somatic mutation

A somatic mutation occurs in an ordinary body cell — skin, liver, blood, anything but the reproductive cells — so it affects only the person in whom it arises and is not passed to children. The change is a private edit to one branch of the body, not to the family blueprint.

When a somatic mutation happens, every cell descended from that mutated cell by division inherits the change, creating a patch of cells called a clone that differs genetically from its neighbours. Over a lifetime our bodies accumulate enormous numbers of these mutations, and most have no noticeable effect.

Somatic mutations are central to cancer, which arises when a cell accumulates a particular combination of changes that let it grow without restraint. They also contribute to ageing and to mosaicism, where one individual is built from cells of more than one genotype. Crucially, because they spare the germline, they vanish when the individual dies rather than entering the next generation.