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The mystery behind a sheep's behind

By Robert Cooke, Chicago Tribune

15 September 2002: Although for some it's a delicate subject, scientists are rejoicing that they've finally found the gene for big butts. Not fat butts. Big, muscular butts -- in sheep.

Named Aphrodite Kallipygos (Greek for beautiful buttocks), these sheep seem to convert food more efficiently into muscle instead of fat. And it shows up most visibly in the derriere.

"These sheep are, in effect, pumping iron without lifting weights," said radiation oncologist Randy Jirtle, at Duke University. "They are converting food into muscle in their hind regions, instead of converting food into fat."

The benefit of finding such a gene, is that it may help explain how fat and muscle tissues get deposited in living animals, including humans.

The studies, reported in the journal Genome Research, indicate that the mutant gene allows an animal to convert food into muscle 30 percent more efficiently than normal. And manipulating the gene may offer ways to enhance meat production.

Unfortunately, all that extra muscle makes lousy meat. There is too little fat, the scientists said, so the meat is tough and tasteless.

"It's not acceptable from the consumer's standpoint," said research geneticist Brad Freking, at the U.S. Department of Agriculture's meat animal research center, in Clay Center, Neb. "But it does give us an avenue to look at the genetics of meat tenderness."

Jirtle, Freking and their colleagues found that the strange gene is expressed, or not expressed, depending on which parent it is inherited from. If a lamb gets the gene from its father, then it builds an enormous, muscular derriere. But if the gene comes from its mother, the animal's rear end is far less bulbous.

The mutation causes each muscle fiber to double in thickness, rather than doubling the number of fibers.

The mutation works via a poorly understood phenomenon called genetic imprinting, which is also seen in some rare human disorders. This involves a few genes getting silenced or activated, depending on which sex they pass through en route to the next generation.

When the gene is "on," Freking said, "these lambs are born looking normal. But in three, four or five weeks it's clear that their musculature is different."

This odd difference was first noticed in Oklahoma about 20 years ago, by a rancher working with show animals. Soon "they were winning carcass contests because their muscling was substantially bigger," Freking said. And observers "were beginning to wonder whether they'd been drugged" to build muscle mass. "But it was a naturally occurring mutation."

Jirtle said he's now comparing the sheep genes to those in opossums and in platypuses, hoping to learn more about the process of genetic imprinting.