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. 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. 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. Unlike tobiano patterning, there is currently no DNA test that can identify an overo gene or gene complex.
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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 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.
Frame overo coloring is controversial because it was originally thought to somehow "cause" lethal white syndrome. However, though there is some type of correlation, as all cases of LWS foals have horses with overo coloring in their pedigrees,[2] the link is not necessarily connected to the visible expression of the color; many overo horses do not carry the LWS gene, and some non-overo horses do. Thus, the existence of a link between frame overo and lethal white is disputed and controversial (see below). Fortunately, there is a DNA test for the gene which can be used to plan breedings and avoid ever producing affected foals.
Lethal white horses should not be confused with genetically white horses, nor with cremellos, perlinos, smoky creams, or "fully expressed" white 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 [3].
[edit] Sabino "Overo"
Sabino is often listed as a type of overo coloring.[4] 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. However, genetically, it is quite different; it appears to be a gene-complex, and one gene, SB-1 has been mapped to the equine KIT gene, in the same general region as the tobiano gene, though Sabino is not created by the same genetic mechanism as tobiano, either. [5]
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, that is, caused by multiple genes. [6] However, there is another theory that Sabino is recessive. [7] One gene that is linked to Sabino coloring, SB1, now can be detected with a DNA test. [8]
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 is created by a different gene that is a dominant gene. (All tobianos must have at least one tobiano parent). 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 and probably carries genes for both patterns at the same time. 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"
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 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.
[edit] See also
- Pinto horse
- American Paint Horse
- Lethal white syndrome
- Tobiano
- Sabino horse
- Tovero
- Equine coat color
- Equine coat color genetics
[edit] References
- ^ "Horse coat color tests" from the UC Davis Veterinary Genetics Lab
- ^ " Overo-Lethal White Foal Syndrome (OLW)", University of Queensland, web page accessed December 1, 2007
- ^ APHA Genetic Equation: Splashed White
- ^ APHA Genetic Equation, "Sabino Spotting"
- ^ Haase B, Brooks SA, Schlumbaum A, Azor PJ, Bailey E, et al. (2007) "Allelic Heterogeneity at the Equine KIT Locus in Dominant White (W) Horses." PLoS Genet 3(11): e195 doi:10.1371/journal.pgen.0030195
- ^ APHA Genetic Equation, "Sabino Spotting"
- ^ Information on recessive Sabino
- ^ "Horse coat color tests" from the UC Davis Veterinary Genetics Lab
- Paul D. Vrotsos RVT and Elizabeth M. Santschi DVM. University of Minnesota Genetics Group. "Stalking the Lethal White Syndrome". Paint Horse Journal. July 1998.
- "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
[edit] External links
- American Paint Horse Association
- Pinto Horse Association of America
- Spotted Saddle Horse Breeders & Exhibitors Association
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