Overo - Frame Overo - "Lethal White"

"Lethal White"

Main article: Lethal white syndrome See also: Dominant white

Foals which are homozygous for frame and thus have lethal white syndrome (LWS) are not albinos. Albinism does not exist in horses. LWS foals are born almost or completely white with pink skin, but have blue eyes, not red ones. The lethal trait is that the nerves of the foal's digestive system are undeveloped and the bowel cannot move food along. All lethal white foals die within 72 hours after birth, and are typically euthanized sooner for humane reasons. A horse can be a carrier of the trait when it carries only one copy of the gene, (that is, heterozygous) and carriers are healthy and show no clinical signs of lethal white syndrome.

Thus, it is recommended by organizations such as the American Paint Horse Association that horses of any pattern with overo ancestors be tested to verify their carrier state before being bred. There is a DNA test for the gene which can be used to plan breedings and avoid producing affected foals. The mutation has not been found in solid-colored horses from breeds without frame patterning. Likewise, a sick foal that happens to be white can be tested to avoid inadvertent euthanization of a non-lethal foal who has a simple case of colic.

Lethal white horses should not be confused with genetically white horses, nor with "fully expressed" white sabinos, any of which may also be completely white with pink skin. Blue-eyed, pink-skinned cremellos and other horses carrying the cream gene do not carry the LWS allele unless they also have an ancestor with overo genetics. The splash white and sabino genetics are not associated with lethal white, though such horses could also be carriers of the frame allele.