Identification of W13 in the American Miniature Horse and Shetland Pony Populations

Abstract

Coat color is a trait of economic significance in horses. Variants in seven genes have been documented to cause white patterning in horses. Of the 34 variants that have been identified in KIT proto-oncogene, receptor tyrosine kinase (KIT), 27 have only been reported in a single individual or family and thus not all are routinely offered for genetic testing. Therefore, to enable proper use of marker-assisted selection, determining breed specificity for these alleles is warranted. Screening 19 unregistered all-white Shetland ponies for 16 white patterning markers identified 14 individuals whose phenotype could not be explained by testing results. In evaluating other known dominant white variants, 14 horses were heterozygous for W13. W13 was previously only reported in two quarter horses and a family of Australian miniature horses. Genotyping known white spotting variants in 30 owner-reported white animals (25 Miniature Horses and five Shetland ponies) identified two additional W13/N American Miniature Horses. The estimated allele frequency of W13 in the American Miniature Horse was 0.0063 (79 N/N, 1 W13/N) and the allele was not detected in a random sample (n = 59) of Shetland ponies. No homozygous W13 individuals were identified and W13/N ponies had a similar all-white coat with pink skin phenotype, regardless of the other white spotting variants present, demonstrating that W13 results in a Mendelian inherited dominant white phenotype and homozygosity is likely lethal. These findings document the presence of W13 in the American Miniature Horse and Shetland pony populations at a low frequency and illustrate the importance of testing for this variant in additional breeds.

Spotting the Pattern: A Review on White Coat Color in the Domestic Horse

“…The introduction of white alleles to new breeds is also promoted by registries opening studbooks to foreign horses with hopes of reducing inbreeding. As an example, researchers identified W13 in American Quarter Horses (AQH) in 2011 [14], but more recently observed this allele in both Shetland ponies and American Miniatures [21], two breeds genetically distant from the AQH. As Shetland ponies are not typically tested for W13, identification of this allele only occurred after genotypes for more common white variants (ex: TO, SB1, W20) failed to explain the depigmentation.…”

A de novo missense mutation in KIT is responsible for dominant white spotting phenotype in a Standardbred horse

“…Due to the association between numerous KIT proto‐oncogene, receptor tyrosine kinase ( KIT ) variants and similar white patterns in horses, the remaining known KIT variants were also screened. Specifically, W1–W4, W6–W9, W11–W19, W21, and W23–W30 were genotyped as described by Esdaile et al (2021) (Capomaccio et al, 2017; Haase et al, 2007, 2009, 2011, 2015; Hauswirth et al, 2013; Hoban et al, 2018; Holl et al, 2010, 2017; Hug et al, 2019; Martin et al, 2021). W31 was genotyped using an allele‐specific PCR with products resolved on the Applied Biosystems 3730XL DNA Analyzer (Thermo Fisher Scientific), and W32 was genotyped using a custom‐designed MassARRAY assay (Agena Bioscience) (Patterson Rosa et al, 2021).…”