Mitochondrial DNA is completely separate from a person's regular DNA. It's a bit of genetic material residing in the mitochondria of every cell in the body, and it is inherited unchanged from generation to generation, through the female line. That means all the descendants--male and female--of a particular woman will have identical mitochondrial DNA, which we call mtDNA. This kind of DNA is extremely useful in forensic work, and separate databases are kept of it.