Mitochondrial DNA is found within the mitochondria organelle of the cell. This DNA has only a single copy and does not undergo any passing or mixing (a phenomenon that occurs in the normal DNA found in the nucleus of the cell). Mitochondrial DNA is inherited only via maternal inheritance as the egg cell is very large and contains a large number of mitochondria, whereas the sperm cell is a very small cell and contains a small number of mitochondria. Thus, the mitochondrial DNA in each individual is essentially identical to that of their mother with the exception of rare mutations that occur randomly throughout the generations, at a low frequency, and accumulate in a certain population.
It has been found that populations and ethnic groups from different geographic regions have their own characteristic mitochondrial DNA types which can be easily identified by scientific means. This fact is used by many researchers of population genetics because it allows for understanding about the early history of the population, tracing the maternal lineage of the subject, and determining the mother’s affiliation with different genetic-ethnic groups. These mitochondrial DNA properties are similar to those of the Y chromosome that documents the paternal lineage and is used in similar cases (see the well-known study of the Priestly dynasty).
Closed and isolated populations established by only a few founders usually have unique genetic characteristics. One of the most genetically researched populations is the Ashkenazi community, with thousands of published genetic studies surveying and identifying this community and attesting to its unique characteristics. The Ashkenazi population, which was established by a few Jews from the Middle East who migrated to Central Europe and from there to Eastern Europe, rapidly grew in size.
Over the years, this population has accumulated genetic markers that distinguish it from other populations of the world and which can be identified by scientific means.
In the context of mitochondrial DNA, more than 40% of Ashkenazi Jews have their own characteristic DNA distinct from that of other populations such that in many cases it is possible to identify whether the mother of a person, their maternal grandmother, and so on, belongs to this population.
The mode of mitochondrial DNA inheritance from mother to offspring is identical to the halakha that determines a person’s Jewishness as passed from mother to child. At the same time, it is important to note that a significant percentage of Ashkenazi Jews carry mitochondrial DNA that is not unique and is found in other populations.