When we think about inheritance, what usually comes to mind is the way our DNA carries information, with parents' genes affecting things like their children's eye colour, height, and intelligence.

We often hear people say: "I take after my father." Indeed, any textbook will tell you that this is how people inherit characteristics from their parents and grandparents.

But, in fact, your life has been directly influenced not just by the genes but by the experiences of your grandparents - what they ate and what they did.

This could mean that your health was partly determined before you were born, even before your parents were born, according to the science of epigenetics.

This new field, "epigenetic inheritance", has transformed our views of why we are the way we are. It suggests that there is a system of switches, able to turn genes on and off, controlled by environmental factors such as nutrition.

When transmitted from one generation to the next, these switches are responsible for some inherited effects.

Marcus Pembrey, a professor of clinical genetics at the Institute of Child Health in London, has worked with Swedish researcher Lars Olov Bygren, who found early evidence of these transgenerational effects in Sweden.

They looked at evidence from the Avon Longitudinal Study, a survey of British children born to 14,000 mothers in the early 1990s.

The results showed that factors such as smoking may have an effect across generations. More than 5,000 fathers that took part either smoked or had been smokers.

Of these, 166 had started smoking in what is known as the slow growth period - the time before the start of puberty when environmental factors have a larger impact on the body, which for boys is usually between nine and 12.

The sons of the men who had smoked during their slow growth period were significantly more overweight than average at nine, at the start of their own slow growth period. These results caused Prof Pembrey to go back to records from Overkalix, a remote town in northern Sweden.

An earlier study in Overkalix showed that starvation or over-eating during the slow growth period would influence a child's descendants' risk of dying from diabetes or heart disease.

The amount of food that the parents and grandparents had consumed was estimated using data on the price of food and success of harvests during the slow growth periods of generations born in 1890, 1905 and 1920.

If food was scarce during a father's slow growth period, then fewer sons died early. The number of sons dying early from all causes was increased if there was a lot of food at the time of the paternal grandfather's slow growth period.

This was not the case for granddaughters, which suggests that a single environmental event can affect descendants in a sex-specific way.

Scientists disagree on the reasons. Men have a Y chromosome and an X, while women have two Xs, and it is unclear whether the answer lies on the chromosomes or is related to other factors.

So, it seems that, as Prof Wolf Reik of the Babraham Institute in Cambridge suggests, the age-old argument about nature versus nurture is no longer relevant.

With the arrival of epigenetics, the two are intertwined within all of us, moulding us into the people that we are, and shaping all aspects of our lives.

And, just as importantly, Prof Pembrey's research allows us an insight into coming generations: a major cause for concern, given that stories about binge drinkers, obesity and younger smokers are never far from the headlines.

"We are all guardians of our genome," says Prof Pembrey. "The way people live and their lifestyle no longer just affects them, but may have a knock-on effect for their children and grandchildren."

# Joanne Marlor of Grange School, Cheshire, came second in the younger category of the Bayer/Telegraph science writer awards, which will be relaunched next week. See www.science-writer.co.uk
# Prof Pembrey is chairman of the Progress Educational Trust, which will discuss the Human Tissues and Embryos Bill on Thursday at the Institute of Child Health, London. Call 020 7278 7870, or email admin@progress.org.uk