Maternal DNA, referred to as mitochondrial DNA or mtDNA, is passed down from mothers to their sons and daughters. It is only carried through the female line, however, so while a son inherits his mother’s mtDNA, he does not pass it down to his own children. It does mean, however, that both men and women can have their mtDNA tested.
mtDNA tests can be used to test your direct maternal lineage - your mother, your mother's mother, your mother's mother's mother, etc. mtDNA mutates much more slowly than Y-DNA, so it is really only useful for determining distant maternal ancestry.
Everyone. mtDNA is passed down from mother's to their sons and daughters, so both sexes can use mtDNA tests to discover information on their distant maternal ancestry.
How mtDNA Testing Works:
Your mtDNA results will generally be compared to a common reference sequence called the Cambridge Reference Sequence (CRS), to identify your specific haplotype, a set of closely linked alleles (variant forms of the same gene) that are inherited as a unit. People with the same haplotype share a common ancestor somewhere in the maternal line. This could be as recent as a few generations, or it could be dozens of generations back in the family tree. Your test results may also include your haplogroup, basically a group of related haplotypes, which offers a link to the ancient lineage to which you belong.
What mtDNA Test Should I Have Done?:
mtDNA testing is generally done in two regions of the genome known as hyper-variable regions: HVR1 (16024-16569) and HVR2 (00001-00576). Testing only HVR1 will produce low resolution results with a huge number of matches, so most experts generally recommend testing both HVR1 and HVR2 for more precise results. HVR1 and HVR2 test results also identify the ethnic and geographic origin of the maternal line.
If you've got deep pockets, a "full sequence" mtDNA test looks at the entire mitochondrial genome. Results are returned for all three regions of the mitochondrial DNA: HVR1, HVR2, and an area referred to as the coding region (00577-16023). A perfect match indicates a common ancestor in recent times, making it the only mtDNA test very practical for genealogical purposes. Because the full genome is tested, this is the last ancestral mtDNA test you will ever need to take. You may be waiting a while before you turn up any matches, however, because full genome sequencing is new enough that few people have yet to take the test.
What About Inherited Medical Conditions?:
It is important to understand that a full-sequence mtDNA test (but not the HVR1/HVR2 tests) may possibly provide information about inherited medical conditions - those that are passed down through maternal lines. If you don't want to learn this type of information, don't worry - it won't be obvious from your genealogy test report, and your results are well-protected and confidential. It would actually take some active research on your part or the expertise of a genetic counselor to turn up any possible medical conditions from your mtDNA sequence.