One tiny piece of our DNA is inherited only down the female line. It is called mitochondrial DNA because it is held as a unique circular strand in small tubular packets known as mitochondria that function rather like batteries within the cell cytoplasm. Some molecular biologists say that, aeons ago, the mitochondrion was a free-living organism with its own DNA, and possessed the secret of generating lots of energy. It invaded single celled nucleated organisms and has stayed on ever since, dividing, like yeast, by binary fission. Males, although they receive and use their mother’s mitochondrial DNA, cannot pass it on to their children. The sperm has its own mitochondria to power the long journey from the vagina to the ovum but, on entry into the ovum, the male mitochondria wither and die. It is as if the man had to leave his guns at the door.