The origin of the spotted gene in the Tennessee Walking Horse breed is something of a mystery.  As you can see from the thumbnails below, spots were limited to sabino and overo markings in roans until the appearance of Nell Overstreet in 1941.  This has left some questions in the minds of geneticists as to the purity of the spotted lines. On the other hand, since the Tennessee Walking Horse breed is a performance breed, the spotted horses who also exhibited the necessary gaits and were able to pass ON these gaits, have been incorporated into the breed registry.