There’s been a lot of discussion recently about older fathers and their DNA. An Icelandic study led by Kari Stefansson, published in Nature and explained more clearly for us lay people here in The Guardian.
Men produce sperm throughout their lives (unlike women, whose eggs are all there by the time they are born), and as they age, men produce a higher (though still very small) number of mutations that pass to their children. Stefansson’s team looked at 78 trios (i.e. father, mother and child) and found that ‘a 20-year-old father transmits, on average, around 25 mutations…whereas a 40-year-old father transmits around 65.’ Every additional year at the time of conception adds an average 2 further mutations.
It’s an interesting finding, even though the number of mutations in the father’s DNA is only a tiny fraction of the total. Journalists have extrapolated from the finding to suggest that it may explain rising levels of certain diseases, such as autism and schizophrenia, because the age at which men father children is rising.
This seems to me rather a leap to make, based on a sample of only 78 fathers – and from Iceland, one of the most homogeneous populations in the world. Nor is it all that clear what role genes play in conditions such as autism and schizophrenia, although there are certainly some problems that are caused by a genetic mutation, such as Fragile-X syndrome.
I can’t comment on the science, or the statistics, but the other assumption in this story is that fathers are now older than they used to be. It’s true, more or less, that first time fathers in 2012 are older than first time fathers in 1900, or 1950, or ever 1980. But the assumption that most children were once born to younger men is wrong.
A hundred years ago, men and women married earlier, had their first child 9 or so months later, and then continued to have children until the mother reached menopause or died, quite possibly in childbirth. If only because they needed help to raise their motherless brood, widowers usually married again, so they might well start another family when they were in their 40s or 50s. Henry Parkes, Premier of New South Wales, married 3 times, and his youngest son Cobden, was born when he was 77.