Malaspina’s results were so surprising that some of her colleagues found them hard to digest. “Reproductive scientists in my department said ‘It can’t be,’” she recalls. Yet she had hit upon a phenomenon that geneticists had recognized for decades: Older fathers are far more likely than younger men to have children with genetic disorders. According to geneticist James Crow of the University of Wisconsin at Madison, paternal age is the source of genetic diseases caused by new dominant mutations.

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