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Overo refers to several genetically unrelated pinto coloration patterns of white-over-dark body markings in horses, 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."〔(''American Paint Horse Association's Guide to Coat Color Genetics'' American Paint Horse Association, 2007. ) Accessed August 19, 2008〕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.〔( "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 Veterinary Genetics Lab〕 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, and there is a DNA test for three mutations, SW-1, SW-2, and SW-3, associated with 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. ==Frame Overo== (詳細は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),〔(" 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 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.〔("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, which is a dominant gene and epistatic 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Overo」の詳細全文を読む スポンサード リンク
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