A chestnut is a red(ish) horse with no black pigment. Chestnuts vary greatly, from pale with a light mane and tail (flaxen), to a deep burgundy that can be mistaken for black. Although there is great variation in this color, most chestnut horses will fall into the middle of this color spectrum and are easy to recognize.
Mutations in the KIT gene cause white spotting patterns in horses, dogs, cats, mice, rats, cattle, humans, and swine. The frequency of KIT gene mutations vary from species to species but, in horses, it is responsible for more white spotting patterns than any other gene. KIT gene mutations are responsible for Sabino 1, Classic Roan, and all the Dominant White patterns (currently at 20). Tobiano can also be added to this group.
A palomino mare with the tobiano pattern. Photo courtesy of CheyAut Ranch
Tobiano is probably the best recognized of all the white patterns. It almost always leaves the legs white. It will cross the top line at the shoulders and hip before other areas and fights to leave the chest, flanks and head colored. It commonly results in a solid head and white tale head. Some theorize that Tobiano doesn't add face white but whether this is true or not is still open for conjecture. When Tobiano is mixed with other patterns we can get a solid white or nearly white horse but rarely, if ever, does Tobiano express to that extent alone. When Tobiano is very minimal (sometimes called Crypt-Tobiano). It will usually express as four white stockings or socks, sometimes with a wither or neck spot. In miniature horses, it's expression can found at it's most minimal, at times leaving only the back two feet white.