An equine chromosome 3 inversion is associated with the tobiano spotting pattern in German horse breeds.

The research article examines the relationship between a chromosomal inversion on equine chromosome 3 and the Tobiano white-spotting pattern in certain German horse breeds.

Overview and Significance of the Research

• The focus of this study is the Tobiano white-spotting pattern, which is considered a desirable trait and is prevalent among various horse breeds. Its genetic basis, however, has been a subject of debate, with several markers being previously associated with the phenotype.
• This pattern is autosomal dominant, meaning horses can carry one or two copies of the Tobiano allele (To) and still express the phenotype. Horses with one or two copies of the allele, called heterozygotes and homozygotes respectively, are phenotypically indistinguishable.

Previous Research and Limitations

• Prior to this study, a Single Nucleotide Polymorphism (SNP), a variation at a single position in a DNA sequence, had been identified in the 13th intron (a non-coding section) of the equine KIT gene, which was believed to be associated with the Tobiano phenotype.
• An indirect gene-testing method, based on this SNP, had been successfully deployed among several horse breeds. Nevertheless, when applied to the Lewitzer breed, the method failed to identify 14% of Tobiano-patterned horses, leading to the realization that this breed might carry a different mutation causing the Tobiano pattern.

Current Research Findings

• The research team took on the task of exploring this proposed alternate mutation. Recent research suggested that the causative mutation for To is, in fact, a significant inversion mutation on equine chromosome 3.
• An inversion mutation occurs when a chromosome breaks in two places and the ensuing segment flips over before re-attaching. Furthermore, it was observed that one of the breakpoints of this inversion is around 70 kb downstream of the KIT gene, suggesting it may disrupt a regulatory element of this gene.
• The researchers obtained genotype data for both the intron 13 SNP and the chromosomal inversion from 204 Tobiano-patterned horses and 24 control animals across multiple breeds.
• The findings confirmed the proposed chromosomal inversion in the KIT gene to be perfectly associated with the To allele in all investigated horses. Thus, the newly identified test for this inversion mutation can accurately identify heterozygous To/+ and homozygous To/To horses from the tested breeds.