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Overo is a pinto pattern caused by the dominant gene O. Overo was formerly thought to be recessive, but statistic analysis of the offspring of overo horses has proven it to be a dominant. Like roan, homozygous OO overo is a lethal; foals who inherit an O gene from each parent usually die in utero or shortly after birth. So almost all overos are heterozygous Oo, with oo horses being non-overo.
Overo is different from tobiano in that the white markings of an overo horse do not cross the topline. Instead, they appear to begin at the belly and spread upward. White legs are not as common in the overo as they are in the tobiano, and there is often a great deal of white on the face. The horse in the photo above is a dark chestnut overo. Note the lack of white crossing the topline, the one solid-colored leg, and the large amount of white on the face. These are all hallmarks of the overo.
Blue eyes are also common in overos, while tobianos usually have brown eyes.
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Buckskin overo (A-CcE-O-). This horse has a bit of white crossing the top of his neck, but the "topline" is only from the tip of the tail to the withers. Overos sometimes do have white crossing their neck, but not their spine or tail. |
A minimally marked black overo foal, in his foal coat. His genotype is aaE-O-. |
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Chestnut overo. CCeeO- |
Palomino overo. CceeO- |
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Black overo |
Dark bay overo |
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Red dun overo. CCD-eeO- |
Sorrel overo |
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Sorrel overo |
Sorrel overo. Note that even with as much white as this mare has, it still does not cross her topline. |
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Chestnut overo. This horse is about 80% white, and still none over the topline, even though his neck is almost entirely white. |
Grey overo, probably born bay. If so, his genotype is A-E-G-O-. |
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Black overo. This mare would be easy to confuse with a tobiano, but she has no white on her legs, nor does her white area cross her topline. |
Black overo, minimally marked |
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Black overo |
Chestnut overo |
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Sorrel overo |
This mare, a liver chestnut, shows two patterns: Overo and rabicano. The overo accounts for her white patches, and the rabicano accounts for the roaning on her flank. Her genotype is eeffO-Rb-. |
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Toveros
Since the tobiano and overo patterns are genetically distinct and inherited on separate sets of genes, it is possible for a horse to inherit both. These horses are called toveros, and show characteristics of both patterns. This usually means tobiano-type body markings and overo-type facial markings.
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At left is a black tovero. He has the regular tobiano body pattern of white over the topline and a colored patch on the flank, but his facial markings indicate that he also possesses the overo gene. His genotype is aaE-O-T-. |
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Chestnut tovero. Little bit of red on the flank and an almost entirely white head. A horse with a colored "cap" on the top of its head is often called "medicine hat," an Indian term. This horse's genotype is eeO-T-. |
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Black tovero. This foal, too, is a "medicine hat Paint." |
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Palomino tovero (CceeO-T-) |
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Dark bay tovero (A-E-O-T-). This foal looks like the Phantom of the Opera. Note that his left eye is blue. "Mismatched" eyes are common in Paints. |
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Another black tovero. |
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Bay tovero. Also a medicine hat Paint. |
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This foal is almost entirely white, except for a lone black patch on his left ear. This is a black tovero. |
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More Politics
The horse at right is a very minimally marked young grey overo, born black. Note that his tail has begun to turn silvery. As you can see, he has no white spots on his body. However, the great amount of white on his face indicates that he is most likely an overo; he carries the overo gene even though he does not express it much.
Sometimes horses like this are caught in a bind. The American Quarter Horse Association, the world's largest breed registry, will not register foals who display "excessive white." There is a diagram showing what areas on the face and legs can show an acceptable amount of white -- any white beyond these areas or any white on the body, and the foal is unregisterable. This rule applies even to foals born to two Quarter Horse parents; such foals are known as "cropouts."
The horse above is of Quarter Horse ancestry, although he is registered with the American Paint Horse Association. The APHA was created for horses of stock horse type who showed pinto markings, since AQHA won't register them. If the horse above had AQHA parents, he would be eligible for AQHA registration, except that he probably has too much white on his face (although the white on his legs is within acceptable boundaries). In that case, although AQHA gives him the back of its hand, he could be registered as a Paint with APHA, which has its own guidelines for acceptable color. Some horses, though, have too much white for AQHA and not enough for APHA. They are thus left without a registry, bringing their value down for no good reason.
APHA has a division called "breeding stock" which is reserved for solid-colored foals born to APHA-registered parents. Pinto patterns don't always breed true, so sometimes solid foals are born to colored parents. APHA will still register them as Paints, and they can be shown in breeding-stock divisions.
