It can be difficult for the amateur --- and sometimes even for the professional horse breeder --- to distinguish between the two different color patterns in pinto or paint horses. To try to simplify things, I have created the following images and descriptions, based on my years of study and research into equine color genetics. I hope it is helpful in clearing up some of the confusion.
Knowing the parents of a particular horse may help you decide which color pattern it is, because tobiano and each of the three patterns of overo are produced by entirely different genes. An overo cannot produce a tobiano unless bred to one, and under normal circumstances, a tobiano cannot produce an overo. The exception to this is the tovero, a horse that carries both overo and tobiano genes, but these are usually recognizable as such. A few examples will be shown below. In the rare event of an apparent overo out of tobiano parents, a HIDDEN overo gene would have to be the culprit. This is how crop-out overos (overo pintos produced from two apparently non-pinto parents) occur, but they are uncommon. Exactly how this happens is still a mystery to geneticists. Suffice it to say that if a pinto horse has one or both tobiano parents, it is overwhelmingly likely to be a tobiano, too. If neither parent is a tobiano, a paint or pinto can be assumed to be an overo. (See *Note below.)
Now for the descriptions:
Tobiano is the most common color pattern in pinto or paint horses, especially in miniatures. It can best be described as having white markings that usually seem to come from the top of the horse and run down toward his hooves. Notice, I said "usually." You will see that word over and over again in this article, because there are very few hard and fast rules in pinto color genetics, and virtually none in the patterns themselves. Exceptions will always occur, and are often rather frequent.
Tobianos usually have white legs and a solid or "normally" marked head. That is to say, they usually have the face markings (or lack thereof) of a solid colored horse; i.e. a star, strip, blaze, etc. Usually they do not have bald or apron faces.
A common misconception is that if the white markings cross the center of the back, between the withers and the tail, then it is a tobiano, if not, it's an overo. This is simply not true. Many minimally marked tobianos do not have white that crosses the back, and many overos (especially splashed white overos) do. Tobianos frequently have their darker color in specific areas: a predominantly dark head, a "chest shield," a "flank shield," and color around the tail head. Usually, the line between color and white is crisp and well defined. Many have two-tone tails, part white, part colored.
The following drawings represent some of the possible tobiano color patterns, but by no means are intended to represent all of them. Keep in mind that these drawings are in black and white, while tobianos or overos can and do occur in any and all color combinations.
Horse #1 has only a tiny white spot in her mane, but she is nevertheless a tobiano. Horses #6 & 7 both are homozygous tobianos, showing the "paw prints" (small, irregular dark spots within their white areas) that these often display. The tiny dark dots on horse #5 are just that --- dots. Only one of her parents was tobiano, so she cannot be homozygous.
Overo is generally the less common color pattern in horses. There are three distinct types of overo, each produced by a different set of genes. The most common is the frame overo. You will see examples below. And, no, "frame" does not mean a mostly white horse that is "framed" in color only along his topline, chest, and head as some seem to believe. As you will see below, a frame overo can be anything from mostly colored with a tiny belly spot to mostly white. A beautiful pattern, that unfortunately often carries the "lethal white" gene. (See **Note below.)
The best way to describe the frame overo is that usually (there's that word again!) the white markings seem to come up from under the belly, as if the horse were rolled over and the white paint poured on. Often times, the white is ragged or splotchy. In frames, it is rare for the white to cross the top line between withers and tail, but it occasionally can in horses with a large amount of white. And white that crosses the back line does not, in any way, keep the horse from being a frame overo. Remember: frame is a distinctive gene, not just the way the white is laid on the horse.
Probably 75% of all frames have bald or apron faces. Most have colored legs, or at least one colored leg (but again, some have all white legs), and most have a dark colored tail. Many have blue eyes, but so do many tobianos (and many of our non-pinto miniatures!), so eye color can't really be a factor in determining color pattern.
The second overo color pattern is the sabino overo. Less understood, perhaps, than frame overo, it is the color pattern you see in "roan" or "pinto" Clydesdales and Tennessee Walkers. Usually (tired of that word yet?) sabinos have high white stockings that streak upwards, especially on the stifle, and the white "sprinkles" up onto the body, usually in a roan-like pattern. Again, they range the whole gambit from mostly dark to mostly white. Sabino horses can even be totally white, especially if both parents are sabinos.
Most sabinos have a wide blaze, some have bald faces, but most do not have the completely white head of the splashed whites. Often times, sabinos have dark eyes.
The third and least studied pattern is the splashed white overo. Considered rare in full sized horses (at least in America) but not uncommon in miniatures, the splashed white overo differs from the others mainly because of the crisp line between white and color and the tendency of the white to cross the back line. Most of them look as if they have been dipped or rolled in white from the hooves up. Many splashed whites have apron faces or completely white heads, often with a "bonnet" or "medicine hat" over their ears. Splashed whites frequently have all-white or half-white tails. They almost always have one or two blue eyes. Some very odd patterns can be seen in splashed white overos.
See below for examples of all of these color patterns. Note that in some cases it is impossible to tell exactly which of the overo color patterns a horse carries. To confuse matters further, it is entirely possible for a horse to carry the genes for more than one overo pattern. For example, a horse could be a sabino/frame. All of these illustrations are taken from actual horses. I'm not making this stuff up, folks!
These horses are all frame overos.
Horse #1 is a crop-out --- a pinto out of 2 non-pinto parents. Since it has no tobiano parent, it must be an overo despite the solid colored head. Both #3 & #5 display some sabino overo characteristics, and may carry that gene as well.
These horses are sabino overos.
Horse #3 is almost all white. His pinto markings show up only as a light roaning, but become very obvious when he is wet. Horse #1 displays the sabino pattern so typical of many Clydesdales, Tennessee Walkers, and Paso Finos.
These are splashed white overos.
Notice how these horses look as if they have been dipped or rolled in white paint. Horse #8 is out of both splashed white parents. Horse #9, despite the fact that the white crosses her back, is a splashed white overo. Her dam is horse #4 and her full sister is horse #6, and the sire is non-pinto.
Tovero is the term usually used for a horse that displays both tobiano and overo characteristics. This can be a difficult pattern to identify at times, and sometimes only the horse's offspring can determine if it is truly a tovero. (That is, if it produces both tobiano and overo foals out of non-pinto mates.)
Most toveros, like most overos, have bald or apron faces. This is frequently the only visible sign of the overo gene they carry. Many are marked on their body and legs just like tobiano pintos. More white than color is typical of toveros, but not a hard and fast rule. Toveros, since they carry two different pinto genes, produce a higher percentage of pinto foals than anything besides homozygous tobianos. (See ***Note below.) A few toveros are shown in the examples below.
These are toveros.
Horse #6 is in fact homozygous for the tobiano gene and also carries an overo gene. His sire was a tobiano and his dam a tovero. He will be able to produce tobianos and toveros, but no solids and no overos, since all of his foals will carry the tobiano gene and thus will display the tobiano pattern.
Difficult to distinguish patterns.
Horse #1 could be easily mistaken for an overo, if not for the fact that both parents are tobianos. This horse is, in fact, homozygous for the tobiano gene. Horse #2 is definitely an overo, but could be either a frame or a sabino. Horse #3 looks like a tovero, but is a full brother to #6 & 9 in the splashed white overo chart. He is Rosa Roca's Willy Bea Star, a splashed white overo by Rosa Roca's Tailor Made, who is a non-pinto bay.
*NOTE: Those of you who have studied equine genetics long enough will know that there have been a FEW apparently solid colored horses that have proven to carry the tobiano gene. However, this occurrence is so rare that most of us will never see one, so it doesn't really pay to even worry about it.
**NOTE: There is a wealth of information about the lethal white gene in frame overos available on the Internet.
***NOTE: Homozygous (pronounced ho-mo-zi-gus) tobianos will always produce tobiano foals (or toveros, if they or their mate carry the overo gene) regardless of what they are bred to, because they carry two tobiano genes, instead of one like most other pintos. They MUST have both tobiano pinto parents in order to be homozygous, but there is only a 25% percent chance that a foal from two tobiano parents will be homozygous. They can sometimes be distinguished by the presence of "paw prints," irregular spots of color within their white areas. But the only way to tell for certain is through progeny or blood testing. The University of California at Davis is one of the primary testing centers, and can be contacted over the Internet. There are no homozygous overos, apparently.