Humans have 2 copies of 23 chromosomes (1 from the mother and 1 from the father) and one chromosome determines a person's sex - 2 Xs for female and 1 X and 1 Y for male. The other 22 are called the autosome chromosomes.
The Y-DNA and Autosomal DNA tests use DNA from these 23 chromosomes but the mtDNA Test uses DNA from another part of the cell.
A human's 23 chromosomes are stored in the nucleus of each cell and around this nucleus there are a number of organelles that perform special functions. The mitochondria are organelles that provide energy to the cell.
Mitochondria are a part of each egg cell and the mitochondrial DNA is the same for all of a woman's egg cells. So all of a woman's sons and daughters will have the same mtDNA in their cells. Since the mitochondria are part of the egg cells they are passed unchanged from a woman to each of her sons and daughters.
So a woman's mitochondrial DNA should be exactly the same as her mother's, maternal grandmother's and the other females in this line for thousands of years – because a woman passes a copy of her mtDNA unchanged to all of her daughters.
However from scientific studies we know that sometimes a part or parts of the mtDNA will randomly mutate or change.
If a daughter receives her mother's mtDNA with a small mutation or change then that daughter will pass on the mutated or changed mtDNA to her daughters who will pass on that mutated or changed mtDNA to their daughters.
For example assume that a woman had 5 daughters and 1 daughter received a mtDNA mutation and the other 4 daughters did not receive any mtDNA mutations. Then the daughter with the mtDNA mutation would pass that mutation on to her descendants. However, the other 4 daughters would pass on the mtDNA without any mutations to their descendants.
So genealogists can use these mutations or changes in the mtDNA to determine the relationships among individuals and create a DNA family tree because each mutation or change is like a branch on a tree that is slightly different from its parent.
Scientists know which sections of mtDNA have changed or mutated most often over the years and so these sections of DNA are used as the markers for the mtDNA testing. The mtDNA test can help identify relatives whose mtDNA is similar and identify ancient migration routes taken by maternal ancestors.
Today most mtDNA testing companies offer their mtDNA Tests for 3 different regions. One test uses 2 of the regions and the other uses all 3 regions that make up the entire mitochondrial DNA - less than 17,000 base pairs. The tests determine which base pair or nucleotide is at each location for the test.
The 4 base pairs or nucleotides are adenine (A), thymine (T), guanine (G) and cytosine (C). For more information about these base pairs see our discussion Use DNA Testing to Determine Your Ancestry.
Based on the test results for the DNA markers an individual's markers will be compared to the markers of other individuals in the testing company's database. Depending on the number of matches estimates can then be made about the relationship of the individuals.
For example, if two people have an exact match for the entire mtDNA (all 3 regions) they most likely will share a common ancestor within the last 22 generations (about 550 years).
Testing companies also provide tested individuals with their mtDNA haplogroup which shows the migratory routes traveled by the individual's maternal ancestors.