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Big-bottomed sheep show rare mutation for muscle

15 September 2002: DURHAM, N.C. - Some sheep have unusually robust and muscular bottoms, thanks to a rare genetic mutation, scientists have found. They hope understanding the mutation will help lead to leaner meat.

Researchers at the U.S. Department of Agriculture and Duke University Medical Center discovered a gene called callipyge, Greek for "beautiful buttocks," in sheep that are able to convert food into muscle 30 per cent more efficiently than normal sheep.

Scientists don't understand why the sheep put their resources into muscle, rather than fat.

Randy Jirtle, a professor of radiation oncology at Duke University in Durham, N.C., and his colleagues found a single change to a DNA factor was unique to the callipyge sheep, and it alone gave rise to the sheep's big-bottom stature.

"These sheep are, in effect, pumping iron without lifting weights," said Jirtle in a release. "They are converting food into muscle in their hind regions, instead of converting food into fat."

Jirtle said the finding may help explain how muscle and fat are deposited in the animals. The sheep's meat is tough, but animal breeders may try to select for tender callipyge sheep.

Geneticists are intrigued by the so-called imprinted gene mutation because of the rare way it is inherited and pops up in specific tissues at defined intervals during an animal's development, the researchers said.

Big-bottomed sheep only inherit a working copy of the mutated gene from their father; the mother's copy is turned off.

The study will appear in the October 2002 issue of the journal Genome Research.