Overo
Encyclopedia
Overo refers to several genetically unrelated pinto
coloration patterns of white-over-dark body markings in horse
s, and is a term used by the American Paint Horse Association
to classify a set of pinto patterns that are not Tobiano
. Overo is a Spanish word, originally meaning “like an egg."There are at least three genetically different spotting patterns which fall under the "overo" classification: frame overo
, sabino
overo, and splash or splashed white
overo.
To complicate matters further, some of the spotting gene patterns can be combined to produce a horse with multiple color traits, such as the Tovero
. The genetics of pinto spotting patterns are still being researched and are not fully understood. Some patterns may be polygenic, dominant
, or incomplete dominants, and spoting can be so minimally expressed
on an individual that the animal is mistaken for a "solid" colored horse. The sabino pattern is one of the most complex, as there has been one gene identified, SB-1, that creates one type of sabino patterning, but not all sabino-patterned horses carry it. There is a DNA
test for the allele
associated with the frame overo pattern, which causes the controversial lethal white syndrome
. There is no current DNA test for splashed white.
Although the condition lethal white syndrome
is also sometimes called "overo lethal white," that term is somewhat a misnomer. Because there are many different ways a horse can be classified as an "overo," and different genes linked to overo patterning, it is not accurate to say the "overo gene" is what "causes" lethal white syndrome. The frame allele is associated with lethal white, but the other genetics, such as horses with sabino and splash alleles, are not. There is a genetic link between splash white and deafness, but the correlation is not yet fully understood.
The frame overo pattern is the most common of the three types of overo patterns recognized in the American Paint Horse
breed. A frame overo horse appears to be any solid base color (bay, black, chestnut, etc.) with white irregular patches added, usually with a horizontal orientation. Markings
are often of jagged shape rather than rounded, the white rarely crosses the back, the lower legs tend to be dark, and the tail is one color, usually dark. The head is often white or bald-faced, and blue eyes are not uncommon. The frame overo pattern usually behaves like a dominant
gene, in that when frame overo horses are crossed on nonspotted horses, about half of the foals come out spotted. There are records of frame overos being produced by two nonspotted parents. There is a theory, however, that these "solid" horses simply may be horses with very minimal expression of overo genetics.
Frame coloring is controversial because it is associated with lethal white syndrome
(OLWS or LWS). LWS occurs when a foal
is homozygous for the Ile118Lys EDNRB mutation, which is considered by many researchers to be "usually responsible" for the frame overo phenotype
. However, other researchers emphasize that overo spotting patterns are phenotypically and genetically heterogeneous, that is, may have multiple sources.
The frame overo gene can be masked by other white patterns, particularly tobiano
, which is a dominant gene and epistatic
to overo. In addition, some carriers of the LWS allele appear to be solid. One theory holds that such horses carry the frame gene, but so minimal in expression that they appear solid. Either way, all LWS foals have horses with frame overo patterning in their pedigrees, and horses carrying a frame allele may not necessarily have a visible expression of the frame overo color.
(LWS) are not albinos. Albinism does not exist in horses. LWS foals are born almost or completely white with pink skin, but have blue eyes, not red ones. The lethal trait is that the nerves of the foal's digestive system are undeveloped. All lethal white foals die within 72 hours after birth, and are typically euthanized
sooner for humane reasons. A horse can be a carrier of the trait when it carries only one copy of the gene, (that is, heterozygous) and carriers are healthy and show no clinical signs of lethal white syndrome.
Thus, it is recommended by organizations such as the American Paint Horse Association
that horses of any pattern with overo ancestors be tested to verify their carrier state before being bred. There is a DNA
test for the gene which can be used to plan breedings and avoid producing affected foals. The mutation has not been found in solid-colored horses from breeds without frame patterning. Likewise, a sick foal that happens to be white can be tested to avoid inadvertent euthanization of a non-lethal foal who has a simple case of colic
.
Lethal white horses should not be confused with genetically white
horses, nor with "fully expressed" white sabinos, any of which may also be completely white with pink skin. Blue-eyed, pink-skinned cremellos and other horses carrying the cream gene
do not carry the LWS allele unless they also have an ancestor with overo genetics. The splash white
and sabino genetics are not associated with lethal white, though such horses could also be carriers of the frame allele.
A splash or splashed white overo pattern appears like a solid-colored horse who has been dipped in white paint, and the color splashed up from the bottom. It is the least common of the overo patterns. The legs and bottom portion of the body are usually white, as is the head, and blue eyes are common. Edges where white and dark color meet are usually crisp and sharp. Recent studies suggest that splashed white may be caused by a dominant gene. Splash overos are more prone to being deaf than other horses.
Outside of the United States, the term "splash" is sometimes applied to horses of apparent Sabino patterning.
Sabino
is listed a type of overo coloring by the American Paint Horse Association
. This terminology, outdated in terms of modern genetics, creates confusion, because other breeds, including many that will not register "pinto" or "paint" horses, may have individuals with sabino genetics. One reason for the terminology confusion is that "overo" was used by Paint horse breeders in the United States to refer to "anything that is not tobiano." Another reason for this terminology is that the term "overo", when used outside of the USA, particularly in Spanish
-speaking countries, refers to horses with the speckled roan
ing patterns typical of horses called sabino
in the USA. In addition, strongly marked sabino horses may mimic either overo or tobiano coloring.
In general, sabino is a generic description for a group of similar white spotting patterns. These include include high white stockings on all four feet, often extending up the legs with jagged edges, a fairly white head, with markings
often extending past the eyes, roaning at the edges of white markings, lip spots, "lacing," and white patches on the lower barrel that may extend to the flanks. A minimal sabino may only have one of the traits associated with sabino
horses. On the other hand, a "sabino-white" is a near-white or completely white horse, and sabino genetics are not linked to lethal white
.
Tobiano
coloring is the inverse of Overo spotting. Tobianos have a vertical spotting pattern, large, rounded spots, more white than dark, white that crosses the back, dark heads, but mostly white legs and white or multi-colored tail. It is created by a different gene from any of the overo patterns and is a dominant gene. All tobianos must have at least one tobiano parent. When a tobiano allele
is present, it is epistatic
and often masks other patterns.
A tovero
horse has pinto spotting patterns that show characteristics of both overo and tobiano
and probably carries genes for both. For example, a tovero might have tobiano body spotting with rounded edges and white across the back, yet have irregular facial markings and blue eyes.
Horses may also have a combination of sabino and frame overo genetics or any other combination of genes, resulting patterns that are difficult to smoothly classify into any one group.
Pinto horse
A pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses frequently have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto...
coloration patterns of white-over-dark body markings in horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
s, and is a term used by the American Paint Horse Association
American Paint Horse Association
The American Paint Horse Association is a breed registry for the American Paint Horse. It is currently headquartered in Fort Worth, Texas. It was founded in 1965 with the merging of two different color breed registries that had been formed to register pinto-colored horses of Quarter Horse bloodlines...
to classify a set of pinto patterns that are not Tobiano
Tobiano
Tobiano is a spotted color pattern commonly seen in Pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is present from birth and does not change throughout the horse's lifetime, unless the horse also carries...
. Overo is a Spanish word, originally meaning “like an egg."There are at least three genetically different spotting patterns which fall under the "overo" classification: frame overo
Lethal white syndrome
Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...
, sabino
Sabino horse
Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...
overo, and splash or splashed white
Splashed white
Splashed white or splash is a horse coat color pattern that produces pink-skinned, white markings. Many splashed whites have very modest markings, while others have the distinctive "dipped in white paint" pattern. Blue eyes are a hallmark of the pattern, and splash may account for otherwise "solid"...
overo.
To complicate matters further, some of the spotting gene patterns can be combined to produce a horse with multiple color traits, such as the Tovero
Tovero
The Tovero coloration is a mix of tobiano and overo colorations in Pinto horses and American Paint Horses. The genetics of pinto coloration are not always fully understood, and some horses have a combination of patterns that does not fit cleanly in either category.Some characteristics of a Tovero...
. The genetics of pinto spotting patterns are still being researched and are not fully understood. Some patterns may be polygenic, dominant
Dominance relationship
Dominance in genetics is a relationship between two variant forms of a single gene, in which one allele masks the effect of the other in influencing some trait. In the simplest case, if a gene exists in two allelic forms , three combinations of alleles are possible: AA, AB, and BB...
, or incomplete dominants, and spoting can be so minimally expressed
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
on an individual that the animal is mistaken for a "solid" colored horse. The sabino pattern is one of the most complex, as there has been one gene identified, SB-1, that creates one type of sabino patterning, but not all sabino-patterned horses carry it. There is a DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
test for the allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
associated with the frame overo pattern, which causes the controversial lethal white syndrome
Lethal white syndrome
Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...
. There is no current DNA test for splashed white.
Although the condition lethal white syndrome
Lethal white syndrome
Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...
is also sometimes called "overo lethal white," that term is somewhat a misnomer. Because there are many different ways a horse can be classified as an "overo," and different genes linked to overo patterning, it is not accurate to say the "overo gene" is what "causes" lethal white syndrome. The frame allele is associated with lethal white, but the other genetics, such as horses with sabino and splash alleles, are not. There is a genetic link between splash white and deafness, but the correlation is not yet fully understood.
Frame Overo
American Paint Horse
The American Paint Horse is a breed of horse that combines both the conformational characteristics of a western stock horse with a pinto spotting pattern of white and dark coat colors. Developed from a base of spotted horses with Quarter Horse and Thoroughbred bloodlines, the American Paint Horse...
breed. A frame overo horse appears to be any solid base color (bay, black, chestnut, etc.) with white irregular patches added, usually with a horizontal orientation. Markings
Horse markings
Markings on horses usually are distinctive white areas on an otherwise dark base coat color. Most horses have some markings, and they help to identify the horse as a unique individual. Markings are present at birth and do not change over the course of the horse's life...
are often of jagged shape rather than rounded, the white rarely crosses the back, the lower legs tend to be dark, and the tail is one color, usually dark. The head is often white or bald-faced, and blue eyes are not uncommon. The frame overo pattern usually behaves like a dominant
Dominance relationship
Dominance in genetics is a relationship between two variant forms of a single gene, in which one allele masks the effect of the other in influencing some trait. In the simplest case, if a gene exists in two allelic forms , three combinations of alleles are possible: AA, AB, and BB...
gene, in that when frame overo horses are crossed on nonspotted horses, about half of the foals come out spotted. There are records of frame overos being produced by two nonspotted parents. There is a theory, however, that these "solid" horses simply may be horses with very minimal expression of overo genetics.
Frame coloring is controversial because it is associated with lethal white syndrome
Lethal white syndrome
Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...
(OLWS or LWS). LWS occurs when a foal
Foal
A foal is an equine, particularly a horse, that is one year old or younger. More specific terms are colt for a male foal and filly for a female foal, but these terms are used until the horse is age three or four. When the foal is nursing from its dam , it may also be called a suckling...
is homozygous for the Ile118Lys EDNRB mutation, which is considered by many researchers to be "usually responsible" for the frame overo phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
. However, other researchers emphasize that overo spotting patterns are phenotypically and genetically heterogeneous, that is, may have multiple sources.
The frame overo gene can be masked by other white patterns, particularly tobiano
Tobiano
Tobiano is a spotted color pattern commonly seen in Pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is present from birth and does not change throughout the horse's lifetime, unless the horse also carries...
, which is a dominant gene and epistatic
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
to overo. In addition, some carriers of the LWS allele appear to be solid. One theory holds that such horses carry the frame gene, but so minimal in expression that they appear solid. Either way, all LWS foals have horses with frame overo patterning in their pedigrees, and horses carrying a frame allele may not necessarily have a visible expression of the frame overo color.
"Lethal White"
Foals which are homozygous for frame and thus have lethal white syndromeLethal white syndrome
Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...
(LWS) are not albinos. Albinism does not exist in horses. LWS foals are born almost or completely white with pink skin, but have blue eyes, not red ones. The lethal trait is that the nerves of the foal's digestive system are undeveloped. All lethal white foals die within 72 hours after birth, and are typically euthanized
Animal euthanasia
Animal euthanasia is the act of putting to death painlessly or allowing to die, as by withholding extreme medical measures, an animal suffering from an incurable, especially a painful, disease or condition. Euthanasia methods are designed to cause minimal pain and distress...
sooner for humane reasons. A horse can be a carrier of the trait when it carries only one copy of the gene, (that is, heterozygous) and carriers are healthy and show no clinical signs of lethal white syndrome.
Thus, it is recommended by organizations such as the American Paint Horse Association
American Paint Horse Association
The American Paint Horse Association is a breed registry for the American Paint Horse. It is currently headquartered in Fort Worth, Texas. It was founded in 1965 with the merging of two different color breed registries that had been formed to register pinto-colored horses of Quarter Horse bloodlines...
that horses of any pattern with overo ancestors be tested to verify their carrier state before being bred. There is a DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
test for the gene which can be used to plan breedings and avoid producing affected foals. The mutation has not been found in solid-colored horses from breeds without frame patterning. Likewise, a sick foal that happens to be white can be tested to avoid inadvertent euthanization of a non-lethal foal who has a simple case of colic
Horse colic
Colic in horses is defined as abdominal pain, but it is a clinical sign rather than a diagnosis. The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. The most common forms of colic...
.
Lethal white horses should not be confused with genetically white
White (horse)
White horses are born white and stay white throughout their life. White horses may have brown, blue, or hazel eyes. "True white" horses, especially those that carry one of the dominant white genes, are rare...
horses, nor with "fully expressed" white sabinos, any of which may also be completely white with pink skin. Blue-eyed, pink-skinned cremellos and other horses carrying the cream gene
Cream gene
The cream gene is responsible for a number of horse coat colors. Horses that have the cream gene in addition to a base coat color that is chestnut will become palomino if they are heterozygous, having one copy of the cream gene, or cremello, if they are homozygous. Similarly, horses with a bay...
do not carry the LWS allele unless they also have an ancestor with overo genetics. The splash white
Splashed white
Splashed white or splash is a horse coat color pattern that produces pink-skinned, white markings. Many splashed whites have very modest markings, while others have the distinctive "dipped in white paint" pattern. Blue eyes are a hallmark of the pattern, and splash may account for otherwise "solid"...
and sabino genetics are not associated with lethal white, though such horses could also be carriers of the frame allele.
Splash Overo
Outside of the United States, the term "splash" is sometimes applied to horses of apparent Sabino patterning.
Sabino "Overo"
Sabino horse
Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...
is listed a type of overo coloring by the American Paint Horse Association
American Paint Horse Association
The American Paint Horse Association is a breed registry for the American Paint Horse. It is currently headquartered in Fort Worth, Texas. It was founded in 1965 with the merging of two different color breed registries that had been formed to register pinto-colored horses of Quarter Horse bloodlines...
. This terminology, outdated in terms of modern genetics, creates confusion, because other breeds, including many that will not register "pinto" or "paint" horses, may have individuals with sabino genetics. One reason for the terminology confusion is that "overo" was used by Paint horse breeders in the United States to refer to "anything that is not tobiano." Another reason for this terminology is that the term "overo", when used outside of the USA, particularly in Spanish
Spanish language
Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...
-speaking countries, refers to horses with the speckled roan
Roan (color)
Roan is a coat color found in many animals, notably horses, cattle and dogs. It is defined generally as an even mixture of white and pigmented hairs that does not "gray out" or fade as the animal ages...
ing patterns typical of horses called sabino
Sabino horse
Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...
in the USA. In addition, strongly marked sabino horses may mimic either overo or tobiano coloring.
In general, sabino is a generic description for a group of similar white spotting patterns. These include include high white stockings on all four feet, often extending up the legs with jagged edges, a fairly white head, with markings
Horse markings
Markings on horses usually are distinctive white areas on an otherwise dark base coat color. Most horses have some markings, and they help to identify the horse as a unique individual. Markings are present at birth and do not change over the course of the horse's life...
often extending past the eyes, roaning at the edges of white markings, lip spots, "lacing," and white patches on the lower barrel that may extend to the flanks. A minimal sabino may only have one of the traits associated with sabino
Sabino horse
Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...
horses. On the other hand, a "sabino-white" is a near-white or completely white horse, and sabino genetics are not linked to lethal white
Lethal white syndrome
Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...
.
Tobiano
Tobiano
Tobiano
Tobiano is a spotted color pattern commonly seen in Pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is present from birth and does not change throughout the horse's lifetime, unless the horse also carries...
coloring is the inverse of Overo spotting. Tobianos have a vertical spotting pattern, large, rounded spots, more white than dark, white that crosses the back, dark heads, but mostly white legs and white or multi-colored tail. It is created by a different gene from any of the overo patterns and is a dominant gene. All tobianos must have at least one tobiano parent. When a tobiano allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
is present, it is epistatic
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
and often masks other patterns.
Tovero and other mixed patterns
Tovero
The Tovero coloration is a mix of tobiano and overo colorations in Pinto horses and American Paint Horses. The genetics of pinto coloration are not always fully understood, and some horses have a combination of patterns that does not fit cleanly in either category.Some characteristics of a Tovero...
horse has pinto spotting patterns that show characteristics of both overo and tobiano
Tobiano
Tobiano is a spotted color pattern commonly seen in Pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is present from birth and does not change throughout the horse's lifetime, unless the horse also carries...
and probably carries genes for both. For example, a tovero might have tobiano body spotting with rounded edges and white across the back, yet have irregular facial markings and blue eyes.
Horses may also have a combination of sabino and frame overo genetics or any other combination of genes, resulting patterns that are difficult to smoothly classify into any one group.
See also
- Pinto horsePinto horseA pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses frequently have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto...
- American Paint HorseAmerican Paint HorseThe American Paint Horse is a breed of horse that combines both the conformational characteristics of a western stock horse with a pinto spotting pattern of white and dark coat colors. Developed from a base of spotted horses with Quarter Horse and Thoroughbred bloodlines, the American Paint Horse...
- Lethal white syndromeLethal white syndromeLethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...
- TobianoTobianoTobiano is a spotted color pattern commonly seen in Pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is present from birth and does not change throughout the horse's lifetime, unless the horse also carries...
- Sabino horseSabino horseSabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...
- ToveroToveroThe Tovero coloration is a mix of tobiano and overo colorations in Pinto horses and American Paint Horses. The genetics of pinto coloration are not always fully understood, and some horses have a combination of patterns that does not fit cleanly in either category.Some characteristics of a Tovero...
- Equine coat colorEquine coat colorHorses exhibit a diverse array of coat colors and distinctive markings. A specialized vocabulary has evolved to describe them.While most horses remain the same color throughout life, a few, over the course of several years, will develop a different coat color from that with which they were born...
- Equine coat color geneticsEquine coat color geneticsEquine coat color genetics determine a horse's coat color. There are many different coat colors possible, but all colors are produced by the action of only a few different genes. The simplest genetic default color of all domesticated horses can be described as either "red" or "non-red", depending...