How the Overo Genes Work
Unfortunately, little is known about the genetics that create these patterns. It is commonly accepted that the overo patterns are under the influence of one or more dominant genes. It appears that each of the patterns may be the result of a dominant gene; but, it is also possible that there is just one gene with other gene modifiers that change the pattern.
There is also some evidence that the genes which produce leg and facial markings may influence the amount of white on an overo. This appears to be true for the sabino and splash white patterns. The frame overo, like the tobiano, is thought to be less sensitive to these genes.
Time and research will eventually answer these questions. For now, here is what the three overo patterns seem to have in common.
All the overo patterns have a large range of expression. At one end of the spectrum they appear mostly white. At the other end, they may be hard to identify from solid horses.
The frame overo has dark color running along the topline. The chest is usually dark as are the legs and the tail.
The color pattern referred to as sabino encompasses a wide range of patterns. It is possible that more than one pattern has been included in this category; but, at the moment, we are going to assume that this is the varied expression of a single gene.
This gene has been designated as the dominant "Sb
The sabino horse characteristically looks like it ran through a vat of white paint while a paint sprayer splatter white creating speckled areas of the body. These roany body spots are usually located on the belly and have crisp jagged edges.
Splashed white is the rarest of the overo color patterns. This pattern looks like a horse ran through a deep vat of white paint. According to Dr. Bowling's recent work, this overo pattern is under the control of a dominant gene identified by the letters "Spl
By the way: Dr. Ann Bowling, Executive Associate Director of the Veterinary Genetics Laboratory UC Davis, CA