Older dads pass down more mutations: the implications
A study this week reported that older men pass on more new mutations to their offspring than do younger men, a fact that could help explain higher rates of disorders such as autism, schizophrenia and others in kids born of older fathers.
The same week, another article by the same group calculated that the mutation rate in fathers doubles between age 20 and age 58.
Why fathers more than mothers? Because many of these mistakes happen as cells divide, and as we get older the rate of errors rises. Since sperm is made all the way through the man’s life and all eggs are pretty much ready before a woman is even born, the difference makes sense.
Bad news? On a personal level, yes, since a DNA mutation -- should it have an effect at all -- is more likely to have a negative effect than a positive one.
And on a societal level, it could be bad too, says Alexey Kondrashov, an evolutionary geneticist at the University of Michigan who wrote a commentary accompanying Stefansson’s report. He’s worried about an issue that has been discussed (often very contentiously) since Charles Darwin’s time: an accumulation of mutations with negative effects in our species.
“Modern human populations are subject to many fewer selection pressures than has been the case throughout human evolutionary history,” Kondrashov writes. “Because deleterious mutations are much more common than beneficial ones, evolution under this relaxed selection will inevitably lead to a decline in the mean fitness of the population.”
Kondrashov thinks the recent rise in autism cases could be a good example of that. (He notes though, that scientists aren’t sure that the recent rise is real, and not based solely on broader recognition of the disorder.) It’s a very loaded issue, he says, but studies such as this are important so that scientists can figure out how big a problem this might be and how fast it may be acting.
The issue may raise hackles, “but you cannot fool nature by [saying] ‘Let’s pretend there is no problem,’ ” he says. “I fully believe that individual human rights are more important than a golden society. But nature doesn’t care. And if 20% of kids become autistic ... what’s going to happen?”
But there’s another way of looking at it, says study senior author Kari Stefansson of the University of Iceland and the company deCODE Genetics. As far as our species goes, mutations like this are the stuff of natural selection. It’s the ultimate source of the variation that evolution acts on, allowing organisms to adapt as conditions change.
“We are much more concerned about the well-being of our children than the survival of our species,” Stefansson says. But “what poses risks to the next generation may actually be very important for the survival of our species going forward.”
And here are some thoughts from Hank Greely, a law professor at Stanford who focuses on ethical and legal issues raised by biological research. “Medicine is all about relaxing selection so that our gene pool is less ‘robust,’ ” he says.
Adaptation means surviving in the environment that you’re in -- and our environment has changed greatly over the many thousands of years human beings have been around, and those environmental changes include advances in medicine. “Now people with Type 1 diabetes can live long enough to reproduce -- this does not strike me as a terrible thing,” he says.
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