Overo
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Overo refers to a pinto coloration pattern of white over dark body markings in horses. There are at least three different genes which fall under the "overo" classification: frame overo, sabino, and splash overo. All but sabino are dominant genes although can be so minimally expressed on an individual that the animal is mistaken for a "solid" colored horse. Overos often have fully blue or partially blue eyes. [1] To complicate matters further, some of the spotting gene patterns can be combined to produce a horse with multiple color traits. The genetics of overo and related patterns are still being researched and are not fully understood.
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[edit] Frame Overo
A horse with frame overo patterning 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, rather than rounded shape, the white rarely crosses the back, the lower legs are normally dark, and the tail is one color, usually dark. The head is often white or bald-faced, and blue eyes are not uncommon. Frame overo coloring is controversial because it was originally believed connected to lethal white syndrome, though subsequent research has found the lethal gene carried by horses with other color patterns. Thus the link between frame overo coloring and lethal white is somewhat disputed and quite controversial (see below).
However, lethal white horses should not be confused with genetically white horses, nor with cremellos, perlinos, smoky creams, or "fully expressed" white tobiano or sabinos, any of which may also be completely white with pink skin.
[edit] Splash Overo
A splash overo pattern appears like a solid colored horse who has been dipped in white paint, and the color splashed up from the bottom. Splash horses may be more prone to being deaf than other horses [2].
[edit] Sabino "Overo"
- Main article: Sabino horse
Sabino is often listed as a type of overo coloring, though genetically it is quite different.[3] One reason for this terminology is that the term "overo" is 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.
The most common markings include high white stockings (often with jagged edges), a wide blaze, often extending past the eyes (crooked blazes are also common), roaning at the edges of white markings, lip spots, "lacing," and small white patches on the belly or flanks. Sabino coloring is thought to be polygenic, caused by multiple genes. [4] However, there is another theory that Sabino is recessive. [5] One gene that is linked to Sabino coloring, SB1, now can be detected with a DNA test. [6]
The minimal sabino may only have one of the traits associated with sabino horses, such as high white, a bald face, or belly spots. On the other hand, a "maximum sabino" is a completely white horse. Because some sabinos can be totally white, it appears that this gene does not carry the lethal white trait.
[edit] Tobiano and Tovero
Tobiano coloring is the inverse of Overo spotting and appears to be caused by a different gene. 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. A tovero horse has pinto spotting patterns that show characteristics of both overo and tobiano. 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.
[edit] "Lethal White"
- Main article: Lethal white syndrome
Regardless of whether there is any link between overo coloring and lethal white, foals which are homozygous for the lethal white gene are not true albinos. They are born completely white with pink skin, but have brown or blue eyes, not red ones. The lethal trait is that the foal's digestive system is undeveloped. All lethal white foals will die a few days after birth if not 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.
[edit] References
- ^ UC Davis web site on Overo genetics
- ^ APHA Genetic Equation: Splashed White
- ^ APHA Genetic Equation, "Sabino Spotting"
- ^ APHA Genetic Equation, "Sabino Spotting"
- ^ Information on recessive Sabino
- ^ The Sabino Pinto test
[edit] See also
- Pinto horse
- American Paint Horse
- Tobiano
- Sabino horse
- Tovero
- Equine coat color
- Equine coat color genetics
