Lethal White Overo: Pinto Patterns and OLWS Risk
By Breedio team
In short
Tobiano, frame overo and tovero are inherited differently, and only frame overo carries a lethal risk. Learn how overo lethal white syndrome works and why both parents must be tested.
Lethal white overo syndrome (OLWS) strikes a foal that inherits the frame overo mutation from both parents: it is born white and dies within days. Two frame carriers have a 1 in 4 chance of producing such a foal at every mating, and a carrier can show almost no white, so a DNA test on both parents is the only reliable check. This guide is part of our horse coat colours guide and explains the main pinto patterns, how each is inherited and which one carries a health risk.
What are the main pinto patterns?
A pinto (called pie in French) is a horse with large white patches over a coloured coat. Several different genes can produce this look, and they do not behave the same way. Three patterns matter most for breeders:
- Tobiano: white that crosses the topline, usually white legs, a head that is mostly dark, and smooth, rounded edges.
- Frame overo: white patches that stay on the sides of the body and do not cross the back, often with a white face and dark legs, with crisp edges.
- Tovero: a mix of tobiano with overo-type patterns, giving a horse with a lot of white and often a coloured head and chest.
Sabino, splash and the leopard complex also produce white patterns. They involve other genes and some of them are not covered by the Breedio calculator, so we mention them only as separate patterns. Remember that the word "overo" is a visual term: a horse called overo can carry frame, sabino or splash, and only frame is linked to OLWS.
How is each pattern inherited?
The table below sums up the genes, the inheritance and the health consequences.
| Pattern | Gene | Inheritance | Health risk |
|---|---|---|---|
| Tobiano | KIT region | Dominant: one copy shows the pattern | None known, homozygous horses are healthy |
| Frame overo | EDNRB | One copy gives the frame pattern (often subtle), incompletely dominant | Two copies cause lethal white foal syndrome (OLWS) |
| Tovero | Tobiano plus another pattern gene | Each gene follows its own rule | Depends on the genes present: a frame copy makes the horse a carrier |
| Sabino | Several genes | Varies, not covered by the calculator | Depends on the variant |
| Splash | Several genes (for example PAX3, MITF) | Varies, not covered by the calculator | Some alleles are linked to deafness; Splashed White 3 homozygotes have not been found and are assumed lethal |
| Leopard complex | LP | Dominant, not covered by the calculator | Two copies cause congenital stationary night blindness |
Tobiano is the easy one. It is dominant, so a tobiano foal needs at least one tobiano parent. A tobiano bred to a solid horse gives about 50% tobiano foals. A horse with two copies is just as healthy as one with a single copy, and it passes the gene to every foal. The pattern sits on top of any base coat, which is why you meet a pie alezan (chestnut pinto), a pie bai (bay pinto) or a pie noir (black pinto).
What is lethal white overo syndrome?
OLWS is a recessive genetic disorder caused by a two base pair change in the endothelin B receptor gene (EDNRB, p.Ile118Lys). The gene was identified in 1998 and was the first horse white-spotting gene found, at the UC Davis Veterinary Genetics Laboratory. The syndrome has been known in Paint Horses since the 1960s.
EDNRB guides the migration of embryonic nerve cells that also contribute to skin pigment. A foal with two mutated copies (O/O) lacks the nerve plexuses in the lower bowel. The result is ileocolonic aganglionosis: the descending colon and rectum are underdeveloped and the gut cannot move its contents.
What does an affected foal look like?
- Fully or almost fully white coat, pink skin and blue eyes at birth.
- No meconium passed in the first 24 hours.
- Colic signs (rolling, thrashing) usually between hours 12 and 16 of life.
- Death within about 48 hours to a few days, from bowel rupture or a build up of toxic waste in the blood.
There is no treatment. Surgical bypass of the lesion has never worked, and enemas or analgesics such as flunixin meglumine only relieve signs for a short time. Veterinarians therefore advise humane euthanasia once the diagnosis is made.
Two cautions apply. First, not every white foal from Paint parents has OLWS: a white foal that is not O/O can also struggle with meconium, and an OLWS foal can still show a few coloured spots. A differential diagnosis, including colonic aganglionosis, should be made before any decision. Second, a DNA test settles the question: a case report in a 16-hour-old white filly out of a solid Quarter Horse mare confirmed the mare as heterozygous and the foal as homozygous for the mutation.
What are the odds for each mating?
OLWS follows simple Mendelian rules. N is the normal allele and O the frame mutation.
| Mating | Solid (N/N) | Frame carrier (N/O) | Lethal white (O/O) |
|---|---|---|---|
| N/N x N/N | 100% | 0% | 0% |
| N/O x N/N | 50% | 50% | 0% |
| N/O x N/O | 25% | 50% | 25% |
The key figure is the 1 in 4 risk (25%) for two carriers, at every pregnancy, whatever happened with the previous foals. A carrier bred to a horse that is clear of the mutation can never produce an affected foal, but half the foals will be carriers. In stud book data the observed rate of lethal white foals is lower than 25%, possibly because of unreported cases or early embryonic loss, so do not count on luck.
Why can a frame carrier show almost no white?
This is the trap. The frame pattern is highly variable: some N/O horses have big, obvious white patches, while others have a small white face, a thin belly spot or nothing visible at all. The pattern can even be hidden by a heavy tobiano or by a coat that is mostly white from other genes.
The figures show how misleading appearance is:
- More than 94% of frame overos are carriers, as expected.
- But among heterozygous horses, over 95% were overo and fewer than 1% were solid-coloured, which means a solid horse can still carry the gene.
- Up to 20% of broodstock from overo lineage without much white also carry it.
The mutation is not limited to Paint Horses. It has been found in Quarter Horses, Thoroughbreds, Miniature Horses, Half-Arabians, cremellos and Appaloosas, and in horses of unknown origin descended from carriers. A solid mare with Paint ancestry deserves the same attention as a flashy overo. Phenotype, the amount of white, is an unreliable way to identify carriers.
Why test both parents before breeding?
Because the risk exists only when both parents carry the allele, and neither can be spotted by eye. Testing one parent tells you nothing about the cross. A test costs about 30 to 55 euros in France and takes 10 to 20 days, far less than a foal lost days after birth.
Practical points:
- Test the stallion and the mare before the breeding season, while the cross can still be changed.
- Test horses with Paint, Quarter Horse or overo ancestry, even when they are solid.
- Send the right sample. Most labs want 20-30 hairs pulled with the roots from the mane or tail (the shaft alone has too little DNA). Blood must be unclotted, refrigerated and delivered within 24 hours.
- Keep the result with the horse's records. A known N/O mare can be bred safely to an N/N stallion.
- Remember the limit of the test. It covers the known EDNRB variant. A rare cross of two N/N horses could still give an affected foal if another mutation were involved.
Carriers are healthy and valuable breeding animals, apart from possible deafness reported in some. The aim is not to remove carriers, only to avoid carrier to carrier matings. In France, the coat on the papers is recorded by the SIRE/IFCE, but the paper coat does not replace a genotype.
How does the calculator flag the OLWS risk?
The Breedio foal coat colour calculator is free and runs in your browser. Enter the coats of the mare and the stallion, add their parents' coats to narrow the range, and add DNA test results when you have them. It returns the foal's possible adult coats with their odds and covers Extension, Agouti, Cream, Champagne, Silver, Dun, Grey, Roan, Tobiano and Frame overo. When both parents may carry frame overo, it warns about the lethal white (OLWS) risk.
A coat does not show every gene, so the result is a range. Grandparents narrow it, and a DNA test confirms it. Once your mating is planned and the mare is in foal, the Gestation Tracker follows her through the roughly 340 days of pregnancy, and you can find the other tools on the Breedio tools page.
Key points to remember
- Tobiano is dominant and harmless, even in two copies.
- Frame overo is the only common pinto pattern linked to a lethal syndrome in the foal.
- Two frame carriers have a 1 in 4 risk per foal, and affected foals die within days.
- A carrier can look solid or nearly solid, so test both parents.
