What does the word "Overo" mean?
Well it truly means "Not tobiano." A horse in APHA who was not tobiano, but had qualifying white was registered as "Overo". The horses who had the splash pattern, the sabino pattern, dominant white, and frame overo were all registered as "overo".
Whats the problem with this you ask? Well, only one of those patterns has a horrible genetic disease associated with it, and lumping them all together can cause confusion over this genetic disease. Two options that would create less confusion, and better systems for registering would be to either have a separate category for tobiano, frame overo, splash white, sabino and dominant white or to have a category for LWO positive horses, and lethal white negative horses within the tobiano grouping and the overo grouping. This would help owners avoid creating more lethal white foals.
And now that we have had that discussion lets talk about Frame Overo/Lethal White.
Frame Overo is known by a few names. Its known as "Frame", Lethal White, LWOS, LWO, OLW, OLWS, and sometimes (though by now you know why this is bad, or you should LOL) just "Overo".
All of those names refer to the exact same gene and that is the "O" gene. It is technically an incomplete dominant gene like the cream gene is. In its heterozygous form it can cause anything from a completely solid animal, to a mostly white animal, with the majority being somewhere in the middle. Its homozygous form is an animal that is 95% or more white, and dies from complications caused by being homozygous for the frame gene.
Frame itself, does not usually cause leg white. How much face white frame will cause is up for debate. I personally believe frame will not cause more than a wide blaze. Others believe it will cause any amount of white on the face including a completely white head. Others even believe that frame will not cause any face white at all. Frame is also one of the only two patterns that can cause blue eyes.
Frames body white tends to move in a horizontal fashion across the horses body. It tends to start on the neck and the sides of the horse too, and then expand from there. Frame also tends to avoid the mid line of the horse, and avoids crossing the back especially between the whithers and the tail.
This is Nite Spot. He is a deceased thoroughbred stallion who was owned by Gestuet Falkenhorst. Notice the way his white has concentrated to his sides and his neck. This is a fairly typical presentation for a more minimal frame.
This is Dillon Ryan owned by Exclusive Equines. He is a 2006 APHA colt. Notice how the white has expanded further over his sides and his neck.
This is one of the loudest versions of the frame overo pattern. This is On Ice a 1982 APHA stallion. Notice how the white has moved horizontally across his body, and has avoided his mid line from whithers to tail.
Now, how do people unknowingly have LWO effected foals you ask since this pattern seems very easy to spot? Well frame overo has the incredible ability to be nearly invisible on horses. For example these three miniature horses (all three are separate horses) from Strebor Mini Horse Ranch, are all LWO positive.