This page is heavily based on the book Equine Color Genetics written by Dr. Phillip Sponenberg, DVM, PhD, published by Iowa State University Press, Ames, in 1996, pages 60 to 78.
Dr. Sponenberg suggests that there are actually three distinct color patterns which are being lumped together under the overo name. He call these three patterns frame overos, sabiano overos and splashed white overos. Each of these patters is genetically different and distinct from one another. Each of these three different overo patterns consistently place and characterize the white. Each of these three different overo patterns is dominent. In the past these were throught to have been recessive and there was some confusion mainly because all three of these patterns were lumped under the same name "overo" when in fact each is a distinct color pattern.
Frame Overos
Characteristics
1. Generally have dark feet.
2. The head is usually marked with extensive white markings, such as a blaze, apron or bonnet face.
3. White spots usually occur on the middle of the sides of the body and neck, these spots rarely cross the topline.
4. The spots are generally clean, although more ragged than tobiano spots.
5. The spots tend to have horizontal arrangement.
6. Blue eyes are very common even when the eye is surrounded by the base coat color (ie. sorrel, black etc. ) rather than the white with unpigmented skin near the eye.
7. The horse may be predominantly dark or white.
Genetics
The frame overo pattern is a dominent allele. In some instances, the frame overo allele may appear to be passed as a recessive allele, but usually due to a minimally marked animal non having any body spots. Cropout horses may imply that there is more than one allele involved, and it may be more complex. These cropout horses do reproduce this frame overo pattern as though the gene were dominant. This suggests that cropout horses may be due to new mutations of this allele.
The frame overo horse may have minimal white to extensive white markings. Horses which have been classified as solid breeding stock horses may be a minimal white frame overo without any body spots, dark legs and extensive white markings on their face, such as a bald face. These horses are expressing the minimum of the frame overo color pattern. The tip-off is usually the combination of abundant facial white with minimal leg white. These horses do not have any body spots, but do produce foals with spots.
The frame overo allele can be responsible for the production of lethal white foals, which die within 48 hours of birth, usually. It has been thought that the lethal white foals are homozygous and have recieved a frame overo from each parent. This may be more complicated because solid horses (by a frame overo parent) crossed with a frame overo have produced lethal white foals. Where these solid horses (by a frame overo parent) really a minimal white frame overos without any body spots or is there another gene or mutation involved?
Sabino Overos
Characteristics
1. Usually involves extensive leg white and facial white.
2. Body spots are usually on the belly, and can either occur as roan areas, speckled areas or rarely as white patches with clean, crisp edges. Sabino horses with clean, crisp edges are often confused with frame overo horses.
3. Most have flecked with white or roaned, especially when there is extensive sabino markings.
4. Sabinos with extensive white markings will usually have roan or speckled areas on the ears, tail base, flank and chest areas.
5. A minimal white sabino will have extensive white marks, on the face and sock, but no white body spots. This is often overlooked, and the horses are thought to be solid.
6. May have blue or partially blue eyes.
Genetics
This gene often acts like a single dominant gene. This color pattern has also produced lethal white foals, while some other white foals are still viable and normal. The sabino color pattern may also include several different distinct patterns which have not yet been researched.
Splash White
Characteristics
1. Splash overos have the appearence of being dipped in white paint.
2. They usually have white legs.
3. The edges between the color and the white is usually crisp and clean. Sometimes the splash overos are really mislabled sabinos with clean edges between the dark and the white coloring. These sabinos also lack the flaking or roan coloring.
4. They often have blue eyes.
Genetics
The splash overo pattern was thought to be recessive but recent studies with overo genetics suggest that the gene is dominant. Homozygotes have not been documented.
Combinations of Paint Patterns
Since each of the paint color patterns are on different genes, including the tobiano, frame overo, sabino overo and splash overo, a horse may have one or more of the different color combinations. If a horse has one of the color patterns and is bred to a solid horse they have a 50% chance of producing colored offspring. If a horse has two of the color patterns (ie a frame overo and tobiano combination) this horse will produce a 75% chance of color when bred to solid horse. If a horse has three of the color pattern combinations (ie. a frame overo, sabino overo, and tobiano combination), this horse will produce 87.5% color when bred to a solid horse. If a horse has all four color patterns they would produce 93.75% colored offspring when bred to a solid horse.