Genetically, a piebald horse begins with a black base coat colour, and then the horse also has an allele for one of three basic spotting patterns overlaying the base colour. The most common coloured spotting pattern is called tobiano, and is a dominant gene. Tobiano creates spots that are large and rounded, usually with a somewhat vertical orientation, with white that usually crosses the back of the horse, white on the legs, with the head mostly dark. Two less common spotting genes are the overo gene, that creates a mostly dark, jagged spotting with a horizontal orientation, white on the head, but dark or minimally marked legs. The sabino pattern can be very minimal, usually adding white that runs up the legs onto the belly or flanks, with "lacy" or roaning at the edge of the white, plus white on the head that either extendings past the eye, over the chin, or both. The genetics of overo and sabino are not yet fully understood, but they have recessive characteristics and can appear in the offspring of two solid-coloured parents, whereas a tobiano must always have at least one tobiano parent.
