Mitochondrial Dna Mitochondrial Analysis

Mitochondrial DNA is found, as the name suggests, in the mitochondria of cells, which are sometimes referred to as cellular power generators. Mitochondrial types of DNA tests are often used in genetic genealogy. Mitochondrial analysis can be used to identify members of a maternal lineage and construct family trees.

Mitochondrial DNA (mtDNA) can also be used in forensics to help identify a human’s remains. mtDNA mutates more frequently than nuclear DNA.

The Biology of Mitochondrial DNA

Mitochondria are organelles located within the cytoplasm of cells. Sometimes they are referred to as the power plants of the cell since they generate most of the cell’s energy. They have their own unique DNA which consists of 16,500 base pairs (bp). This DNA encodes 13 protein coding genes (or polypeptides), 22 transfer RNA (or tRNA) and 2 ribosomal RNA (or rRNA). There are 100-10,000 copies of mtDNA per cell. mtDNA was one of the first genomes to be sequenced.

mtDNA is inherited only from one’s mother. Paternal mitochondria in sperm are destroyed after fertilization. Therefore, one can only trace direct matrilineal genetic lines through mitochondrial analysis.

mtDNA does not recombine like nuclear DNA. It recombines with copies of itself within the same mitochondria.

Uses for Mitochondrial DNA

Mitochondrial DNA analysis is used in various areas of genealogy. Different types of DNA tests exist which allow individuals to identify their ancestors.

mtDNA is used to trace maternal lineage. This is accomplished by sequencing one or more of the hypervariable regions (HVR’s). The 440 bp sequence of HVR1, for example, is compared to the same control regions of other individuals of a control group or other subjects. This information can help to determine what haplogroup from which a person descended.

Mutations in mtDNA are linked to maternally inherited diseases which are thought to cause age-related pathology. In contrast, Y chromosomal DNA is paternally inherited and only passed from father to son.

Researchers construct evolutionary trees to track ancestry back hundreds of generations using mtDNA. Phylogenetics is the study of genetic relationships of individuals or groups within a species. It compares rapidly mutating mtDNA in animals to study relationships within species. It identifies evolutionary relationships (phylogeny) between closely related species. Nuclear DNA, conversely, mutates less frequently and is used to compare species with other more distantly related species.

In forensics, mtDNA analysis is helpful in identifying human remains. Although mtDNA is not unique to a specific individual, it can be used with other evidence, such as circumstantial or anthropological evidence, to identify an individual or differentiate them from another suspect.

mtDNA can be used to exclude potential unidentified remains with missing persons. It is especially useful with older remains since it has a higher number of copies per cell than nuclear DNA.

Limitations and Reliability of Mitochondrial DNA Testing

Limitations and reliability are a concern in every type of DNA testing. One major concern with DNA testing is the number of loci which are compared. The match is unreliable unless enough loci are used. Not all DNA sequences are unique; an adequate number of loci must be determined and matched.

In genealogy, evidence from mitochondrial analysis can confirm more traditional genealogy research, such as birth records and public records. If the genealogist hits a “road block” in his or her research (as inevitably happens when tracing family trees) mitochondrial analysis can provide the clues necessary to pick up the trail of your ancestors.

If mitochondrial analysis has a limitation in the field of genetic genealogy, it is that the DNA test only traces matrilineal descent. Fortunately, DNA from the male Y-chromosome is used in different types of DNA tests to trace patrilineal ancestors. Between the two types of DNA tests, an individual can gain valuable insight into his or her genetic inheritance.

Resources

Dolan, M., and J. Felch. (2008). The verdict is out on DNA profiles. Retrieved September 10, 2008, from the LA Times Web site: articles.latimes.com/2008/jul/20/local/me-dna20

Ingman, M. (2001). Mitochondrial DNA clarifies human evolution. Retrieved September 9, 2008, from the Action Bioscience Web site: www.actionbioscience.org/evolution/ingman.html