Tobiano is a pinto pattern caused by the T gene. It's a dominant gene; TT and Tt are tobiano, while tt is solid. TT in it's Homozygous form will throw 100% Tobiano color pattern everytime to it's offspring. Tobiano can be distinguished from other pinto patterns because a tobiano horse has white that crosses its spine somewhere between the withers and the tail (topline), as if the white had been poured on from above. Tobianos also almost always have white stockings on all four legs; often the white socks blend into the white on the body. A solid-colored leg on a tobiano is almost unheard of.
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 black framed overo. Note the lack of white crossing the topline, and he has a 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.
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.
Bay tovero. Also a medicine hat Paint.
Dark bay tovero (A_E_O_T_).
Note that his left eye is blue. "Mismatched" eyes are common in Paints.
