Moira acknowledged it with a nod, then went on: "Everyone has some genetic defects. When you are breeding more or less randomly within a large population, there’s a tendency for those errors to be swamped by the law of averages. Everything sort of works out. But when two people sharing the same defect mate, their offspring is likely to have that defect as well, and over time we see the usual unpleasantness that we all associate in our minds with inbreeding."