Almost all the genetic code that humans need is packed into twenty-three pairs of chromosomes that we all carry inside the nuclei of our cells. There is one exception and that is the mitochondrial DNA, or mtDNA, which stays outside the cell nuclei. Each person inherits his or her mtDNA exclusively from his or her mother (the father also carries mtDNA passed on by his own mother, but he doesn’t pass it on to any of his children, male or female). The twenty-three chromosomes together with the mtDNA comprise a person’s genome.
