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Tobiano

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. A horse that is homozygous for tobiano, symbolized as TOTO, will always produce offspring that are tobiano regardless of the mate. For breeders interested in producing tobiano offspring, it is obvious that a horse that is homozygous for tobiano is desirable in a breeding program.

Requirements for Tobiano Zygosity Analysis

The minimum requirement for a horse to be homozygous for tobiano, is that it must have two tobiano parents. The new tobiano analysis requires the following information for complete analysis:

  • Pedigree: three-generation pedigree with color and patterns marked
  • Photos of the subject horse: good quality photos and both sides preferred. Digital photos are accepted.

Test Results

Allow 2-6 business days for results.

 

Detailed Tobiano Information

Determining if a Horse is Homozygous for Tobiano

There is no direct DNA diagnostic test available for the tobiano gene, but progeny testing provides the next best information. A horse that produces any non-tobiano colored foals is not homozygous for tobiano. A horse bred to at least 10 solid colored mates that has only tobiano foals is statistically 99.9% certain to be homozygous.

Many horses that are homozygous for tobiano have a secondary spotting pattern of small clustered dark spots with roan edges breaking through in the white areas of the body. These spots, sometimes called "ink spots" or "paw prints" are often, but not always, associated with homozygous tobianos.

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.

Tobiano Zygosity Testing

There is no direct test at this time for the tobiano gene, however there are closely linked markers that can be used to determine if a horse carries one or two copies of the tobiano chromosome. Previously, the blood markers, Albumin (AL) and Group-specific component (GC) were used for this linkage analysis. VGL is now using hair samples to test for six closely linked DNA markers, thus eliminating the need for blood samples.

Requirements for Tobiano Zygosity Analysis

The minimum requirement for a horse to be homozygous for tobiano, is that it must have two tobiano parents. The new tobiano analysis requires the following information for complete analysis:

  • Pedigree: three-generation pedigree with color and patterns marked
  • Photos of the subject horse: good quality photos and both sides preferred. Digital photos are accepted.
Linkage Analysis

The tobiano gene has been mapped to horse chromosome 3. It has long been thought that the KIT gene that maps to this region is involved with this and other white spotting patterns. Recent research by Samantha Brooks and collaborators has confirmed a hypothesis by Dr. Ann T. Bowling that Tobiano is associated with a chromosomal rearrangement (inversion). The inversion includes KIT and may disrupt the normal functioning of this gene which results in the spotting pattern. This rearrangement significantly reduces recombination such that Tobiano and markers closely linked to it are inherited together. This forms the basis for the linkage analysis used to determine zygosity of this white spotting pattern.

More information on tobiano and genetics

Bowling, Ann T. HORSE GENETICS. CAB International, Wallingford, UK. 1996.
This book covers the following: Basic genetics, extensive reviews on coat color, parentage testing, medical genetics, karyotyping and chromosomal abnormalities, genetics of performance traits, pedigree and breeding schemes.

Bowling, A.T. & A. Ruvinsky (eds). THE GENETICS OF THE HORSE. CAB International, Wallingford, UK. 2000.
This book covers many aspects of horse genetics including systematics and phylogeny of the horse, genetics of morphological traits and inherited disorders, molecular genetics, immunogenetics, genetic aspects of disease in horses, cytogenetics and physical gene maps, linkage and comparative maps, reproductive and modern breeding technologies, behavioral and developmental genetics, genetic conservation, genetics of performance traits, conformation, locomotion and physiological traits, genetic improvement and standardized genetic nomenclature for the horse. Chapter 3 "Genetics of Colour Variation" by A.T. Bowling reviews basic colors, color dilution genes, white and white spotting genes.

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.

 

 

 
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