A new study looking at the link between genetic mutations and parental age has turned a long-standing assumption on its head: It's the father's age, not the mother's, that raises the risk for new genetic mutations in their children, including autism and schizophrenia.
Older fathers transmit more new DNA variations than younger fathers, researchers found. The mother's age has no bearing on the risk for these disorders.
The findings may explain why childhood autism rates are rising, scientists said Wednesday. The increasing age of fathers could account for as many as 30 percent of the cases, the New York Times reported.
The risk of chromosomal abnormalities, like Down syndrome, increases for older mothers, the Times noted, but for complex developmental and psychiatric problems, the key to genetic risk lies in the aging sperm, not the egg, the study found.
Researchers analyzed the genetic makeup of 78 Icelandic families in which the parents had no signs of a mental disorder, yet gave birth to children diagnosed with autism or schizophrenia. It was the father's age that was crucial to the genetic risk for the disorders.
"Conventional wisdom has been to blame developmental disorders of children on the age of mothers," lead researcher Kari Stefansson of the Icelandic firm deCODE Genetics, told Reuters. "(But) our results all point to the possibility that as a man ages, the number of hereditary mutations in his sperm increases."
Stefansson even called older men's sperm "mutational bombs."
The study, published online in the journal Nature, found that the average child born to a 20-year-old father had 25 random mutations that could be traced to the father's genes. For each year the father aged, the number of mutations increased by two, reaching 65 mutations for the children of 40-year-old men -- essentially a doubling of the mutation rate every 16 years.
The average number of mutations from the mother was 15, regardless of her age, the study found.
Reproductive experts say the study underscores the fact that fathering a child at an advanced age is not without risk.
"We used to think men can wait as long as they want to have children, but that's not true. If they do, they stand a higher risk of having a child with a genetic defect," Harry Fisch, M.D., of Weill Cornell Medical College of Cornell University, told WebMD.
Dads-to-be might even consider collecting their sperm at a young age and storing it for later use, a biologist with the University of Michigan told Bloomberg News.
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