That is the problem with the word "overo". It literally means "a horse that has body white that is probably not tobiano". It could be caused by any of these:
Sabino (one testable mutation, at least one more still not found)
Splash (three testable mutations, at least one more probably still not found)
Dominant White (11 known strains, five testable, probably many more out there)
So that is a total of ten testable mutations that cause patches of body white and are not tobiano, with at least nine more that are not testable yet. As you can imagine, overo covers a lot, and only one of those genes causes a lethal white foal.
well that makes a ton of sense. Its so complicated. I keep reading and studying on the color genetics (im slightly addicted) and I know more than most of the people around me but man I don't even come close to knowing all of it and it changes so often with all the new scientific findings. I may have to test my mare out of curiosity to see if she has frame or not. Im fairly sure she has Sabino but im not sure on frame, she has a big bald face, white legs, and a white spot on her side about 5 inches long and maybe an inch or two wide.... The mare I am considering purchasing I believe has Sabino, Rabicano, frame, and who knows what else... I think if I bred either one of them I would be doing all the tests just so I know what I might expect.