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."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.
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.