The overo patterns
The overo group of patterns has become hopelessly complicated due to the lumping of several genetically distinct patterns under the single name of "overo." Because at this point the nomenclature is hopeless, the best we can do is to forge ahead and try to understand these patterns and how they fit into the overall Paint Horse picture. The key concept is that the term "overo" covers three genetically distinct patterns.
Frame overo spotting
Frame overo is one of the overo patterns. The name "frame" refers to the usual appearance, which is of white patches centered in the body and neck and framed by colored areas around them.
The usual frame pattern has a horizontal arrangement, and does not cross the topline as does tobiano. The head is usually quite extensively marked with white, and the eyes are commonly blue.
The feet and legs of frame overos are usually dark, although white feet and minor white leg marks are as common on frame overos as they are on nonspotted horses. The white areas on frame overos are usually crisply and cleanly delineated from the colored areas, although some have a halo or shadow of pigmented skin under white hair directly at the boundary.
The frame overo pattern occurs in a limited range of horse breeds. It seems to appear only in breeds that have Spanish ancestry, including the Paint Horse.
The genetics of frame overo has only recently been documented. Frame overo behaves as a dominant gene. It is common to mate frame overo horses to nonspotted horses, and about half of the foals come out spotted.
On many occasions, though, there are records of frame overos being produced by two nonspotted parents. This is typical of a recessive gene, and it is not logical to have both a recessive and a dominant control over the same pattern. A look at the parents of these "cropouts" sometimes reveals that one or the other is oddly marked. These oddly marked horses usually have bald faces but colored feet, which is a very unusual combination in horses.
Some of these horses are genetically frame overo, but have failed to get a body spot. They are essentially very dark frame overos--so dark that the spots are all gone from the body. They still have the gene, however, and can still produce frame overo-spotted offspring.
This phenomenon may not account for all the cropouts. For example, the occurrence of the frame overo pattern in the Thoroughbred breed (the racehorse Tri Chrome, for instance) seem to be new examples of this gene in a breed that previously did not have it.
Waiting to see what these cropouts produce will be the final test. Because previous cropouts have reproduced as if they had a dominant gene, there is no reason to expect any differently from the more recent spotted cropouts from solid-colored parents.
At the whiter extreme, the frame overo pattern is responsible for lethal white foals. It is the pattern most closely associated with these foals.
Recent characterization of the gene involved in the lethal white foals has confirmed that the foals with two doses of the gene are white, and die soon after birth from gut innervation abnormalities. Horses with only one dose are frame overos, and survive.
This documentation is important for breeders of Paint Horses. With DNA tests now available for the frame gene (and the lethal white foals that can accompany it) it is possible to test breeding horses. Those with the gene can be mated to horses without it, resulting in about half frame and half nonspotted foals, but avoiding completely the production of lethal white foals.