Horse Color Genetics: Overo and Lethal White Syndrome

Review: E=black, A=bay, G=gray, CR=cream, D=dun, TO=tobiano, F=flaxen, Z=silver.

The first question you may be asking is, "What is overo?" Overo is a pinto coat color in which the large splotches across the body appear to be rather jagged at the edges. In the color overo, the white never crosses the back, so the horse is mostly whatever is the background color. Can you see the difference between the two horses below? The top one is overo, the bottom tobiano.

Similarly to tobiano, any color can be the background: chestnut, bay, black, palomino, buckskin, dun, roan. In genetics, modifiers like overo and tobiano can be in any combination with other colors. The abbreviation for overo is O. The horse below is a dun overo; see the stripe down it's back? I'm not sure whether it is a dunskin or a bay dun.

Another characteristic of overo is generally they have quite a lot of white on their face, extending past what you might call a 'normal' marking. Usually, when the white covers the eyes, the eye is blue.

Overo is rather strange compared to some of the other genetic color modifiers. Unlike other colors, homozygous in frame overo's do not exist. I will get more into what happens if it is in a minute.

There are several different kinds of overo. All the horses above are called 'frame overo'. The other kinds are splash and sabino; sometimes a horse can receive the different kinds of overo, all mixed in at once. The different overo's are actually all different genes, but the other overo's are still associated with overo.

The horse below is a splash overo. Splash overo's basically look like they have been dipped in white paint. Their legs, face, and sometimes their belly and the bottom part of their neck and chest may be white, although generally the minimum is white legs and a lot of white across the face. The horse below has a lot of white and the fact that he is a splash is very obvious.

Sometimes the fact that a horse is a splash isn't apparent, or people don't know it. If a horse has what is called a 'bald face', which is not an uncommon marking, it might carry the splash gene and they just aren't aware of it. The horse below is splashed white. The abbreviation for splashed white overo is SPL.

Sabino overo is a little confusing, as the exact terminology is not very well established. Generally, sabino horses may have a lot of markings, or very minimal markings. White markings are very lacy, even more so than the frame overo, and may even look a bit like roan. Abbreviation for sabino is SB1. The horses below are sabino. The last horse is very minimal sabino; it might even be sabino and splashed white, I don't know.

For both splash and sabino pinto's, whether they are homozygous or heterozygous might show by the amount of white is on their bodies.

Now, back to why frame overo can never be homozygous.
Scientists are struggling to understand exactly why this happens, but if a foal receives two dominant frame overo alleles, something in the stomach doesn't fully develop all the way. The foal is born pure white, and appears completely normal and may function normal for a little while, but after whatever food runs into where the stomach wasn't developed, it has absolutely nowhere to go. Usually, these foals are put down after not too long; they will eventually (within 48 hours) die of, basically, colic. This is called Lethal White Syndrome, or Overo Lethal White Syndrome (LWS, OLWS).

It is very sad, but it is the truth. It can be very dangerous to breed two overo's together, because if both of the frame overo genes are inherited, well, you get the idea. It is safer to breed an overo with a solid color and pray it inherits the one frame overo gene from that particular parent. It is strange that there are as many frame overo's as there are.
Some owners of stallions who are overo that is not frame will still have their horse tested to make sure that there is no frame overo that they aren't aware of (like a combination of different overo's) and will advertise if their horse has it or not.
If breeding two frame overo's together, the chance that it will have LWS is 25%. It is a pretty basic equation; here are the possible outcomes (keep in mind that both parents would be heterozygous) (these are not taking into account the back color): OO (LWS), Oo, Oo, or oo. The reason there are two is, say, the dominant is from the dad and recessive from mom, and the second would be vice versa.