Children born of older fathers are at greater risk of developing a rare bone defect, say researchers.
Apert syndrome leads to malformed hands and feet
It is well-known that as a man ages, the chance of mutations in the genetic code of his sperm cells increases.
This in turn increases the chance of passing a genetic weakness onto the child.
Children with a disorder called Apert syndrome are born with distorted skulls, and often webbing joining the fingers and toes.
This strikes approximately one in 70,000 children born in the UK - but the chances of the defect among the children of older fathers may be much higher.
To account for all the extra cases born to older fathers, mutations of the gene that causes it - called FGFR2 - would have to be frequent events as men age.
But scientists from Oxford University found that something else was going on.
They found the particular mutation that caused Apert syndrome did not happen very often - but when it did, it had a profound effect on the way sperm was produced.
The mutation - in a "stem" cell whose job it is to produce sperm-making cells - is actually highly advantageous to the stem cell.
It makes the cell divide more quickly, and, over time, populate the testicle with many more copies of the damaged gene.
Professor Andrew Wilkie, from the Weatherall Institute of Molecular Medicine at the University, said that the effect was almost like that of a cancer, with rogue cells multiplying and producing sperm with the mutated gene.
He told BBC News Online: "It turns conventional wisdom on this subject on its head."
Prof Wilkie said, in theory at least, it might lead to ways of helping to avoid the birth of children with Apert's syndrome.
"It is not unrealistic to expect that some rich 60-year-old US businessman onto his second or third marriage and still feeling like a spring chicken might want to check for this."
He said a few other conditions caused by similar genetic defects - the best known being achondroplasia, which causes the development of extremely short arms and legs, might be caused by this effect.