What do a pie invented almost 2,000 years ago by the Roman statesman Cato the Elder and the organ most intimately connecting a mother and her unborn child have in common? They are both called placenta (and in some places, both are still eaten). "Placenta" comes from the Greek word plakous, meaning flat cake.

Mice and various other mammals often eat their placenta after giving birth. Human placenta apparently smells like liver and tastes like chicken, and is reportedly great raw or in a stew with garlic and red peppers. Mouthwatering though that may be, what really gives food for thought is a paper in Nature this week in which scientists describe work on genes active in the placenta.

We have two copies of each gene, one from our mother and one from our father. Both copies are used as we grow, and their effects are blended. The same is true for the genes at work in the placenta, but for some of them, so-called imprinted genes, only one copy of the gene is active; the other copy is inactive or "silent" -- it might as well not be there. Sometimes the paternal copy of the gene is switched on, sometimes the maternal copy. And when the mother's and father's genes both want a say, there can sometimes be a conflict of interest.