De Novo Mutations Matter

Every child is born with around 70 brand-new mutations found in neither parent — fresh typos in the genetic code. Older fathers tend to pass on more of them, since sperm-making cells keep dividing for life and pick up new errors with age. Most of these mutations land in harmless spots and do nothing at all.

Once in a while, though, a brand-new mutation hits a critical gene and causes a condition no one saw coming — a genetic disorder in a family with no history of it. This is why some disorders seem to appear out of nowhere. Every conception carries a bit of genuine novelty: not just a reshuffle of old genes, but something truly new.