The Genetic Mutations Behind Dwarfism in Horses

Scientists categorize dwarfism in horses as being either proportional or disproportional, but both types are caused by genetic mutations.

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Genetic Mutations Behind Dwarfism in Horses
In Miniature Horses with disproportional dwarfism, some of the body parts are smaller than others, putting the individual visibly out of proportion. | Photo: Courtesy of The Peeps Foundation
Scientists categorize dwarfism in horses as being either proportional or disproportional. Proportional dwarfism is exactly like it sounds: Everything is smaller. With disproportional dwarfism, however, only some of the body parts are smaller, putting the individual visibly out of proportion.

Dwarfism results from a genetic mutation affecting growth. Proportionate dwarfism appears to be linked to the wither height geneHMGA2 found on equine chromosome 6. Swiss geneticist Mirjam Frischknecht, PhD, of the University of Bern and of Agroscope in Avenches, announced her discovery of the gene’s connection with wither height in ponies in 2015.

The following year, Swedish researchers revealed the harmful effects of short stature homeobox (SHOX) gene mutations in Shetlands and Miniature Horses. When the SHOX gene or the nearby cytokine receptorlike factor 2 (CRLF-2) gene is deleted, the legs grow crookedly, in a condition known as skeletal atavism. Sometimes confused with true dwarfism, skeletal atavism causes Miniature Horses to have splayed limbs that are sometimes so severe the animal must be euthanized, says Carl-Johan Rubin, PhD, of Uppsala University, in Sweden.

Meanwhile, John Eberth, MS, PhD candidate at the University of Kentucky’s Gluck Equine Research Center, in Lexington, has been conducting genetic studies on true disproportionate dwarfs in Miniature Horse herds. Having grown up on Miniature Horse farms and experienced firsthand the “tragedy” of dwarfism, he says he was determined to find the mutations responsible for the condition

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Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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