Frame Overo (LWO)
Frame Overo is a highly desirable white pattern gene. All Frame Overo horses carry a single inherited copy of the the Ile118Lys EDNRB mutation. This mutation causes pigment loss, producing white markings on certain areas of the horse. While the mutation produces visually desirable horses, it is also linked to a fatal condition known as Lethal White Overo or (LWO), whereby a foal is born almost pure white in appearance, and dies within its first few days of life. Correct breeding can avoid this occurrence.
LWO occurs when a horse inherits two copies of the mutated gene, one from both parents. Whereas horses with just one copy of the gene will live normally and exhibit the desirable pattern. A horse with two copies of the mutated gene will suffer intestinal abnormalities caused by undeveloped nerves of the foal's digestive system. These animals die within the first 72 hours of being born and are typically euthanized sooner for humane reasons.
Frame Overo horses which carry just a single copy of the gene, will pass one copy of it to their foals approximately 50% of the time when bred. Therefore, when breeding an Overo horse to a solid non-Overo horse, the foal can only inherit one copy. However, if two Overo horses are bred together they could potentially BOTH pass the Overo gene to the foal, meaning it inherits two copies. Horses which inherit two copies of Frame Overo will suffer the Lethal White condition. Proper mating must be carried out to ensure that two frame Overo horses do not breed. This will prevent any risk of the foal inheriting two copies of the mutated gene.
Animal Genetics offers DNA testing for Frame Overo. The genetic test verifies the presence of the Ile118Lys EDNRB mutation, and presents results as one of the following:
|O/O||Homozygous||A Lethal White foal which carries two copies of the frame Overo gene would test as homozygous for frame Overo. Since no living frame Overo horse more than a week old will test as being homozygous, it applies only to horses in the Lethal White condition.|
|n/O||Heterozygous||Horse carries just a single copy of frame Overo, and therefore exhibits the desirable frame Overo markings. Since frame Overo is a dominant gene, the coat pattern should be present in all horses with a single copy of the mutated gene.|
|n/n||Negative||Non-Overo or 'solid' horse|