One copy of the frame overo gene (heterozygous-nO) is necessary to get the white patches.
Two copies of the gene (homozygous-OO) is fatal and known as Lethan White Overo. Foals who are born with Lethal White Overo are usually all white at birth and die within 72 hours of birth. Lethal White Overo causes the intestinal system to not develop properly so food is not digested. Since overo is a desirable coat color and requires one frame overo gene, careful breeding must be done to prevent Lethal White Overo. A genetic test for Lethal White Overo exists.
Overo is present in a number of breeds including Paints, Thoroughbreds, Quarter Horses, Miniature Horses and others.