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 nonspotted 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 nonspotted 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 "nonspotted 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 nonspotted 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.

Basic tobiano genetics

The basic genetic control of the tobiano pattern is straightforward. The pattern is due to a dominant gene. As a result, every tobiano horse has at least one tobiano parent, and the pattern never skips a generation.

The only possible exception to this rule is the minimally marked tobiano horse. Some of these are misclassified as nonspotted, even though they do have the tobiano gene. In reality, they are indeed genetic tobianos despite the lack of body spots.

Most tobianos are heterozygous, meaning that they have only one dose of the tobiano gene. When these horses are mated to nonspotted animals, the expectation is that half of the foals will be spotted and the other half will be nonspotted. Actual foal crops vary around this 50/50 ratio, as governed by the rules of statistics.

When tobianos are mated to other tobianos, the expectation is that 25 percent of the foals will be nonspotted, 50 percent will be heterozygous, and 25 percent will be homozygous (meaning they have two doses of the tobiano gene).

The homozygous horses are quite useful, because they will always produce spotted foals when mated to a nonspotted horse. The obvious question is whether there are any tip-offs as to whether a horse is heterozygous or homozygous.

Some observers have noted that the presence of "bear paws," or smudgy roaned regions in the pattern are a sure tip-off to homozygous tobiano horses.

Understanding tobiano genetics

A variety of details about the genetics of the tobiano pattern make it especially interesting. The tobiano locus is part of a linkage group that includes other genetic loci. (Locus, with the plural loci, is simply a Latin term meaning "location' or "address." Do not let the terminology intimidate you!).

The specific and individual combination of variants at these loci tend to be passed as an intact group, although they will occasionally be recombined. The loci that are linked include tobiano, roan, extension, and various bloodtyping proteins including esterase and albumin.

The variants at tobiano include tobiano and nonspotted. The variants at roan include roan and nonroan. The variants at extension include chestnut and non-chestnut. (These non-chestnut horses are usually bay or black, or some derivative of those colors.) The loci for the bloodtyping proteins each have a few variants, which can all be picked up on a routine bloodtype.

The mechanisms of the linkage can be demonstrated by some hypothetical crosses. One that is fairly easy to understand is the result of crossing a bay tobiano (heterozygous at both loci) onto a large group of chestnut or sorrel mares. If these genes were not linked, the foals would be expected to come out 25 percent chestnut, 25 percent bay or black, 25 percent chestnut tobiano, and 25 percent bay or black tobiano. Each combination is equally represented, as is expected when most genes are recombined and passed on to the next generation.

The equal mixture does not happen, though, because the loci are linked and this prevents recombination of the variants. What really would happen in such a cross has never been directly described, but from the above cross, two different situations are possible. If the tobiano and chestnut are the two variants linked in the spotted parent, then the result is nearly 50 percent bay or black foals, and nearly 50 percent chestnut tobiano foals. Very few bay or black tobianos will be produced, and there will be very few non-spotted chestnuts.

The linkage results in only certain combinations being produced, and prevents the complete and random mixing of the combinations as would be expected if the genes were not linked.

Because the genes can be linked in different combinations, consider the same cross but with the tobiano and the bay/black linked. In this situation the foals would be nearly 50 percent bay/black tobiano, and nearly 50 percent non-spotted chestnut. The reverse of the linkage has also reversed the result in the foal crop.

The roan gene would behave similarly in a breeding horse. A tobiano roan of any color would--depending on the linkage status--produce an unequal mixture of foal colors. If the roan and tobiano were not the linked variants, then about half of the foals would be roan and half would be tobiano. Very few would be the roan/tobiano combination or solid colors. If the roan and tobiano were linked, then about half of the resulting foals would be the combination of tobiano and roan, and half would be solid-colored, with few non-roan tobianos and few non-tobiano roans.

The situation with esterase and albumin is similar, which has some potential practical benefit to breeders of tobiano horses. The esterase and albumin variants can be determined by bloodtyping, and are tightly linked to tobiano. As a result, in many cases it is possible to determine if a horse is heterozygous or homozygous for tobiano by bloodtyping the animal and some of its close relatives. This is done through the usual bloodtyping labs.

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