OLWS
Overo Lethal White Syndrome
recessive traitWhat is OLWS?
Overo Lethal White Syndrome (OLWS), also called Frame Overo Lethal White, occurs when a foal inherits two copies of the Frame Overo (O) allele. These foals are born completely white with blue eyes and appear healthy at birth, but their enteric nervous system did not develop normally — the intestines lack the nerve cells needed for normal movement. Affected foals cannot pass manure and die within hours to days of intestinal complications. Single-copy (Frame Overo) horses are healthy and display the distinctive white frame pattern prized in Paint Horses.
How is it inherited?
OLWS follows an autosomal recessive lethal pattern specific to the Frame Overo coat pattern gene (EDNRB). Breeding two Frame Overo horses together produces a 25% chance of a Lethal White foal. A Frame Overo horse bred to a solid (N/N) horse cannot produce an affected foal.
What do the results mean?
| Result | Status | What it means |
|---|---|---|
| N/N | Non-Overo (solid) | No Frame Overo allele. Horse has no frame patterning from this gene. Breeding two N/N horses produces no risk of Lethal White foals. |
| N/O | Frame Overo (carrier) | One Frame Overo allele. Horse is healthy and typically displays the frame overo coat pattern. Can produce Lethal White foals only if bred to another N/O horse. |
| O/O | Lethal White (fatal) | Two Frame Overo alleles. Foal is born all-white and cannot survive. This result is only seen in newborn foals — not in adult horses. |
No Frame Overo allele. Horse has no frame patterning from this gene. Breeding two N/N horses produces no risk of Lethal White foals.
One Frame Overo allele. Horse is healthy and typically displays the frame overo coat pattern. Can produce Lethal White foals only if bred to another N/O horse.
Two Frame Overo alleles. Foal is born all-white and cannot survive. This result is only seen in newborn foals — not in adult horses.
What should buyers know?
For buyers, N/O horses are perfectly healthy and can be ridden at any level. The test result matters only for breeding: avoid pairing two N/O horses to eliminate the risk of a Lethal White foal. N/N horses carry no OLWS risk regardless of breeding partner.
Which breeds are affected?
Exclusive to horses carrying the Frame Overo pattern — primarily American Paint Horses and horses of Paint descent. Not a concern in breeds where frame overo patterning does not occur.
This page is for general education only. Always consult a veterinarian or equine geneticist before making purchasing or breeding decisions based on genetic test results.
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