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Lethal White Overo: What you should know

an classic example of frame expressed on a chestnut horse

A classic example of frame on a chestnut horse. Notice how the white is confined to the barrel, has jagged edges, and leaves the legs solid. Photo Courtesy of Ancient Oaks Sport Horses

a black and white homozygous tobiano horse who is also a frame carrier

Homozygous tobiano stallion with frame. He is homozygous for black as well. An example of how frame can "hide" in other patterns. Photo courtesy of Emily West

an example of minimal frame on a black horse

A good example of just how minimal frame can be. Although rare, it is necessary for breeders to be acknowledge the fact that frame can be extremely minimal. He has been DNA tested positive for Frame (LWO). Photo is courtesy of Celtic Mist Miniature Horses

A lethal white foal  Homozygous for frame

The sad result of a frame to frame breeding. This foal was humanly euthenized when he began showing signs of distress. In this case the owner knew her mare was positive for frame but had been assured by the stallion owner that his stallion was negative. As you can see that was not the case.

Frame is a white pattern in horses that puts white on the barrel and the face of a horse. It tends to leave white markings with jagged edges but no "halos" (areas of dark skin under white hair) or roaning. Frame can cause blue eyes and face white but prefers to leave the legs solid. Horses positive for frame can have leg white, but it is usually added by a separate white pattern. Any color horse can be a frame carrier and frame's expression can very greatly from a "solid" animal to one who is wildly marked. Frame will rarely cross the top line of the horse without another white pattern being present. Because of frame's wildly varying expression, as well as its ability to "blend" with other patterns, it can be very difficult to tell if a horse carries frame simply by visual inspection. Frame is the gene responsible for Lethal White Overo Syndrome. It is therefore important, for everyone who may breed a frame positive horse, to understand how frame is inherited and how it affects foals.

Lethal White Overo foals are born almost solid white with blue eyes. They are born with an under-functioning digestive track which results in severe colic 24-72 hrs after birth. Only foals who carry two copies of the frame gene will be "Lethal White" therefore, both parents must be carriers. Since, 25% of frame to frame breedings will result in a lethal white foal, never take someones word that their stallion is negative for frame. If your mare is positive for frame always ask to see the stallions test results. If you are considering buying a mare and the owner states she is negative for frame again, ask to see the test results. If they really have tested they won't mind sharing the result. If they haven't tested can test yourself. The only case where a test would not be necessary is if a horse's sire and dam have both tested negative for frame. Since neither parent carried a copy of the frame gene the foal (or horse as the case maybe) cannot be positive for frame.

Breeding frame to frame will never increase your chances of getting a colored foal. It does increase your chances of a dead foal. In any frame to frame breeding there is a 25% chance of getting a frame negative foal, a 50% chance of getting a frame foal, and a 25% chance of getting a dead foal (ie. Lethal White). In frame to solid breedings these numbers change to 50% frame foal and 50% negative frame foal. Notice the lack of a dead foal. The 50% chance of frame stays the same. If your mare (or stallion for that matter) carries frame and you want to maximize your chances of getting white pattern on your foal, breed your frame pattern horse to a tobiano or sabino (who is negative for frame of course). It is possible to get a solid white foal that is not Lethal White Overo. These foals are dominant white, maximum sabino, or carry multiple patterns. The frame pattern is caused by a missense mutation in the gene EDNRB. Testing is available from both UC Davis and Animal Genetics Visit the Frame Photo Gallery

Breeds that Carry Frame*

Possible- Based on phenotype

American Saddlebred
American Warmblood
Australian Draught Horse
Australian Stock Horse
Missouri Fox Trotter
National Show Horse
Quarter pony

Known Breeds- based on phenotype and testing

American Paint Horse
American Quarter Horse
American Indian Horse
American Shetland Pony
Miniature Horse
Spanish Mustang
Spotted Kentucky Mountain Saddle Horse
Spotted Saddle horse
Tennessee Walking Horse

*This list was complied by CMHorses on this forum thread