Many modern horse geneticists would argue that there are at least three types of Overo genes responsible for three main patterns: Frame Overo, Sabino and Splashed White. It is believed that one, two or all three of these patterns can be present in the genetic make-up of a single horse – perhaps due to mutation(s) in the (presumably) original "Sabino" gene. This would explain why up to 80%+ of an Overo horse's foals will be coloured, when bred to a solid horse, for some overos. This is particularly the case for those with a large percentage of white in their coats, believed to be caused by different overlapping patterns.. Were a single gene responsible for the colour pattern, as is the case with Tobianos, the ratio of pass-through to progeny would be a straightforward 50%. The multiple gene theory would also explain the higher diversity in "Overo"-type vs Tobiano patterns.

That said, today's research has yet to substantiate this multiple Overo gene theory. Until it does, APHA only formally recognises an "Overo" pattern category. This is basically a classification for horses with "non-Tobiano" white markings. Frame, Sabino and Splashed White characteristics of "Overo" are referenced. But it is not possible to formally categorise a horse by these indicative markings today, since a "Sabino" can look like a "Frame Overo", and a "Frame Overo" like a "Splashed White", etc. Further, there is likely more than one "Overo gene" behind such patterns – and some of these genes may be recessive or otherwise influenced by other genes. Genetic testing would be the only way to ensure that these patterns were correctly catalogued. As only the "Frame" Overo gene has been isolated, no further genetic test for Overo genes is available.

Regardless, I've attempted to describe and illustrate patterns on the webpages that follow, for Frame Overo, Sabino and Splashed White

As a side note, and to confuse things even further, some Paint Horse professionals believe that chestnut or sorrel coloured horses are more likely to display a "loud" form of the Overo pattern (a'la Hidalgo), at least in some horse families. Indeed, non-sorrel / chestnut loudly marked Overo Paints (particularly Frame Overo) are difficult to find, particularly at many well-regarded Paint Horse breeders which concentrate on conformation vs coat colour. This would suggest there could also be some sort of link between colour gene(s) and the Overo gene(s).