiano. When a tobiano heterozygous stallion is bred to a solid-colored mare, there is a 50% chance that the resulting foal will be tobiano (see "Dominant Trait Inheritance" on page 60); other offspring will be solid. When bred to a heterozygous tobiano mare, there is a 75% chance that the foal will exhibit the tobiano pattern.

Overo Pattern

According to Sponenberg, the overo pattern is becoming known as "non-tobiano" rather than as a color classification. Frame, sabino, and splashed white are three common terms that describe different color patterns that are seen in the genetic overo. In contrast to the tobiano, a frame overo is a mostly solid colored horse with white, horizontal patches on the side of the neck and/or belly, but white rarely crosses the back between the withers and the tail. Typically, the white areas have ragged edges, and the head usually contains a lot of white. The eyes can be brown, blue, or one of each, but blue eyes can occur if colored hair surrounds the eye.

The most common type of overo is called a "frame overo," which occurs when the topline, chest, legs, and tail are all dark, and the white markings occur in horizontal patches on the horse's sides with white on the face. A sabino type has four white legs, a white face, jagged markings, and considerable roaning throughout the rest of the body. This pattern can be found in a variety of breeds, including Clydesdales, Thoroughbreds, Arabians, and Tennessee Walking Horses. While the extent of white is extremely variable, the level of expression might be under some genetic control. Breeders often mate a sabino-patterned horse with a lot of white to a horse with dark feet in an attempt to control the white on the legs and body.

Very similar to the sabino is the splashed white overo. These horses have four white legs, a white belly, white face, and usually blue eyes, but not the roaning seen in the sabino. This pattern is normally found in Finnish draft horses and Welsh ponies.

A combination of tobiano and overo patterns, commonly called a tovero, is a mostly white horse with minimal color. The "medicine hat" paint is not a separate pattern, but rather a name for the specific arrangement of color. This is a largely white horse with color on the ears or ears and eyes, chest, flank, and the base of the tail. It can result from crossing two very white sabinos, a frame overo with a tobiano, a sabino with a frame overo, or a sabino with a tobiano.

Breeding for the overo pattern is not as simple as breeding for tobiano. For a long time it was considered a recessive trait, but recent research has countered that idea, demonstrating that at least one form of overo is an autosomal dominant trait, which shows direct transmission from parent to offspring as a dominant gene. This gene (O) is associated with the frame overo pattern. Its other allele, non-overo, has been designated o. There are other forms of overo caused by other genes that do not follow the dominant pattern, because it is possible to get an overo-colored horse from a mating of two solid-colored animals. By studying the ancestors, you might find minimal white markings that explain a "sudden" expression of white. It also might be a result of a new gene mutation.

These overos that just "pop up" usually produce the pattern with the same regularity as those known to have the dominant gene. Evidence from the American Paint Horse Association shows that a cross between an overo and a solid usually results in an overo offspring, which implies the trait's dominant nature.

The genotype for overo is heterozygous (Oo), and homozygous recessive (oo) results in a solid color. However, breeding for the overo pattern does come with a risk, as homozygous dominant (OO) is a lethal gene. When a foal is OO, he is born with a syndrome called overo lethal white syndrome (OLWS). The overo gene is found in Paints, miniature horses, half-Arabians, Thoroughbreds, and Quarter Horses.

Foals born with OLWS are mostly white with blue eyes. Initially, they might appear normal, but soon begin to colic because they cannot pass manure. The reason for this is that OLWS causes the intestines to be underdeveloped and contracted because of a failure of embryonic cells that form the gastrointestinal system. Interestingly, these same cells play a role in determining coat color. Veterinarians have tried surgery to bypass the damaged intestines, but so far that treatment has not been successful. Therefore, euthanasia of the foal is recommended because the foal will die of colic caused by fatal constipation.

Since no treatment for OLWS exists, prevention is the key. University of Minnesota researchers have found the mutated gene and developed a test for the defective allele. It is recommended that horses which carry the OLWS gene not be mated to one another, so that an OLWS foal will not be born.1

Appaloosa

There are a variety of patterns associated with the Appaloosa breed including spots over the entire body (leopard), spots on the rump, roan, snowflake, and even a broadly white pattern called fewspot. According to Bowling's book, it is believed that a single, incompletely dominant gene gene, Lp, is responsible for all Appaloosa patterns. Modifying genes are probably responsible for the different patterns because the patterns have not been true-breeding. The Lp gene also causes characteristic mottling around the muzzle and stripes on the hooves. A stallion with any Appaloosa pattern can produce offspring with all of the patterns. However, homozygosity for Appaloosa (LpLp) is associated with the leopard and fewspot patterns. Some Appaloosas with mottled roaning patterns can be mistakenly identified as roans, but roans have more evenly dispersed white hairs.

While the genetics of many coat colors have been determined, there are still unanswered questions. Also, like other characteristics, breeding for color is not always an exact science. Even when we understand how a color trait is inherited, we cannot predict which foal will inherit the genes for color or where the color will appear on the foal. Knowing the pedigree and coloring of the ancestors can improve your chances of getting what you want, but the final answer will not be seen until the foal is born.


REFERENCES

Church, S. "Overo Lethal White Syndrome Update," The Horse, November 2001. Article #3010 at www.TheHorse.com.


FURTHER READING

Bowling, A. Horse Genetics. United Kingdom: CAB International, 1996.

Santschi, E. Overo Lethal White Syndrome. www.xcodesign.com/aaep/displayArticles.cfm?ID=55.

Sponenberg, D.; Beaver, B. Horse Color. College Station: Texas A&M University Press, 1983.

Sponenberg, D. Equine Color Genetics. Ames: Iowa State University Press, 1996.


 

C
c
C
CC
Cc
c
Cc
cc

DOMINANT TRAIT INHERITANCE

Autosomal dominant traits are those where one gene (C) completely masks the other (c). Thus, any animal with a C in its genotype (genetic code) will look the same for this trait. A mating between animals heterozygous for this trait (Cc) will result in the offspring percentages at left, with 75% showing the dominant trait--25% being homozygous dominant (CC), 50% being heterozygous (Cc), and 25% being homozygous recessive (cc).


MAPPING HAIR COLOR GENES

Hair color and color patterns are popular criteria used for selecting horses by many breeders. Indeed, many registries exist on the basis of coat color and many breeds include horses with distinctive colors and color patterns. Hair color genes are also attractive candidates for gene mapping research in horses for several reasons. The gene effects are readily identified by casual observation. We usually know the mode of inheritance for hair color traits, e.g., whether they are dominant or recessive. Best of all, breeders select for color traits and are happy to share families segregating for these interesting genes. In contrast, genes causing diseases or influencing performance are difficult to evaluate, have complex modes of inheritance, and breeders are less inclined to identify potentially detrimental qualities in their breeding stock.

Consequently, the study of coat color genes in horses is proving to be valuable for evaluating the efficacy of the horse gene map. Unfortunately, another reason to study hair color genetics is that some of these genes also have an adverse effect on health as a result of homozygous embryonic lethality (roan, dominant white) and teratogenic (causing developmental defects) effects (overo lethal white syndrome). The more we learn about the genetics of coat color, the better we can promote the health and welfare of horses.

Genes are organized on chromosomes, which are complexes of protein and DNA in cells. Horses have 32 pairs of chromosomes, including the two sex chromosomes, X and Y. Scientists have designated horse chromosomes with the nomenclature ECA1, ECA2, ECA3, and so on to ECA31, plus ECAX or ECAY. With relatively few exceptions, the same type and number of genes are found in people, mice, and other vertebrate species as in horses. The genetic differences between horses and other species appear to be the result of many changes in the structure and regulation of genes recognizable in all species. Therefore, we can use genetic information from other species, including color differences and DNA sequences, to make inferences for the horse.

To study hair colors in horses, scientists have collected DNA from horse families, recorded the inheritance of color genes, determined which chromosome contains the hair color gene, selected a likely gene by reference to the human or mouse gene map, then DNA sequenced that gene to determine whether it is responsible for the hair color. Major hair color genes are listed in the above table, along with information about gene mapping and gene identification. So far seven hair color genes have been mapped to chromosomes, and the actual gene and gene mutations are known for four of them. When the chromosome has been identified, and especially when the gene has been identified, scientists can more accurately advise breeders about the inheritance of hair color genes in individuals. Using similar approaches, gene mapping scientists hope to identify genes that influence the health and performance of horses.--Ernie Bailey, PhD

HORSE COAT COLOR GENES
COLOR LOCUS
SYMBOLS
CHROMOSOME
GENE
Agouti
"A, a"
ECA22
ASIP
Appaloosa
"Lp, lp"
unknown
unknown
Champagne Dilution
"Ch, ch"
unknown
unknown
Cremello
"Cr, cr"
ECA21
MATP
Dominant White
"W, w"
unknown
unknown
Dun
"D, d"
unknown
unknown
Extension (chestnut)
"E, e"
ECA3
MC1R
Gray
"G, g"
ECA25
unknown
Overo (dominant, frame)
"O, o"
ECA17
EDNRB
Overo (recessive)
not defined
unknown
unknown
Roan
"Rn, rn"
ECA3
unknown
Sabino
"Sb, sb"
unknown
unknown
Silver
"Z, z"
unknown
unknown
Tobiano
"To, to"
ECA3
unknown

About the Author

Stephanie J. Corum, MS

Stephanie J. Corum received a MS in animal science from the University of Kentucky in Lexington. She has worked in various aspects of the horse industry, including Thoroughbred and Arabian racing, for nearly 20 years. More information about her work can be found at www.theridingwriter.com. She has also published the illustrated children's story Goats With Coats. Currently she and her husband own Charisma Ridge, a small horse farm in Maryland, and she competes in dressage.

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