Frame is one of the white patterns and once (and sometimes still) called 'overo' or 'frame overo' when people did not yet discern between most white patterns. Its precise history is unknown, but it likely originated somewhere in the United States not too long ago. Therefore, it is only present in American breeds, and horses descending from these breeds. The pattern has since spread to other regions in the world, including Europe and Australia, when horses with the pattern were sold and/or transported overseas.
Frame is a popular pattern, but not one which should be bred for lightly. It can appear as little as a white blaze, but it may also capture almost the entire side of a horse, while leaving the dorsal and outer edges untouched, which has the horse looking 'framed', hence the name.
Horses with one copy of the frame gene have the pattern, even when it is not always discernible, making it dominant. As a result, it is inherited in a straight line, and never skips a generation. It can, however, be very cryptic, as it can mimic normal white markings like a blaze, and sometimes appear as only a single blue eye. Whereas this would not be much of a problem with other white patterns - despite it sometimes being severely frustrating for breeders looking for loud patterns - with frame this is a very serious factor.
While horses with one copy are completely healthy, horses with two copies of the gene will die a painful death within 72 hours after birth. There is no cure for these foals, and humanely euthanizing them is the only option. This genetic condition is called Lethal White Syndrome, Overo Lethal White, OLW (or variations thereof), and can only be prevented by careful and selective breeding. More information on OLW can be found below.
Due to its origin, frame is only present in American breeds, or horses descending from these lineages. Examples include the American quarter horse, the paint horse, American miniature and mustang horse. Horses without American heritage are free of frame and OLW.
Frame can be a very cryptic white pattern. In its most minimal stage, a horse will have as little as a white marking on the head, or a single blue eye, making it nearly impossible to identify. In these cases, especially when the horse is breeding material and descending from frame parentage (or unknown heritage with American influences), it is strongly recommended to test the horse for frame to prevent OLWS.
Horses with a more expressed pattern can have broad blazes, extending to the side of the head, possibly with one or two blue eyes. In these cases, there is often some white on the side of the neck and/or on the belly. The most extreme expressions include a nearly completely white side, from neck to hindquarters. Edges are often torn, but can have a more lacy appearance.
The legs, topline and the area behind the ear are nearly always excluded from white and maintain their colour, unless another white pattern is present that does influence these areas.
Note: when a horse has one or more white patterns besides frame, the areas of exclusion may be white caused by these genes. In such cases, frame can be very hard to distinguish, and owners should maintain vigilance and test for frame to exclude a foal with OLWS, if they wish to breed the horse.
Overo Lethal White Syndrome
Breeding with frame horses brings the risk of the genetic disorder OLW or OLWS. The frame white pattern is caused by a mutation of EDNRB, a gene with several functions. It not only affects the pigment in hairfollicles, creating the white pattern, but it is also important in the development of the gastrointestinal tract. Unborn foals homozygous for the mutation develop normally inside the mare's uterus, and appear healthy when born. But when they start eating by themselves, the digestion at the end of the gastrointestinal tract is broken, causing the foal to die a very painful death within 72 hours.
The syndrome can only appear when two horses carrying one copy of the frame gene are bred together. In 25% of the case, foals will not carry frame, while 50% will carry both the pattern and the mutation. The last 25% will have two copies of the gene, and therefore suffer from OLW. These foals are nearly always born entirely white and will appear healthy for the first hours, until they become ill due to their abnormal metabolism.
Horses without frame, do not cary the mutation, nor can they produce foals with the syndrome. Breedings where one horse carries the pattern, while the other does not, can result in foals with no frame for 50% of the time. The other half will be carries of frame and will show the pattern (though it may not always be obvious), but will not suffer from the disorder. These breedings are safest if one wishes to produce a foal with the frame pattern, but minimise the risk for OLW.
For this reason, all frame x frame breedings should be avoided.
As frame can be very cryptic and appear as nothing but a blaze, or can be hard to identify due to the presence of other white genes, it is of the utmost importance untested horses with frame in their pedigree are tested before mating. This not only helps in identifying risky combinations, but it may also help to ascertain a white born foal has no OLW (when it has been tested at least one of the parents carries no frame), and therefore will not develop the symptoms, nor should they be humanely euthenized as a precaution.