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How OLWS is Inherited

Paint horse stallionHow is the overo lethal white syndrome inherited? It's simple. Two copies of the gene makes for a lost foal; but one copy from each parent is the only way this happens. If breeding stock is tested, and only two clear, or carrier to non carrier matings take place, their foals are safe. Two carriers will always have a 25% chance, each time, of having a foal that will be a lethal white foal.

How does this work? This means each foal from two carriers has the same chance; not one in four of the mare's foals, but each foal she delivers. On average, this means that if the mare were to have one hundred foals sired by the same stallion, twenty-five would be non-viable foals, fifty would be carriers, and another twenty-five foals would be clear.

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Overo Lethal White Syndrome

Paint mareOvero Lethal White Syndrome (OLWS) is a condition that occurs in newborn foals. The condition is genetic, caused by a recessive gene, and both parents *must* carry a copy of the gene for a foal to be born with this defect. Horses that carry this gene are most commonly overo white patterned horses (frame overos), but there are exceptions.

The defective gene has been found in American Paint Horses, American Miniature Horses, Half-Arabians, Thoroughbreds, and horses formerly called cropout Quarter Horses (foals born to registered Quarter Horse parents that had too much white to qualify for registration with the American Quarter Horse Association. This rule was removed from the AQHA at the 2004 Convention). The mating of two horses carrying the recessive gene will statistically result in a 25% chance of a lethal white foal.

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