Mitochondrial inheritance

Mitochondria are the cell's power plants and they carry their own small genome, separate from the one in the nucleus. At fertilisation the egg supplies essentially all of them; the sperm's few are degraded and cleared.

So this one loop of DNA descends mother to child only, generation after generation, unshuffled — where nuclear DNA is remixed every time, mitochondrial DNA is copied, with only the slow drift of mutation to mark the passage of time. That makes it a clock and a thread at once: it tells you how far back, and it tells you along which branch. A tradition that traces descent through the mother and a molecule that is transmitted through the mother are, by any measure, an odd thing to find in the same sentence.