Mitochondria in the cell body contain the only non-nuclear DNA in humans.  They perform an important function in the biological process of the cell.  The mitochondria is identical in all the cell’s of an organism and each mitochondriun is identical to all the other mitochondriun in a cell.  Because the mitochondria of the sperm cell are consumed by the cell during fertilization, only the egg passes mitochondria onto the offspring.  Thus, the Mitochondria is passed down from a mother to a child.  And as there is no “pair” of mitochondrial DNA like for chromosomes in the nucleus, and the mitochondrial DNA does not participate in meiosis cell division (used to create eggs and sperm cells), there is no recombination.  Thus the mitochondrial DNA is very stable for hundreds of generations.

Mitochondrial DNA (often abbreviated to mtDNA) is the shortest of all the DNA strands, consisting of only around 16,000 base-pairs.  By contrast, the shortest chromosome, on the other hand, #22 is about 50 million base-pairs.

Because Mitochondrial DNA is so short, and passes from generation to generation relatively unchanged, it can really only be used for population studies to understand the movement of people’s thousands to tens of thousands of years ago.  The only benefit in Genetic Genealogy is that if you believe you have two people descended down the same matrilineal line, then they should have the same Mitochondrial DNA.  A hundred or so SNP’s have been identified in the Mitochondrial DNA which are used to differentiate, slightly, people within the population.  Unlike shared segments in the Autosomal and X chromosomes, single SNP value differences are considered a marker for a difference between two individuals.