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The tobiano white spotting pattern is a trait controlled by a dominant gene. The pattern is clearly marked and characterized by white across the spine that extends downward between the ears and tail. The skin underlying the white spots is pink and under the colored areas it is black. The eyes are usually brown, but one or both may be blue or partially blue. The head is dark, with white markings like those of a solid colored horse. Usually, all four legs are white below the hocks and knees. The spots are generally regular and distinct as ovals or round patterns. The tail can be two colors—a characteristic seldom seen in horses that are not tobiano. A tobiano can be predominantly dark or white.
The tobiano gene has two alternative states (alleles). The dominant allele, TO, produces the tobiano pattern and the recessive allele, to, is non-tobiano (called N by VGL). A horse that is homozygous for tobiano, symbolized as TO/TO, will always produce offspring that are tobiano regardless of the mate. For breeders interested in producing tobiano foals, it is obvious that a horse that is homozygous for tobiano is desirable in a breeding program.
As most owners do not want to wait for progeny information from a very young horse and secondary spotting is not absolutely associated with tobiano homozygosity, a test is available that can help predict the likelihood that a horse is homozygous for tobiano.
Unlike other white spotting patterns caused by specific changes in DNA sequence of the genes, Tobiano is associated with a large chromosome inversion that affects the function of the gene KIT. The inversion associated with the Tobiano pattern was identified by researchers at the University of Kentucky. VGL now offers a direct test for Tobiano which is available along with our other coat color and pattern diagnostic assays.
Tobiano results are reported as:
|N/N||No copies of Tobiano detected.|
|N/TO||1 copy of Tobiano detected.|
|TO/TO||Two copies of Tobiano detected.|
Reference: S.A. Brooks, T.L. Lear, D.L. Adelson, E. Bailey. A chromosome inversion near the KIT gene and the Tobiano spotting pattern in horses. Cytogenet Genome Research 119:225-230 (2007)