Each cell of the human body contains organelles called mitochondria. Mitochondria were probably originally parasitic bacteria. Through evolution they became symbiotic with human cells, and finally an integral part of the cell. They help cells use oxygen. The DNA in mitochondria is separate from the DNA in the nucleus of the cell. The DNA in the nucleus is inherited from both parents, but the DNA in mitochondria is inherited only from the mother.
(YouTube: Gene Tree)
Mitochondrial DNA (mtDNA) passes to a woman’s children without any contribution from the father. That is, mtDNA does not recombine as nuclear DNA does. So, my mtDNA will match my mother and her children exactly. But, because I am male, I don’t pass on my mtDNA to my children. My sisters, on the other hand, will pass on our mtDNA unchanged to their children.
Mitochondrial DNA passes to each generation unchanged except for any mutations. The mutation rate for mtDNA is very low; perhaps one mutation at a given spot every 10,000-12,000 years. Therefore, mtDNA changes very slowly over many generations.
These characteristics make mtDNA useful to both geneticists and genealogists. It is a useful tool for understanding the remote origins of the human race, for tracing pre-historic population movements, and for matching people who belong to the same female line.