Overo

Overo refers to several genetically-unrelated pinto coloration pattern of white over dark body markings in horses. Overo is a Spanish word, originally meaning “like an egg."^[1]There are at least three genetically different different spotting patterns which fall under the "overo" classification: frame overo, sabino overo, and splash or splashed white overo.^{[2] [3]} 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 overo and related pinto patterns are still being researched and are not fully understood. The frame and splash traits may be either polygenic, dominant, or incomplete dominants, although they can be so minimally expressed on an individual that the animal is mistaken for a "solid" colored horse. The sabino pattern is even more complex, as there has been one gene located, SB-1, that creates one type of sabino patterning, but not all sabino horses carry it. Unlike SB-1 and tobiano patterning, there is currently no DNA test that can identify a single overo gene or gene complex other than a test for the allele associated with the frame overo pattern which causes the controversial lethal white syndrome.

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; there are overo horses that do not carry the lethal white gene.^[1]

Frame Overo

Frame overos have sharply-defined, irregular, horizontally-oriented white patches. As here, they are often bald-faced and white patches seldom cross the topline, creating a "frame" of non-white coat.

The frame overo pattern is the most common of the overo patterns.^[1] 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.^[2] 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.^[2]

Frame overo coloring is controversial because it is strongly associated with lethal white syndrome (OLWS or LWS).^[4] 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.^[5] However, research is not yet conclusive, and other researchers emphasize that overo spotting patterns are phenotypically and genetically Heterogeneous, that is, may have multiple sources.^[6]

The frame overo gene can be masked by other white patterns, particularly tobiano, which is epistatic to overo.^[7] In addition, some carriers of the LWS allele appear to be solid. One theory holds that such horses are overo, but are so minimal in expression that they appear solid.^[2] Another study suggests that this is simply evidence of variable penetrance of the LWS mutation.^[7] Either way, all cases of LWS foals have horses with overo patterning in their pedigrees, and so the link is not necessarily connected to the visible expression of the color.^[4]

"Lethal White"

Foals which are homozygous for the lethal white gene are not true albinos. They are born almost or completely white with pink skin, but have blue eyes, not red ones. The lethal trait is that the foal's digestive system is 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 completely healthy and show no symptoms of any kind.^[1]

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 ever producing affected foals.^[2] The mutation has not been found in solid-colored horses from breeds without white patterning.^[7]

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 overo and sabino patterns have a low correlation to LWS.^[5]

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.^[1] The legs an 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.^[1] Splash overos are more prone to being deaf than other horses.^[8]

Sabino "Overo"

A Sabino, called "sabino overo" by some registries

Sabino is listed a type of overo coloring by the American Paint Horse Association.^[9] 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."^[1] Another reason for this terminology is that the term "overo", when used outside of the USA, particularly in Spanish-speaking countries, to refer 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.^[1]

In general, sabino is a generic description for a group of similar white spotting patterns.^[6] 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.^[1] A minimal sabino may only have one of the traits associated with sabino horses.^[1] On the other hand, a "sabino-white" is a near-white or completely white horse, and sabino genetics are not linked to lethal white.^[6]

Tobiano

The tobiano pattern

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.^[7]

Tovero and other mixed patterns

A Mustang with multiple overo characteristics, including a "medicine hat."

This horse has mostly frame overo characteristics, including a horizonal spotting pattern, white face, and dark tail, but it also has sabino traits of roaning, jagged white leg markings and lip spots. Thus, its coloring suggests it may carry both frame overo and sabino genetics.

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.

See also

• Pinto horse
• American Paint Horse
• Lethal white syndrome
• Tobiano
• Sabino horse
• Tovero
• Equine coat color
• Equine coat color genetics

References

• Paul D. Vrotsos RVT, Elizabeth M. Santschi DVM (Jul 1998). "Stalking the Lethal White Syndrome". Paint Horse Journal. http://www.apha.com/breed/lethalwhites03.html. 
• "Horse coat color tests" from the UC Davis 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

• "Splashed White horses, Min to Max expression" Web Site Accessed August 18th, 2008
• American Paint Horse Association
• Pinto Horse Association of America
• Spotted Saddle Horse Breeders & Exhibitors Association

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