You inherit your mitochondrial DNA only from your mother

Most of your DNA sits in the cell nucleus, shuffled half-and-half from both parents. But there is a second, separate genome living inside your mitochondria — the structures that power your cells. This mitochondrial DNA (mtDNA) is tiny: about 16,500 base pairs carrying just 37 genes, every one essential for the mitochondria to do their job. And it obeys an unusual rule of inheritance.

Offspring inherit their mitochondria — and so their mitochondrial DNA — from their mother.

The egg supplies essentially all of an embryo’s mitochondria, while the few hundred that ride in on the sperm are tagged for destruction and broken down soon after fertilization. So mtDNA passes down the maternal line almost unchanged — your mother’s, her mother’s, hers — a thread running back across thousands of generations. Because it skips the reshuffling that scrambles nuclear DNA each generation, it makes an unusually clean record of ancestry and ancient human migrations. Trace every living person’s mtDNA backward and the lines converge on a single woman, nicknamed “Mitochondrial Eve,” thought to have lived in Africa well over a hundred thousand years ago — the most recent common matrilineal ancestor of all humans alive today. She was not the only woman of her era, nor the first; she is simply the one whose unbroken daughter-line never once failed to produce a daughter.

Because mtDNA can carry harmful mutations, that maternal handoff also passes on mitochondrial diseases, which can strike the brain, muscles and heart. To prevent them, regulators in the United Kingdom approved mitochondrial donation — popularly the “three-parent baby” — in which a mother’s nuclear DNA is placed into a donor egg with healthy mitochondria. The donated mtDNA makes up less than 1% of the child’s genetics, but it is enough to break the chain of disease.