The Tobiano (pronounced: tow be yah' no) coat pattern usually involves some white on all four legs and rounded white spots on the body with sharp, clean edges. The head of the horse is usually colored and will not have white caused by the Tobiano gene. The white on the body will generally cross the top-line of the horse.
Although white often incorrectly referred to as adding color it is actually a deletion of color. Tobiano is the result of a chromosomal inversion, affecting regulatory regions of the KIT gene. The KIT gene has been found to be involved in numerous horse coat patterns such as Sabino1, Roan and Dominant White.
The Tobiano coat pattern is governed by the dominant KIT gene. This means that only one copy is required to express (heterozygous-nT) for the Tobiano coat pattern to be present. Homozygosity of the Tobiano gene (TT-two copies of the Tobiano gene) may show visual clues ("ink spots" or "paw prints") but only genetic testing will tell you more conclusively that the horse is homozygous for the Tobiano gene. When there is no presence of the Tobiano gene (homozygous negative-nn), the Tobiano coat pattern is not possible.
In 2002, researchers at the University of Kentucky, led by Dr. Samantha Brooks, located a SNP within the KIT gene that is closely linked to the Tobiano pattern. Since then, additional markers have been found within the KIT gene by researchers here at Animal Genetics that we also use to determine Tobiano zygosity.
Most recently, researchers at the University of Kentucky discovered a large chromosomal inversion on chromosome 3 near the KIT gene of Tobiano horses. This inversion, which has only been found in Tobiano horses, is believed to disrupt the normal functioning of the KIT gene and cause the Tobiano pattern.
Brooks SA, Lear TL, Adelson DL, Bailey E. A chromosome inversion near the KIT gene and the Tobiano spotting pattern in horses. Cytogenet Genome Res. 2007;119(3-4):225-30. Epub 2008 Feb 1.
Brooks, SA, RB Terry, and E. Bailey A PCR-RFLP for KIT associated with Tobiano spotting pattern in horses. Animal Genetics 2002:33(4):301-3.
Animal Genetics offers DNA testing for the Tobiano gene. The genetic test verifies the presence of the chromosomal inversion and presents results as one of the following:
|T/T||Homozygous||Positive for dominant Tobiano gene mutation, carrying two inherited copies of Tobiano. Will always pass Tobiano to foals. For breeding purposes, homozygous Tobiano horses are highly desirable as they are guaranteed to produce Tobiano foals regardless of their mate.|
|n/T||Heterozygous||Positive for the dominant Tobiano gene mutation, carrier of a single inherited copy of Tobiano. Horse's base color may be modified to varying degrees by the Tobiano markings.|
Since Tobiano is only responsible for the white markings of a so called "colored" horse, the test does not determine the horse's base-color. This is determined using the red/black factor test. The two tests in conjunction not only verify the likelihood of Tobiano being passed to foals, but also the likelihood the foals will be piebald or skewbald.