Dominant white color gene in horses is likely lethal if inherited from both parents

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Some of the ponies tested in the study. Images: Esdaile et al. https://doi.org/10.3390/genes12121985
Some of the ponies tested in the study. Images: Esdaile et al. https://doi.org/10.3390/genes12121985

A dominant gene responsible for a white coat color in horses, which is likely lethal if inherited from both parents, has been identified at a low frequency in American Shetland and miniature horses.

Researchers, writing in the journal Genes, said the results highlight the importance of testing for this variant in additional horse breeds.

Coat color and was one of the first genetic traits to be investigated in horses.

Six variants in five genes in horses have been shown to dilute pigment. These are known as cream, pearl, champagne, dun, mushroom and silver. Variants in seven genes (KIT, EDNRB, TRPM1, MITF, PAX3, RFWD3, STX17) have been documented to cause white patterning in domestic horses.

These are several different but overlapping phenotypes, including sabino-1, tobiano, and dominant white.

Scientists with the University of California, Davis, Veterinary Genetics Laboratory screened samples from 19 unregistered all-white Shetland ponies for 16 white patterning markers. The samples were provided from one ranch.

Elizabeth Esdaile and her fellow researchers identified 14 individuals whose coat color could not be explained by screening for conventional markers.

They then looked for other dominant white variants, finding that the 14 horses were heterozygous for the W13 gene. W13 has previously been reported only in two quarter horses and a family of Australian miniature horses.

They also tested hair samples from 30 owner-reported white animals, comprising five Shetlands and 25 miniature horses, which were held at the laboratory. Two of the miniature horses were found to be heterozygous for the W13 gene.

Later, the study team tested randomly selected samples from 59 Shetland ponies and 80 American Miniature Horses with unknown phenotypes held at the laboratory to determine the frequency of the W13 allele within the two breeds. The allele was not detected in the 59 Shetland samples, but was found in one of the miniatures.

“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,” they said.

In all detected cases, the W13 allele had been inherited from only one parent, meaning they were all heterozygous. No homozygous W13 individuals – that is, having inherited the gene from both parents – were identified. Homozygosity is likely lethal, the researchers said.

All W13 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.

Discussing their findings, the researchers said the 16 horses whose all-white coats could not be explained by variants routinely tested at the laboratory could be explained by being heterozygous for W13.

The study team noted that the Shetland ponies reported with the W13 allele were unregistered, and that they did not detect it when screening the 57 registered Shetland ponies. “It is possible,” they said, “that the unregistered Shetland ponies in this study may have some American Miniature Horse introgression. Additional genotyping of all-white registered Shetland ponies is necessary to fully investigate W13 within the breed.”

Previous studies had identified the W13 allele in only six animals. The first report identified two heterozygous individuals as Quarter Horse-Peruvian Paso crosses, with pedigree analysis suggesting the variant was of Quarter Horse ancestry.

Subsequently, W13 was identified in a family of four Miniature Horses in Australia. Notably, the sire had a white spotting pattern that was not a completely white coat, which contrasts with the phenotype and photographic records of the W13 ponies reported in this study.

The authors cautioned that while the W13 allele frequency in Shetlands and miniature horses is low, the likelihood that it is lethal if inherited from both parents supports the use of W13 genetic testing for breeding selection.

“This is especially important when other known variants do not explain coat color phenotype and when breeding two all-white Miniature Horses and/or Shetland ponies to minimize chances of embryonic lethality.”

The study reflects the largest number of animals identified with the W13 variant to date, they said, adding that it may also be found in other horse breeds yet to evaluated. Screening for W13 in other breeds and investigating potential origins is warranted, they said.

The study team comprised Esdaile, Angelica Kallenberg, Felipe Avila and Rebecca Bellone.

Esdaile, E.; Kallenberg, A.; Avila, F.; Bellone, R.R. Identification of W13 in the American Miniature Horse and Shetland Pony Populations. Genes 2021, 12, 1985. https://doi.org/10.3390/genes12121985

The study, published under a Creative Commons License, can be read here

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