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Impressive’s Flaw
Muscle disease traces to popular sire
Edward R. Winstead

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Impressive certainly lived up to his name. Before his death in 1995, the chestnut quarter horse sired 2,250 foals.

Unfortunately for his offspring, Impressive had a potentially lethal defect in a gene that helps muscle cells operate smoothly. The defect put thousands of Impressive's descendants at risk for a muscular disease known as HYPP, or hyperkalemic periodic paralysis. The same mutation is associated with a variety of symptoms: some carriers show no noticeable effects, while others experience muscle spasms and breathing problems. In severe cases, a horse can have episodes of paralysis and die suddenly.

In the early nineties, researchers identified the gene involved and developed a DNA test that breeders have used to identify carriers of HYPP and reduce the incidence of the disease. With the help of horse pedigrees, a team of geneticists traced the flaw to Impressive. "Had anyone known, Impressive would never have been used as a stud," says James Mickelson, a horse researcher at the University of Minnesota.

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