The Genetic Equation
by the Amercian Paint Horse Association
Tobiano spotting
The tobiano pattern is one of the four spotting patterns that characterize Paint Horses. It is also present in many other breeds, from ponies to draft horses, worldwide. The name “tobiano” is itself unusual, and has an interesting history. In Argentina it is the habit to name unusual colors after horses or people who connect the color to a specific event or individual. In the case of the tobiano horse, that event was the rescue of Buenos Aires by Brazilian General Tobias during a military action that took place in the 1800s. Many of the troops accompanying Tobias were mounted on tobiano spotted horses from Brazil. The color had occurred rarely in Argentina before this event (and was lumped in with all the other spotting patterns as overo), but became firmly associated with Tobias and his troops afterwards, and ended up taking his distinctive name. The tobiano pattern occurs in many breeds worldwide. It is common in pony breeds,some draft breeds, and even occurs in some of the warmblood breeds. In many breeds it is disallowed, but still may have been present in the foundation horses from which some of these breeds were derived.
The characteristics of tobiano
The general characteristics of tobiano spotting in horses are that the feet and varying portions of the legs are usually white, the head usually has no more white than expected in a non-spotted horse, and the spots usually cross the topline somewhere between the ears and tail. The spots are usually crisply delineated from the colored areas and usually have a vertical arrangement to them. The eyes on tobianos are usually dark. Tobiano horses can vary from quite dark, with small amounts of white, to quite white, with little remaining color. The darker individuals sometimes have so little white spotting as to be confused with non-spotted horses. The minimally spotted tobianos are interesting, because they are essentially tobianos that did not get spots. Such horses will produce just like a spotted horse, though, and this is the cause of some tobianos seeming to pop up out of nowhere. A clue to these “non-spotted tobianos” is that they tend to have a large amount of white on the lower leg, but little white on the head. This combination is otherwise rare on horses, because on non-spotted horses the leg and facial white tend to vary together such that horses with a great deal of white on the head usually have a large amount of white on the feet, and vice versa. In the middle portion of the range of tobiano spotting there is no problem telling tobianos from other Paint patterns. They are quite distinctive. At the whitest extreme, many tobianos are all white except for a colored head. This pattern is sometimes called “Moroccan,” although the connection to Morocco or its horses is tenuous at best. Other details of the tobiano pattern include the fact that on many tobianos the border between the white and colored areas consists of pigmented skin overlain by white hairs. The result is usually a bluish cast to the border, almost like a halo or a shadow. Another peculiarity of some tobianos is the presence of “ink spots” in the white patches. These spots are small, generally round spots.
The Overo Patterns
The overo group of patterns has become hopelessly complicated due to the lumping of several genetically distinct patterns under the single name of “overo.” Because at this point the nomenclature is hopeless, the best we can do is to forge ahead and try to understand these patterns and how they fit into the overall Paint Horse picture. The key concept is that the term “overo” covers three genetically distinct patterns.
Frame overo spotting
Frame overo is one of the overo patterns. The name “frame” refers to the usual appearance, which is of white patches centered in the body and neck and framed by colored areas around them. The usual frame pattern has a horizontal arrangement, and does not cross the topline as does tobiano. The head is usually quite extensively marked with white, and the eyes are commonly blue. The feet and legs of frame overos are usually dark, although white feet and minor white leg marks are as common on frame overos as they are on nonspotted horses. The white areas on frame overos are usually crisply and cleanly delineated from the colored areas, although some have a halo or shadow of pigmented skin under white hair directly at the boundary. The frame overo pattern occurs in a limited range of horse breeds. It seems to appear only in breeds that have Spanish ancestry, including the Paint Horse. The genetics of frame overo has only recently been documented. Frame overo behaves as a dominant gene. It is common to mate frame overo horses to nonspotted horses, and about half of the foals come out spotted.
On many occasions, though, there are records of frame overos being produced by two nonspotted parents. This is typical of a recessive gene, and it is not logical to have both a recessive and a dominant control over the same pattern. A look at the parents of these “cropouts” sometimes reveals that one or the other is oddly marked. These oddly marked horses usually have bald faces but colored feet, which is a very unusual combination in horses. Some of these horses are genetically frame overo, but have failed to get a body spot. They are essentially very dark frame overos—so dark that the spots are all gone from the body. They still have the gene, however, and can still produce frame overo-spotted offspring. This phenomenon may not account for all the cropouts. For example, the occurrence of the frame overo pattern in the Thoroughbred breed (the racehorse Tri Chrome, for instance) seem to be new examples of this gene in a breed that previously did not have it.
Sabino spotting
The sabino pattern is nearly as common in Paints as the frame overo and tobiano patterns. The sabino pattern is usually the one called “overo” in South America, so terminology is confusing at best. The term “sabino” in literal Spanish means pale or speckled, and in Mexico and Argentina this term is used to describe fleabitten grey horses, or other mottled patterns. In Europe, and increasingly in the United States, sabino is used to describe a unique and interesting pattern of white spotting in horses.
Sabino horses usually have four white feet and white legs. The white usually extends up the legs in ragged patches, and then extends onto the horse’s body from the belly. The head is usually fairly white, and the eyes are commonly blue. Many sabino horses have eyes that are partially blue and partially brown. Flecks, patches and roan areas are common on sabinos, in contrast to the frame overos that are usually more crisply marked. Sabino occurs in a large number of breeds worldwide, including Paints, Thoroughbreds, Clydesdales and many others. The sabino pattern is the usual culprit when spotted foals appear in breeds that frown on them, such as the British pony breeds and the Quarter Horse.
The sabino pattern is also a great imitator, and some of these horses are nearly perfect mimics of tobiano or frame overo. This adds to the confusion of the sabino pattern. When the sabino pattern is minimally expressed, the horse usually has four white socks and a blaze. Usually there is some betrayal of the fact that these are not the usual white marks on horses, due to some ragged edge or narrow and long extension up the leg. Some sabinos will also have odd white patches on the knee or hock, removed from the main portion of the lower white mark. A few sabinos do have a dark foot or two, although most have four white feet. Minimally marked sabinos are easily confused with truly nonspotted horses.
Another extreme is the sabino that is patched, but not roaned. These can easily be confused with frame overos, especially if they have a dark foot or two. Most patched sabinos have smaller, more ragged patches than typical of frame overos. In some cases it is impossible to distinguish between horses that are truly sabinos and the frame overos that also happen to have white markings on their feet in addition to the frame overo pattern.
The whitest of the sabinos are nearly or entirely white. Some retain color only on the ears. Others are indeed white all over. One of the whiter ranges of expression includes color on the ears, chest, and tail base. These are the medicine hat Paints of the native tribes from the Great Plains. Most sabinos that are largely white are very speckled and roaned, and some can be confused with Appaloosas. Some sabinos are quite white and survive, which points to this being entirely different from the frame overo that results in lethal white foals when homozygous. Sabino, by itself, is not associated with lethal white foals. The sabino pattern is confusing genetically. In many, or most, families it appears to be transmitted as a polygenic trait rather than as a single gene. Many horses appear to transmit it roughly in the percentage that they are themselves white. That is, a sabino medicine hat is likely to produce a higher percentage of spotted foals (or at least foals registerable as spotted) than is a minimally marked sabino.
