Gregor Mendel's remarkable studies of peas revealed a straightforward pattern of genetic inheritance. But since then we've learned that matters are not always so simple: many identifiable traits result from complex interactions between multiple genes and environmental factors. Current work in genetics aims to quantify the contributions of specific variations in each gene to the mix. But pinpointing the right spot in an animal's huge genome can be a nightmare, even in organisms that have had their genome completely sequenced.

The humble pig, meanwhile, has only recently received its own genetic map — a first-generation sketch of what its genome sequence might look like. Imagine, then, how difficult it must have been to find one specific nucleotide that controls 15–30% of the variation in muscle mass seen between pigs. But that's just what Anne-Sophie Van Laere et al. report elsewhere in this issue (Nature 425, 832–836; 2003).

The authors find that a change from a guanine nucleotide to an adenine at a specific point in the IGF2 gene can add 3–4% more meat to a pig. The nucleotide change disrupts the regulation of IGF2, increasing its expression threefold in muscle but not at all in liver, another main organ of expression. As the IGF-II protein stimulates muscle growth, the result is a pig with more muscle and less fat — a boon for meat production. And, as the change occurs in part of the gene that does not actually code for protein, it suggests that researchers should not study just the protein-coding bits when looking for important genetic differences between individuals.

Of course, farmers don't need to know the exact mechanism involved; they spotted this physical trait years ago and selected for it in breeding schemes. Thus, Van Laere et al. show that the muscle-favouring alteration has swept through commercial pig populations, but is not present in Asian or European wild boars tested. Now that the pig is lining up with other farm animals such as cows and chickens to have its genome sequenced, we should be able to find other genetic contributions to bulked-up pigs.