Overo

If two horses with the frame overo gene are bred together, there is a 25% chance the foal will have lethal white syndrome.

[1] A frame overo horse appears to be any solid base color (bay, black, chestnut, etc.)

[2] Frame coloring is controversial because it is associated with lethal white syndrome (OLWS or LWS),[8] the equine version of Hirschprung disease.

[9] LWS occurs when a foal is homozygous for the Ile118Lys EDNRB mutation, which is considered by many researchers to be "usually responsible" for the frame overo phenotype.

[10] However, other researchers emphasize that overo spotting patterns are phenotypically and genetically heterogeneous, that is, may have multiple sources.

One theory holds that such horses carry the frame gene, but so minimal in expression that they appear solid.

[12] 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.

Sabino is listed a type of overo coloring by the American Paint Horse Association.

One reason for the terminology confusion is that "overo" was used by Paint horse breeders in the United States to refer to "anything that is not tobiano.

[9] A tovero horse has pinto spotting patterns that show characteristics of both overo and tobiano and probably carries genes for both.

For example, a tovero might have tobiano body spotting with rounded edges and white across the back, yet have irregular facial markings and blue eyes.