The overo patterns
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
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
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,
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