Sabino 1 (SB1)

General Description

Sabino 1 is a variant of the KIT gene that causes the sabino (or sabino overo) spotting pattern. Sabino 1 is an example of incomplete dominance, indicating that horses that inherit one copy of the allele have different phenotypes than horses with two copies. Heterozygous horses (SB1/N) generally have extended white markings on the legs and face, sometimes with a belly spot and roaning (white hairs) throughout the body. The borders of the white markings tend to be fairly jagged. Sabino horses commonly have dark colored eyes. Homozygous horses (SB1/SB1) are born nearly to completely white.

The sabino spotting pattern is found in many breeds of horse, however not all horses with this phenotype have the SB1 allele. For example, several draft horse breeds have spotting patterns that are nearly identical to SB1/N horses, such as the classic Clydesdale markings. Many of these sabino-like patterns do not produce completely white offspring.

Genotype and Phenotype (Color Names)

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Maximum sabino / sabino white (SB1/SB1)

[Color] sabino (SB1/N)

* Spotting patterns generally follow after the color of the pigmented regions, for example “chestnut sabino” or “grullo sabino.”

* Sabino is sometimes called “sabino overo” or simply “overo.”

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

Gene Information

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 effects associated with KIT mutations have ever been documented in the horse. The SB1 mutation alters a splice junction, which then results in the absence of a portion of the protein.

References

Brooks SA and Bailey E. “Exon skipping in the KIT gene causes a Sabino spotting pattern in horses.” (2005) Mamm Genome. 16: 893-902.