Male lions are among the most sexually active of mammalian beasts. In captivity they have even been known to mount female tigers. The resulting liger offspring make their parents look like pussy cats, often exceeding twelve feet in length and doubling parental weight. In contrast, if you mate a male tiger with a female lion, the resulting tigon is considerably smaller. What makes the liger so big and tigon so small? If a lion and tiger mate, why should it matter which is mum and which is dad? Didn’t you learn in biology lessons that the genetic contribution from both parents was equal? Similar effects are apparent when you cross a horse with a donkey. The mule born of a mare and a jack (male donkey) is visibly different to the hinny born of a jenny (female donkey) and a stallion. Clearly your parents can have different influences on the way your genes work.

The dogma of the genetic age is undergoing a quiet revolution. We’re starting to think less about gene sequences and more about how genes behave in the context of their environment. Bryan Turner (University of Birmingham, UK) reckons that the genome is a bit like a record. You don’t play it all at the same time. The controls on your hi-fi allow you to listen to different tracks, and turn up the volume as you please. As you develop, selected parts of your genome (aka genes) get played at different volumes in response to environmental cues. When we make eggs and sperm the volume knobs get reset. When sex cells fuse, and two genomes become one, a lot of reprogramming goes on in the egg. A fertilised nucleus bundles two metres of DNA into a tiny nuclear kingdom a few millionths of a metre wide.