- Category: OLWS
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How 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.
Translating this to a mare's actual production, she may produce several full siblings in a row that are carriers, and her last few foals all clear. Or, her first four foals may all be lost, dying painfully only days after birth. To really understand the way it works, take a hundred pennies, and toss them, count the number one times they come up heads or tails. You may see streaks of ten heads in a row; likewise, your mare may produce several healthy foals, too; and then the next, a lethal white. Without applying the 25% rule to *each* birth, you can easily misunderstand how the genetic dice roll. With a bunch of pennies, it's easier to see.
Luckily, the genetic test takes the worry of OLWS out of your breeding decision. Outstanding examples of the breed may be used safely within anyone's breeding program, from any bloodline, provided you use non-carriers with the carrier mare, or stallion. The rules shown in the Punnitt square below work the same for all carriers of any breed, family, or background.
With the high cost of producing a healthy foal today, it only makes sense to save ourselves and our mares the expense and heartbreak of losing a foal we've waited so long for. We have an affordable and proven test we can rely on. Remember, "a healthy foal is the goal!"