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

Dwarfism results from a genetic mutation affecting growth. Proportionate dwarfism appears to be linked to the wither height gene—HMGA2 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