A genetic "time-bomb" in the testicles increases a man's risk of fathering children with serious diseases as he ages, say scientists.
New evidence shows that defective sperm containing spontaneous harmful mutations can mimic cancer by reproducing at an unusually rapid rate.
Over time, this raises the risk of a child being conceived who suffers from a potentially lethal abnormality.
The researchers focused on a condition called Apert syndrome that harms skull and limb development and tends to occur in children of unaffected parents.
Although the disease can be traced to mutations in a gene called FGFR2 that arise spontaneously in newly forming sperm, this is only part of the story, the British team found.
The incidence of cases was 1,000 times higher than it should be given the rarity of the mutations. And tests on the sperm of fathers of children with and without Apert syndrome showed that both groups had mutations linked to the condition.
The missing piece of the jigsaw turned out to be that the FGFR2 mutations also caused affected sperm to make extra copies of themselves. Like cancer tumours, they multiplied and accumulated over time to make up a greater proportion of a man's total sperm supply.
Lead scientist Professor Anne Goriely, from Oxford University, said: "The process that gives rise to Apert syndrome happens in every man, meaning any couple could have a child with Apert syndrome, regardless of the health of the parents.
"I also found that older men tended to produce more of these Apert mutations."
Similar "selfish genes" that ensure their own survival are thought to play a role in other sperm-related diseases in children, said the researchers, whose findings appear in the journal Proceedings of the National Academy of Sciences.
Lethal forms of dwarfism were one example of a severe abnormality caused by spontaneous mutations in the sperm of "normal" men.
Each of a man's testes resembles a "massive tangle of spaghetti" containing up to 400 metres of interwoven tubes within which the sperm are made, said Professor Andrew Wilkie, another member of the Oxford team.
Describing the work, he said: "We developed techniques to pinpoint the abnormal regions of these tubes, and using a laser powered microscope, we could isolate these regions. This allowed us to perform detailed genetic analysis and we were able to identify mutations associated with severe diseases in 13 of the 14 testicles analysed."
Older paternal age is also associated with some cancers, autism and schizophrenia.
Future research will attempt to identify other "selfish" sperm mutations and their links to disease.
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