A later version of this post, updated with new photos, can be found here.
Frame is another type of spotting gene in horses, formerly lumped into overo and sometimes called frame overo. It has nothing to do with the KIT locus, unlike tobiano and sabino-1.
Frame involves patterns of white which do not usually include roaning, though frame may occur in conjunction with other genes that cause roaning. In frame, the white areas tend to be arranged horizontally on the sides of the horse, and almost never cross the back. Frame is also almost the only pattern in which the legs remain pigmented, though normal leg markings may occur.
Like all spotted horses, frame horses may vary from mostly colored to predominantly white. A frame horse will almost always have a wide blaze or bald face, and an apparently unspotted horse with a bald face but no white leg markings is likely to be a minimally marked frame. As the white expands the head may become mostly white and the white areas on the sides may expand to cover most of the horse, with the spine and legs being the last areas to lose color.
Frame is due to a single allele, frame, at a locus called endothelin receptor b (EDNRB) on equine chromosome 17. The locus has two known alleles, frame and wild-type. Frame horses, some of which are so minimally marked as to look solid, have one frame and one wild-type allele.
Breeding two frame horses together may produce lethal white foals, with two frame alleles. Such foals are born white, and the part of their nervous systems that control the lower intestinal tract does not develop properly. They normally die within 72 hours of birth, though most are euthanized as soon as they are recognized. Most breeders avoid mating two frame horses together in order to avoid the production of such foals.
The frame allele can be tested for. Such testing has demonstrated that some genetically frame horses appear to be solid colored. Whether this is due to a suppressor gene or genes or is simply the extreme end of random variation of amount of white is unknown.
I do not have a good photo of a frame horse, but many of the web sites linked to have good examples. Note that many have more than one spotting gene, and some look as if splash (to be discussed next week) might be present..