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Olw stands for the phrase Lethal White Overo, a mutation of a gene that controls the development of the digestive system. It produces the typical overo paint pattern when heterozygous. However, if the foal inherits this mutation from both parents it is born all white and with an incomplete digestive system. It dies shortly after birth of impaction colic. The shorthand name for the mutation is Frame, because it often leaves a picture frame of pigmented hair around white patches on the sides of the horse.

This homozygous splash horse with frame shows that particular expression. There are others.

AngeloPro

Frame works in concert with other mutations that cause white spotting, either splash or the Kit white spotting genes (but very subtly with tobiano). A horse with no white patterns at all may be carrying Olw without any affect on the appearance of the horse; only a gene test will verify its presence. For this reason, frame is sometimes called a ninja gene.

Olw makes very pretty markings paired with W2, just don't breed Olw with Olw, as if foal gets both copies it's dead