Gene of the month - Callipyge

Move over Kim Kardashian, because our gene of the month is the curvaceous Callipyge, Greek for "beautiful buttocks". It was first spotted back in the early 1980s on a farm in Oklahoma, when the farmer noticed some sheep in his flock with particularly big, muscular bottoms. Tracking down the Callipyge gene itself turned out to be quite tricky, because it is inherited in an unusual way known as 'paternal polar overdominance' - the only known example of this phenomenon in mammals. Animals only get big bums if they inherit a normal copy of the gene from their mother, and a version from dad that has just one single DNA letter change.

Agricultural researchers are very interested in Callipyge because the mutation enables sheep to convert food into muscle 30 per cent more efficiently than their small-bottomed siblings. And as well as the potential for breeding meatier livestock, scientists think it could also shed light on human size and shape, helping our understanding of the genes (with a g) that affect whether we can fit into our jeans (with a j).