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Tobiano is a spotted color pattern commonly seen in Pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is present from birth and does not change throughout the horse's lifetime, unless the horse also carries the gray gene. (see below)
Other spotting patterns seen in Pinto horses include overo and the sabino markings. In the United Kingdom, Tobianos are frequently referred to as "Coloured" or as Piebald (black and white) or Skewbald (white and any base color other than black). However, Bay and white horses are also referred to as Tri-Coloured.
Tobiano traits generally include the following:
Tobiano is a dominant gene. Therefore, one parent must be a tobiano for the pattern to occur, and the coat pattern will occur with a single copy of the Tobiano gene present (i.e. the horse is heterozygous for Tobiano). Furthermore, when a horse is homozygous for Tobiano coloring, all of that horse's offspring will be spotted, with only a few exceptions: If either parent passes the dominant gray gene to the foal, then its spots will be visible while it is young, but will gradually become lighter until finally, as the gray gene acts upon all coat colors, the entire horse's coat fades. In the case of horses born Tobiano but turn gray, the skin will retain pigmented and unpigmented skin beneath its hair that may produce "ghost" markings. A homozygous Tobiano that also carries a dilution gene, such as a Pinto with a base color of palomino or buckskin may not reliably produce spotted offspring if bred to another horse with a dilution gene, as a double-dilution may "wash out" the base color.
The Tobiano gene itself is not linked to lethal white syndrome. However, some Tobiano horses may be carriers of the gene if they have overo ancestors, and thus have produced affected offspring when bred to another horse that is also a carrier. In some cases, a horse which carries both Tobiano and Overo genetics may display white markings that combine both patterns, and are referred to as Toveros.
Genetic testing is available to help determine the presence of the Tobiano gene. Originally, there was no direct test for Tobiano, the early Tobiano tests utilized six genetic markers, including a marker on the KIT gene on Horse chromosome 3, which were closely linked to Tobiano but could not be used to officially prove homozygosity. However, in 2008 it was determined after some years of research from the University of Kentucky, that Tobiano coat pattern is the result of a chromosomal 'inversion' that affects regulatory regions of the KIT gene. This inversion was finally mapped for genetic testing and is now offered by some laboratories as a more direct Tobiano assay. Testing is most often intended to determine Homozygosity, but is also useful in cases where a horse has greyed out, or if a horse has an unexplained loss of pigmentation.