That year has finally arrived: whole-genome sequencing of dozens of Icelandic families has at last provided the evidence that eluded Haldane. More­over, a study published in Nature finds that the age at which a father sires children determines how many mutations those offspring inherit2. By starting families in their thirties, forties and beyond, men could be increasing the chances that their children will develop autism, schizophrenia and other diseases often linked to new mutations. “The older we are as fathers, the more likely we will pass on our mutations,” says lead author Kári Stefánsson, chief executive of deCODE Genetics in Reykjavik. “The more mutations we pass on, the more likely that one of them is going to be deleterious.”

Haldane, working years before the structure of DNA was determined, was also correct about why fathers pass on more mutations. Sperm is continually being generated by dividing precursor cells, which acquire new mutations with each division. By contrast, women are born with their lifelong complement of egg cells.

Stefánsson, whose company maintains genetic information on most Icelanders, compared the whole-genome sequences of 78 trios of a mother, father and child. The team searched for mutations in the child that were not present in either parent and that must therefore have arisen spontaneously in the egg, sperm or embryo. The paper reports the largest such study of nuclear families so far.