Overo
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Overo refers to several genetically unrelated
pinto Pinto is a Portuguese language, Portuguese, Spanish language, Spanish, Sephardi Jews, Jewish (Sephardic), and Italian language, Italian surname. It is a high-frequency surname in all List of countries and territories where Portuguese is an officia ...
coloration patterns of white-over-dark body markings in
horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...
s, and is a term used by the
American Paint Horse Association The American Paint Horse Association (APHA) is a breed registry for the American Paint Horse. It is currently headquartered in Fort Worth, Texas.
to classify a set of pinto patterns that are not
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 almost always present from birth and does not c ...
. ''Overo'' is a Spanish word, originally meaning "like an egg".''American Paint Horse Association's Guide to Coat Color Genetics'' American Paint Horse Association, 2007.
Accessed August 19, 2008
The most common usage refers to frame overo, but
splashed white Splashed white or splash is a horse coat color pattern in the "overo" group of spotting patterns that produces pink-skinned, white markings. Many splashed whites have very modest markings, while others have the distinctive "dipped in white paint" ...
and sabino are also considered "overo". "Genetic Equation: The overo patterns" ''American Paint Horse Association''.
Web page accessed August 19, 2008
"Horse coat color tests"
from the
University of California, Davis The University of California, Davis (UC Davis, UCD, or Davis) is a public land-grant research university near Davis, California. Named a Public Ivy, it is the northernmost of the ten campuses of the University of California system. The institut ...
Veterinary Genetics Lab
A horse with both tobiano and overo patterns is called
tovero The Tovero (also known as Tobero)PHAA
Retrieved on 13 January 2009 c ...
. Frame overo, splashed white, and sabino are three separate pinto patterns, genetically unrelated, that are grouped together under the name "overo". The frame overo pattern tends to have white spots on the flanks and face, surrounded by a dark "frame" of color. If two horses with the frame overo gene are bred together, there is a 25% chance the foal will have
lethal white syndrome Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are bo ...
. Splashed white or splash overo is a group of patterns that tend to have white on the underside, as if a horse ran through white paint with its head lowered. So far 6 alleles on two genes have been discovered to cause splashed white patterns. There is a correlation between the splashed white pattern and deafness. Sabino or sabino overo is a group of patterns which typically has white on the legs, face, and sometimes also belly, often with white ticking or roaning. Several alleles on the KIT gene are known to cause sabino or sabino-like patterns.


Frame overo

The frame overo pattern is the most common of the three types of overo patterns recognized in the
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 ...
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 Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are bo ...
(OLWS or LWS)," Overo-Lethal White Foal Syndrome (OLW)"
, University of Queensland, web page accessed December 1, 2007
the equine version of Hirschprung disease. LWS occurs when a
foal A foal is an equine up to one year old; this term is used mainly for horses, but can be used for donkeys. More specific terms are colt for a male foal and filly for a female foal, and are used until the horse is three or four. When the foal i ...
is
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
for the Ile118Lys EDNRB mutation, which is considered by many researchers to be "usually responsible" for the frame overo
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
. However, other researchers emphasize that overo spotting patterns are phenotypically and genetically
heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
, that is, may have multiple sources."Horse Coat Color Tests" ''Veterinary Genetics Laboratory,'' University of California, Davis.
web page accessed August 21, 2008
The frame overo gene can be masked by other white patterns, particularly
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 almost always present from birth and does not c ...
, which is a dominant gene and
epistatic Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is dep ...
to overo. Epistatic means that when both genes are present, this is the one expressed. 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 Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
for frame and thus have
lethal white syndrome Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are bo ...
(LWS) are not
albino Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and pink or blue eyes. Individuals with the condition are referred to as albino. Varied use and interpretation of the term ...
s. 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 The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
are undeveloped and the bowel cannot move food along. All lethal white foals die within 72 hours after birth, and are typically
euthanized Animal euthanasia ( euthanasia from el, εὐθανασία; "good death") is the act of killing an animal or allowing it to die by withholding extreme medical measures. Reasons for euthanasia include incurable (and especially painful) conditi ...
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 Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
) 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 The American Paint Horse Association (APHA) is a breed registry for the American Paint Horse. It is currently headquartered in Fort Worth, Texas.
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 Colic or cholic () is a form of pain that starts and stops abruptly. It occurs due to muscular contractions of a hollow tube ( small and large intestine, gall bladder, ureter, etc.) in an attempt to relieve an obstruction by forcing content out ...
. Lethal white horses should not be confused with non-lethal forms of
white White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...
, such as
dominant white Dominant white (W) is a group of genetically related coat color alleles on the KIT gene of the horse, best known for producing an all-white coat, but also able to produce various forms of white spotting, as well as bold white markings. Prior ...
, which may be completely white with pink skin. Blue-eyed, pink-skinned
cremello 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 ...
s and other horses carrying the
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 ...
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.


Splash overo

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 "overo"

'' Sabino'' is listed a type of overo coloring by the
American Paint Horse Association The American Paint Horse Association (APHA) is a breed registry for the American Paint Horse. It is currently headquartered in Fort Worth, Texas.
.APHA Genetic Equation, "Sabino Spotting"
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 US, particularly in
Spanish Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many Latin American countries **Spanish cuisine Other places * Spanish, Ontario, Can ...
-speaking countries, refers to horses with the speckled roaning 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 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 ''Lethal White'' is the fourth novel in the ''Cormoran Strike'' series, written by J. K. Rowling and published under the pseudonym Robert Galbraith. The novel was released on 18 September 2018. Background ''Lethal White'' is preceded by ...
.


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 almost always present from birth and does not c ...
coloring is the inverse of overo spotting. Tobianos have a vertical spotting pattern, large, rounded spots, white that crosses the topline, dark heads, but mostly white legs and white or multi-colored tail. Typical examples tend to have more white than dark, whereas typical frame overo horses tend to more dark than white, though there are many exceptions in both cases. It is created by a different gene from any of the overo patterns and is a
dominant gene In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
. All tobianos must have at least one tobiano parent. When a tobiano
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
is present, it is
epistatic Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is dep ...
and often masks other patterns.


Tovero and other mixed patterns

A
tovero The Tovero (also known as Tobero)PHAA
Retrieved on 13 January 2009 c ...
horse has pinto spotting patterns that show characteristics of both overo and
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 almost always present from birth and does not c ...
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.


Gallery

File:Miniature Horse Runs Through the Pasture.jpg, Miniature frame overo File:Overo.jpg, Frame overo foal


See also

*
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 through ...
*
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 ...
*
Lethal white syndrome Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are bo ...
*
Equine coat color Horses 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 diff ...
*
Equine coat color genetics Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. The "base" colors of the horse are determined by the Extension locus, which in recessive form (e ...


References

*
"Horse coat color tests"
from the
University of California, Davis The University of California, Davis (UC Davis, UCD, or Davis) is a public land-grant research university near Davis, California. Named a Public Ivy, it is the northernmost of the ten campuses of the University of California system. The institut ...
Veterinary Genetics Lab
"Introduction to Coat Color Genetics"
''from'' Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis. Web Site accessed January 12, 2008


External links



Web Site Accessed August 18, 2008
American Paint Horse Association

Pinto Horse Association of America

Spotted Saddle Horse Breeders & Exhibitors Association
{{Equine coat colors Horse coat colors