mtdna sequencing

mtdna sequencing

The completion of human mtDNA sequencing project was the first step in allowing scientists to unravel the secrets contained in our DNA sequencing. Further over the past few years DNA testing has become affordable and easy to do. This has spawned the practice of performing DNA testing for Genealogical purposes which is called Genetic Genealogy.

One of the first genetic genealogy studies was conducted in the late 1980s by scientists with the Department of Biochemistry at the University of California, Berkeley. These scientists Rebecca L. Cann, Mark Stoneking and Allan C. Wilson studied a newly discovered kind of DNA. Mitochondrial DNA (mtDNA sequencing) is contained not in the nucleus of our cell, but in the mitochondria organelles of our cells. These scientists chose to study Mitochondrial DNA (mtDNA) because of its three unique properties which they explain as:

DNA sequencing

First, mtDNA sequencing gives a magnified view of the diversity present in the human gene pool, because mutations accumulate in this DNA several times faster than in the nucleus. Second, because mtDNA sequencing is inherited maternally and does not recombine, it is a tool for relating individuals to one another. Third, there are about 1016 mtDNA molecules within a typical human and they are usually identical to one another (Cann 31).

They extracted and compared mtDNA from "147 people, drawn from five geographic populations" (Cann 31). The researchers discovered that "All these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa" (Cann 31). Their findings also agree with the archaeology record as Cann explains "Studies of mtDNA suggest a view of how, where and when modern humans arose that fits with one interpretation of evidence from ancient human bones and tools" (36).

mtDNA

Swedish researchers Max Ingman, Henrik Kaessmann, Svante Paabo and Ulf Gyllensten critical of these findings conducted their own study in 2000. They claimed that "almost all studies of human evolution based on mtDNA sequencing have been confined to the control region, which constitutes less than 7% of the mitochondrial genome" (Ingman 708). Further they argued that the prior methods of analysis where "providing data that are ill suited to estimations of mutation rate and therefore the timing of evolutionary events" (Ingman 708). So they decided to study the complete mtDNA sequence from 53 people of various races.

Surprisingly their attempt to discredit the previous research failed as they also came to roughly the same conclusions. They conceded to the likely hood of a common ancestor shared by all the subjects despite being "geographically unrelated" (Ingman 712). They estimated "The age of the most recent common ancestor (MRCA) for mtDNA, on the basis of the maximum distance between two humans...to be 171,500" (Ingman 712) instead of the earlier estimate of 200,000 years ago. But they refused to align their findings with archeologists by stating "Whether the ancestors of these six extant lineages originally came from a specific geographic region is not possible to determine" (Ingman 712). Lastly they agreed on the potential of genetic genealogy by summarizing:

    Our results indicate that the field of mitochondrial population genomics will provide a rich source of genetic information for evolutionary studies. Nevertheless, mtDNA is only one locus and only reflects the genetic history of females. For a balanced view, a combination of genetic systems is required. With the human genome project reaching fruition, the ease by which such data may be generated will increase, providing us with an evermore detailed understanding of our genetic history (Ingman 712).

Their call for a more balanced view was shortly answered because in 2000 a team of researchers from the Department of Genetics at Stanford University lead by Peter A. Underhill published their results of studying Y-chromosome DNA. Only males have the Y-chromosome which has unique properties as explained by Underhill:

    Binary polymorphisms associated with the non-recombining region of the human Y chromosome (NRY) preserve the paternal genetic legacy of our species that has persisted to the present, permitting inference of human evolution, population affinity and demographic history (358).

Their report was based upon "the analysis of 1062 globally representative individuals" (Underhill 358). They concluded that the subjects "represent the descendants of the most ancestral patrilineages of anatomically modern humans that left Africa between 35,000 and 89,000 years ago" (Underhill 358).

So far genetic genealogy research has focused on these two kinds of DNA. As mentioned previously mtDNA is passed along the maternal line and Y-Chromosome DNA is passed along the paternal line. These two kinds of DNA effectively encompass all of our ancestors. Yet they provide no information about our ancestors inside the encompassed area. For example our maternal grandfather (mother's father) couldn't contribute any mtDNA or Y-Chromosome DNA to our mother. Yet he did contribute a third type of DNA called autosomal DNA. This type of DNA has yet to be studied for Genetic Genealogy purposes because of its inherent difficulties.

The main reason autosomal DNA is just now being studied is because scientists aren't sure how to determine which autosomal DNA came from mom and which came from dad without testing one or both of our parents. This situation is illustrated by the mathematical equation X = Xm/2 + Xd/2 where our autosomal DNA (X) is half of our mom's (Xm/2) and half of our dad's (Xd/2). By testing ourselves we identify our autosomal DNA but can't determine which part came from mom or dad. Additionally testing one of our parents is necessary to determine exactly which parent contributed which part of our autosomal DNA. This type of testing is currently used for Paternity and near relationship testing. But quickly becomes impractical after a few generations because of the difficulty of obtaining DNA samples from probably deceased ancestors.

Conclusion

analyzing mtDNA

Genetic Genealogy is the science of analyzing mtDNA sequencing for genealogical purposes. Studies have shown that we all stem from a common female and male ancestor. Because this emerging science is so new, benefits of this research are still being identified. Currently I believe Genetic Genealogy offers three categories of benefits.

First is entertainment value. Finding out you're related to famous people like George Washington, Julius Caesar or Genghis Khan is just plain fun. Imagine the bragging rights and small-talk fodder this provides at social gatherings.

Second is scientific value. Current studies have corroborated other scientific findings such as the human archaeological record. Medical sciences will benefit from correlating mtDNA sequencing studies with family genealogies to isolate hereditary diseases.

Third is relatedness value. Finding out you're related to a wealthy individual like Bill Gates may entail a financial windfall. Most importantly of all is the ability to reunite families. Millions of displaced war torn families and adopted children can now turn to Genetic Genealogy to find their relatives.

Article Source: mtdna sequencing