Tobiano (TO)

General Description

Tobiano is a large inversion of equine chromosome 3 (ECA3) that likely disrupts the regulation of the KIT gene. Tobiano is a dominant trait, signifying that horses with two (TO/TO) or one (TO/N) copy of the allele both display the spotting pattern. Typically, tobiano horses have large white patches on the limbs and body, usually with smooth borders and crossing over the topline. The eyes are usually dark. There is a large variation in expression, generally ranging from white present only on the legs, to horses nearly completely white. Rare Tobiano individuals have no visible white markings.

There are no reported negative health impacts with the Tobiano allele. However, chromosomal inversions are well known to carry a reduction in fertility in many species due to difficulty producing gametes (eggs or sperm) with the correct number of chromosomes. Brooks et al. stated that a 20% to 50% reduction in fertility (as might be expected from a chromosomal inversion) may not be noticed in horses due to modern breeding techniques. There have been no published studies investigating fertility rates in Tobiano horses.

Genotype and Phenotype (Color Names)

[Color] Tobiano (TO/_)

* Spotting patterns generally follow after the color of the pigmented regions, for example “Black Tobiano” or “Amber Champagne Tobiano.”

* In some breeds, horses with at least one Tobiano allele and any overo pattern are called “Tovero.”

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Genetic Region Information

Tobiano (ECA3) is located just outside of KIT about 100kb downstream.

KIT is a tyrosine kinase receptor that is vital for normal development. Mutations in other species have led to white spotting, anemia, sterility, and certain types of tumors. However, no negative health affects associated with KIT mutations have ever been documented in the horse. The TO inversion is located in the regulatory region of KIT, thus likely disrupting normal gene regulation. There are two inversions in KIT reported in the mouse, Rump-white and W-sash, both of which result in vertically oriented white patches with smooth borders.

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Questions and Developments/Discoveries

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Ever hear someone say that a horse can be W20/W20 OR W20/N, TO/N,  but never homozygous for one AND heterozygous for the other??  Due to a genetic phenomenon known as linkage it is rare, but it does happen! (see images of horses with W20/W20 and To/n)

How?  Here's the deal -

While W20 (a simple Single Nucleotide Polymorphism or "SNP" change) is within the KIT gene of the equine genome. The sequence change responsible for Tobiano (ECA3) is located just outside of KIT about 100kb downstream.  The two changes happened independently on different copies of the ECA3.   Tobiano is a different kind of variation called an "inversion" whereby a section of the genome (a long chain of base pairs) has been completely "flipped" and reinserted into the chromosome (inverted).  Once in a great while, a chromosome carrying that inverted region will swap in a portion of the KIT region that contains a W20 mutation from another ECA3 by a process known as recombination.  Therefor, a horse with W20/W20 and Tobiano can pass either the single, "normal" W20 alone or the linked combination of W20/Tobiano to it's offspring as one unit.

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References

Brooks SA et al., “A chromosome inversion near the KIT gene and the Tobiano spotting pattern in horses.” (2007) Cytogenet Genome Res. 119: 225-30.

Haase B et al., “An equine chromosome 3 inversion is associated with the tobiano spotting pattern in German horse breeds.” (2008) Anim Genet. 39: 306-9.

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