New research has demonstrated that genes can change, identical twins with the same genetic inheritance can turn out completely different and the impact of environmental influences can be passed down the generations.
The new science of epigenetics has shown that in addition to nature and nurture, what makes us who we are is also determined by biological mechanisms that can switch genes on or off. These epigenetic (above the gene) "light switches" can affect characteristics as fundamental as autism and sexual orientiation. But they are also subject to environmental influences and thus, in theory, within our control.
Professor Tim Spector of Kings College, London, who has undertaken the most detailed twin studies in the world, cited the case of Iranian conjoined twins Ladan and Laleh, who shared identical genes and environment and yet had completely different personalities.
"Up to a few years ago I believed genes were the key to the universe. But over the last three years, I have changed my mind," he said at the launch of a book Identically Different: Why You Can Change Your Genes, which challenges the view that an individual's genetic inheritance is immutable.
Studies of the effects of famines in Holland in the 1940s, in China in the 1950s and in the US over a century ago show they changed the lifespan and obesity rates in subsequent generations. They switched on genes that increased the accumulation of body fat in times of plenty, in order to improve survival chances in times of famine.
In the modern world, with calorie-dense fast or prepared foods more freely available than at any time in history, the seeds of the current obesity epidemic may thus have been sown in the 19th century.
"The risk of obesity can come not just from your own environment or your mother's, but higher up [the ancestral chain]," said Professor Spector.