Children whose grandmothers smoked have a heightened risk of asthma even if their mothers avoided the habit, a study has shown.
Evidence that the lifestyle sins of the grandmother can skip a generation and be visited upon her grandchildren comes from a large Swedish study.
It adds to growing evidence that "epigenetic" environmental influences can alter genetic activity in ways that may be inherited, leading to health problems that emerge in offspring.
Smoking is one environmental factor known to affect the activity, or "expression", of genes.
Dr Caroline Lodge, one of the study authors from the University of Melbourne in Australia, said: "We found that smoking in previous generations can influence the risk of asthma in subsequent generations. This may also be important in the transmission of other exposures and diseases."
Rates of childhood asthma have increased rapidly in the last 50 years, and experts suspect environmental exposures may be behind the trend.
For the new study, researchers analysed data on 44,853 Swedish grandmothers who were asked if they smoked during pregnancy.
Use of asthma medication was also recorded in 66,271 grandchildren and the two sets of results compared.
The investigation showed that if grandmothers smoked while pregnant, the risk of their grandchildren having asthma was raised by 10% to 22%. This was the case even if the children's mothers did not smoke.
Co-author Professor Bertil Forsberg, from Umea University in Sweden, who presented the findings at the European Respiratory Society's annual congress in Amsterdam, said: "The next stage for the research team is to investigate the potential inheritance of asthma risk through the male line, by assessing the risk of asthma in grandchildren whose grandmothers smoked whilst pregnant with their fathers.
"The findings also encourage research into inherited disease risks for other environmental exposures."
About 5.4 million people in Britain suffer from asthma, which is believed to kill three people a day, according to the charity Asthma UK. In 2011 there were 1,167 asthma-related deaths in the UK, including 18 children aged 14 or under.
More than a million UK children are receiving treatment for the condition.
Asthma prevalence is believed to have plateaued since the late 1990s after previously rising steeply. However, the UK still has one of the highest rates in Europe.
Dr Lodge said: "For us to understand more about the asthma epidemic, we require a greater understanding of how harmful exposures over your lifetime may influence the disease risks of generations to come. Additionally, researchers in this area need to be aware, when interpreting the asthma risk from current exposures and genetic predisposition, that individuals may carry an inherited, non-genetic risk from exposures in previous generations.
"This knowledge will help to clarify the findings concerning current risk factors in asthma research."
Dr Samantha Walker, director of research and policy at Asthma UK, said: "Smoking not only increases your risk of having a potentially fatal asthma attack, it is also a contributing factor in causing people to develop asthma.
"This study suggests that there may also be much longer-term effects as through genetic changes, smoking could also affect the risk of developing asthma in subsequent generations.
"More investigation is needed to better understand this, which is why people with asthma need to see more investment into asthma research. Asthma is a complex condition so studies like this one help us to understand the roles that genes play and therefore how we can find better treatments and ultimately a cure."