Frame Overos, or "Overos" as they are commonly called, are generally associated with some of the liveliest colour markings in horses.This Paint pattern has been relatively geographically restricted, occurring mainly in horses with ancestry in North America. It is believed to have evolved from a mutation in the Sabino gene. The Frame gene often produces among the most striking and vibrant patterns in paints/pintos, particularly for those horses with more evenly balanced (50:50) white/colour markings.
The Frame Overo pattern manifests itself in many forms, but it is often represented by irregular white patches that spread more-or-less horizontally along the neck and body.
- Head: The head often has white markings which can be large and/or asymmetrical. An "apron" or "bald" face is not uncommon -- and may be the only white feature in a minimally marked overo. "Moustache" markings, representing a patch of dark skin on the upper lip, are not unusual.
- Top line: The white will almost never cross over the top line except in maximally marked horses (and even then, this generally occurs only at the top of the neck line). This is what often gives the horse its "framed" appearance, when standing profile.
- Tail and mane: These are usually fully coloured, though particularly "loud" horses may have some white hairs.
- Hooves and lower legs: Hooves are often dark, as are the lower legs, unless the Sabino gene is also present; it often is thought to be. But even so, unless the coat pattern has a high percentage of white, generally, at least one lower leg will be solid vs white.
- Eyes: Eyes can be blue ("wall") and/or brown, green or amber.
- Spotting pattern: Body patterns tend to be irregular vs symmetrical. The marking usually contrasts sharply against the white, with few / no shadows or jagged edges, even on the legs... unless another overo gene is present.
As with Tobianos, Frame Overo markings are triggered by a dominant gene. Any horse with one of these two genes (i.e. heterozygous) will display a Frame Overo pattern, if only in a "minimal" form. One parent must be a Frame Overo in order to produce a Frame Overo (or Tovero) foal.
When breeding a heterozygous Frame Overo to a solid horse, there is, in theory, a 50% chance that the foal will also carry the Frame gene. However, as this gene is believed to occur along with other "Overo" genes, some such horses will throw a higher percentage of coloured foals (not necessarily Frames) when bred to a solid horse. Some Frames can appear entirely white, or entirely solid, though they are capable of producing coloured foals.
A Frame Overo does not exist in a homozygous state. A Frame Overo bred to another Frame Overo gives a theoretical 25% chance of producing a homozygous foal. Born pure white, these foals are doomed. All are born with severely defective intestinal track which cannot be corrected, and other serious deformities are also common. As with the so-called Lavender Foals in Arabs, these Lethal White offspring fail to thrive beyond a few days, if not hours. Fortunately, there is a DNA-based test to detect whether the horse is carrying this gene, dubbed the Overo Lethal White Gene (OLWG), so that such Frame-to-Frame matings can be avoided.