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Mutation gives sheep beautiful buttocks DNA change between genes is behind big-bottomed sheep.

By John Whitfield, Nature

15 September 2002: Scientists have identified a mutation that gives some sheep huge, hard bottoms. Understanding how the mutation works could give rise to leaner, meatier sheep and provide insights into inheritance.

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One changed DNA letter produces 'callipyge' sheep, which amass muscle instead of fat around the pelvis, says geneticist Randy Jirtle of Duke University in Durham, North Carolina. The change probably alters the activity of nearby genes.

Some humans might share the trait, Jirtle speculates. "They'd have relatively large rear ends, and absolutely no fat - like sprinters," he says. Other genetic variation in the same region might influence a person's risk of obesity.

A ram called Solid Gold, born 20 years ago, was the first callipyge sheep - the name comes from the Greek for 'beautiful buttocks'. All born since are his descendants. "They're Arnold Schwarzenegger-shaped," says Jirtle, "and their meat's tougher than hell."

Why the sheep put their resources into muscle rather than fat is not understood. The most likely explanation, says geneticist Anne Ferguson-Smith of the University of Cambridge, UK, is that the mutation affects the control of a nearby gene called DLK.

DLK is normally switched off before birth, but is overactive in the muscle tissue of callipyge sheep. This could be why the sheep are normal at birth, but bulk up as they age. "DLK seems to be intimately involved in the balance between making muscle and fat," says Ferguson.

Bottoms up

It's not aesthetics that attracts researchers to callipyge sheep. "The loin and legs are the most valuable cuts," says meat scientist Sam Jackson of Texas Tech University in Lubbock. "Anything that's going to increase their size and value will attract attention." He predicts that breeders will begin trying to select tender callipyge sheep.

Geneticists, on the other hand, study the sheep in the hope of understanding the strange way in which large bottoms are passed down through the generations. Sheep are only callipygous if their father is; mutant mothers do not pass the trait on. And two big-bottomed sheep will have snake-hipped offspring. How the two mutants cancel each other out is still a mystery.