Overo refers to a pinto coloration pattern of white over dark
body markings. Frame Overo, Sabino, and Splash Over are the three different
genes that fall under the "Overo" classification. All but Sabino are
dominant genes although can be so minimally expressed on an individual that
the animal is mistaken for a "solid" colored horse. The eye colors are often
fully or partially blue. To confuse you even more, some of the spotting gene
patterns can be combined to produce a horse with multiple color traits. The
genetics of overo and related patterns are still being researched and are
not fully understood.
A horse with frame overo patterning appears to be any solid base
color (bay, black, chestnut, etc.) with white irregular patches added,
usually with a horizontal orientation. Markings are often of jagged, rather
than rounded shape, the white rarely crosses the back, the lower legs are
normally dark, and the tail is one color, usually dark. The head is often
white or bald-faced, and blue eyes are not uncommon. Frame overo coloring is
controversial because it was originally believed connected to lethal white
syndrome, though subsequent research has found the lethal gene carried by
horses with other color patterns. Thus the link between frame overo coloring
and lethal white is somewhat disputed and quite controversial (see below).
However, lethal white horses should not be confused with genetically white
horses, nor with cremellos, perlinos, smoky creams, or "fully expressed"
white tobiano or sabinos, any of which may also be completely white with
A splash overo pattern appears like a solid colored horse
who has been dipped in white paint, and the color splashed up from the
bottom. Splash horses may be more prone to being deaf than other horses. A
lot of people see Splash Overo horses completely opposite from the Tobiano.
It's very common for the Splash Overo Horses to have 4 white legs.
Sabino is often listed as a type of overo coloring, though
genetically it is quite different. One reason for this terminology is that
the term "overo" is used outside of the USA, particularly in
Spanish-speaking countries, to refer to horses with the speckled roaning
patterns typical of horses called Sabino in the USA.
The most common markings include high white stockings (often with jagged
edges), a wide blaze, often extending past the eyes (crooked blazes are also
common), roaning at the edges of white markings, lip spots, "lacing," and
small white patches on the belly or flanks. Sabino coloring is thought to be
polygenic, caused by multiple genes. However, there is another theory that
Sabino is recessive. One gene that is linked to Sabino coloring, SB1, now
can be detected with a DNA test.
The minimal Sabino may only have one of the traits associated with Sabino
horses, such as high white, a bald face, or belly spots. On the other hand,
a "maximum Sabino" is a completely white horse. Because some Sabinos can be
totally white, it appears that this gene does not carry the lethal white
Tobiano and Tovero
Tobiano coloring is the inverse of Overo spotting and appears to
be caused by a different gene. Tobianos have a vertical spotting pattern,
large, rounded spots, more white than dark, white that crosses the back,
dark heads, but mostly white legs and white or multi-colored tail. A tovero
horse has pinto spotting patterns that show characteristics of both overo
and tobiano. For example, a tovero might have tobiano body spotting with
rounded edges and white across the back, yet have irregular facial markings
and blue eyes.