Mitochondria produce the energy that powers cells. The organelles carry their own DNA, separate from the nuclear DNA that contains most of the cell’s genes. That’s not their only quirk. You inherit your mitochondrial DNA from your mother, because an embryo retains only the mitochondria that came from the egg, not the sperm. However, about one in 200 women carry defective mitochondrial DNA, and their children can develop serious or fatal diseases such as cyclic vomiting syndrome, which results in bouts of nausea and fatigue, and Leigh syndrome, in which babies gradually lose muscle strength and control.