Frame overo is a highly desirable white pattern gene. All frame overo horses carry a single inherited copy of a gene 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). Foals born with LWO are almost pure white in appearance and die within the first few days of life. Correct breeding can avoid this occurrence.
LWO is a recessive genetic disease. This means that a horse requires two copies of the mutated gene from both parents in order to be affected by LWO. Horses with only one copy of the mutated gene present the trademarked spots of frame overo and are not affected by LWO. These horses are known as carriers of the disease, however. A horse with two copies of the mutated gene will suffer intestinal abnormalities caused by undeveloped nerves in the foal’s digestive system. These animals die within the first 72 hours of being born and are typically euthanized sooner for humane reasons.
When breeding for a frame overo horse, it is extremely important to know the genomes of the horses being bred. This will prevent two carriers from being bred together. When breeding a frame overo horse to a non-overo horse, there is a 50% chance of the foal inheriting the mutation and developing the frame overo pattern. This is the safest way to breed frame overo horses.
If two frame overo horses (two carriers) are bred with one another, there is a 25% chance per foal born that they will inherit LWO, the fatal disease. There is a 50% chance per foal born that they will develop the frame overo pattern. As the chances of producing a frame overo horse are the same when breeding an overo horse to another overo horse as they are when breeding an overo horse to a non-overo horse, it is highly advisable to breed the latter option. Breeding a carrier to a non-carrier will eliminate the chance for the foal to develop LWO while maintaining the same odds of producing an overo horse.
A missense mutation in the endothelin-B receptor gene is associated with Lethal White Foal Syndrome: an equine version of Hirschsprung disease.
Metallinos DL, Bowling AT, Rine J. Mamm Genome. 1998 Jun;9(6):426-31. [PMID: 9585428]
|O/O||Homozygous Lethal; Horse born with two copies of LWO will experience failure to thrive and will typically be euthanized by 3 days old.|
|n/O||Heterozygous: Horse has one copy of the Frame Overo (LWO) gene. Horse will typically exhibit the Frame Overo pattern in varying degrees and can pass this gene on to any offspring.|
|n/n||Negative: Horse is negative for the Frame Overo (LWO) gene.|