Mitochondrial DNA (mtDNA) and Your Ancestry

The sequence of your mtDNA reveals a wealth of information about your ancient roots.  People with similar ancestry will have similar mitochondrial DNA sequences.  Someone whose ancestors came from Africa will have a different spectrum of DNA changes in their mtDNA than someone of Western European descent, for example. 

Why sequence mitochondrial DNA?  Mitochondrial DNA has a number of features that have led to its widespread use in studies of human evolution, migration, population histories and genealogy:

• mtDNA traces an unbroken maternal line back through time for generation upon generation, going back much further than the oldest written record.  It is passed from a mother to each of her children, male and female.  The mitochondrial DNA in your body is a copy of your mother's mitochondrial DNA, as well as her mother's (your maternal grandmother), and her mother's (your maternal great-grandmother) and so on. Both men and women have mtDNA, but only the daughters can pass it on to the next generation. 

• In addition to maternal inheritance, mtDNA does not recombine: the molecule stays intact through the generations.  Because it does not recombine, the linear order of information on the mitochondrial DNA is maintained. 

• There is only one type of mtDNA per person.  This is in contrast to nuclear genes, where there are two copies of each gene, one from each parent.   The two copies can have slightly different DNA sequences.

• There are multiple copies of mtDNA in each cell  making it easier to obtain mtDNA for analysis. All of the copies are identical.

• Mitochondrial DNA has a high mutation rate, allowing its use as a "molecular clock" over the time scale of our migration out of Africa and our expansion around the globe.

As your mtDNA was passed down through the centuries, it occasionally acquired minor alterations.  These mutations are for the most part completely  benign, or they would not still be around.  With the ability to sequence your mtDNA we can now read these mutations and see how your particular mtDNA has changed over time.  These changes tell a story about your ancient roots, about where you come from.  If two people have similar mtDNAs, they will have a common maternal ancestor in the not-too-distant past. 

There are two types of information that can be gleaned from the sequence of your mtDNA.  The first is your haplogroup.  Haplogroups are defined by mutations.  For example, mutations at positions 11467, 12308, and 12372 define haplogroup U.  People with similar mitochondrial mutations - reflecting a common ancestry - are classed together in the same haplogroup.  Mutations that are used to define haplogroups tend to be rather common, since their function is to classify people into groups of a reasonable size.  Very rare mutations are not very helpful for such classification purposes.

Haplogroups are named with letters: A, B, C etc.  Sub-groups are identified with numbers, then letters, then numbers, as needed (D, D5, D5a, etc).  Because previous studies have identified geographical regions where a given haplogroup is well represented, knowing your haplogroup gives you an immediate insight into your roots. 

Everyone belonging to a particular haplogroup can trace their lineage back to a single person.  Every person in haplogroup H, for example, carries a replica of the mtDNA from a woman - a "clan mother" - who lived about 8,000 years ago, most likely in Europe.  Brian Sykes' book "The Seven Daughters of Eve" explores this idea in depth, imagining the world that the various clan mothers lived in.  The clan mothers were not the only people alive at the time, but they were the only ones to have direct maternal descendants that are living to the present day. The other women in the tribe, or their descendants,  either had no children at all or had only sons.

The clan mothers themselves had ancestors, and their lines of descent can be traced back to the mother of all human mtDNA, known as "Mitochondrial Eve".  She lived in Africa about 150,000 - 200,000 years ago.

The second type of information is gathered from the rare mutations.  Whereas the diagnostic mutations help to class your mtDNA into similar clusters, or haplogroups, the rare mutations help define your individuality.  These rare changes, sometimes called personal polymorphisms, are useful for genealogical purposes.  If a certain polymorphism in your mtDNA is not present in any other of the thousands of mtDNA that have been sequenced, it will be useful for identifying your "DNA cousins".