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A sperm age calculator can tell men how “old” their sperm are, using clues from DNA analysis, and has revealed some of the effects of smoking on sperm.

While a woman’s age has long been known to affect the health of her offspring, we have only recently begun to understand how a father’s age can have similar effects. Older fathers are now known to pass on more genetic mutations to their children than older mothers do. And children of older fathers are more likely to have autism and schizophrenia.

“The hope is that we could potentially screen people and say, ‘your sperm is really old’, and identify risks for the offspring,” says at the University of Utah.

Growing evidence suggests that older dads might pass on health risks through epigenetic tags on the DNA in their sperm. These tags alter how active genes are, and lifestyle factors such as diet and smoking are known to make epigenetic changes that may affect the next generation.

Ageing is also a factor. Jenkins and his colleagues have studied the sperm of 350 men, looking for these genetic switches. So far, the team have found changes at 147 points in the genome that seem to be linked to a man’s age.

Using this information, the team have created a “calculator” that assesses the state of the DNA of a man’s sperm at these 147 sites. Their analysis can predict a man’s age with approximately 95 per cent accuracy, and identify whether a man’s sperm have aged prematurely.

Jenkins and his colleagues have found that smokers have much older-looking sperm. “For a 40-year-old man who smokes, our calculator would calculate him to be 44 or so,” says Jenkins, who will present his findings at the end of the month at the annual meeting in San Antonio, Texas.

The researchers don’t yet know if these changes are responsible for the increased risks of autism and schizophrenia in the children of older men, but at Manchester Metropolitan University, UK, suspects this is the case.

“The emphasis on smoking has always been on the mother’s side,” Carroll says. “But it’s becoming more evident that exposures in men can alter health. It’s not just about how sperm swim or what they look like – there are changes at the molecular level which can affect the offspring.”

Because sperm is produced throughout a man’s life, there’s a chance that men with old sperm might be able to reverse some of the damage, says Carroll.

“If you have a male in his 30s or 40s who has a sperm age in the 50s or 60s, you’d have to look at why that is,” says Carroll. “If you could change those factors, you could possibly reverse the clock.”