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Older fathers pass on more genetic mutations to their children than older mothers do, according to study that investigated the genomes of thousands of Icelandic parents and children. The researchers behind the work hope to understand how such mutations put children at risk of rare diseases.

New mutations are genetic alterations that appear for the first time in eggs rather than being carried for generations. They are key drivers of evolution but some can be harmful.

“An extraordinarily large percentage of rare diseases in children are rooted in mutations that are not found in their parents,” says Kári Stefánsson of deCODE genetics, a genetics company based in Iceland. “It’s important to figure out where these mutations are coming from.”

To find out, Stefánsson and his colleagues sequenced the genomes of 14,688 Icelanders. The team used two different approaches that involved comparing the genome sequences of individuals with those of their parents, children and siblings.

“If a sequence is not present in the parents but is present in the child, then it’s new,” says Stefánsson. They discovered that 80 per cent of new mutations come from the father, and that the number of mutations increases in line with the age of the parents.

Divide and mutate

It makes sense that age affects the sex cells of men more than women. Women are thought to be born with all the eggs they will ever have. Although these cells age, they are not thought to divide. Men, on the other hand, are continually making sperm – and every cell division carries the risk of creating a new genetic mutation.

These mutations won’t all be harmful. We’re all born with at least 70 new mutations, and most of these don’t affect the way our bodies and brains work. “The vast majority of mutations don’t matter, says at the University of Essex. “There might be the occasional mutation that is deleterious, or the extremely occasional mutation that is beneficial.”

He compares the impact of mutations to swinging a hammer at a car engine. “There’s a chance that you will improve its function, but it’s much more likely that the hammer will either just bounce off or break something.”

Other research has shown that older fathers are more likely to have children with autism and schizophrenia, but the team do not know if the mutations they observed are linked to either disorder.

Stefánsson’s team found certain hotspots in the genome where new mutations appeared to cluster – although the implications of this is not yet clear.

Read more: Testicular time bomb: Older dads mutant sperm; Warn people of genetic health risks says decode boss