Mitochondrial DNA Testing (mtDNA)

mtDNA is the “mother” of genealogical DNA testing!

Mitochondrial DNA (mtDNA) can greatly benefit your DNA research, once you understand its potential applications.

Test results can help determine a common maternal ancestor, narrow your research focus, and provide insight into the ancient origins of maternal family lines.

It can aid adoptees looking for clues to their maternal family and can be combined with autosomal DNA test results to narrow down potential cousin matches and shared ancestors.

Let’s take a closer look at mtDNA and learn more about its unique possibilities!

What is Mitochondrial DNA?

Mitochondrial DNA is found outside the nucleus of each of your cells.

Almost every cell in your body contains hundreds to thousands of copies of the mtDNA molecule.

This differentiates mtDNA from autosomal DNA, Y-DNA, and X-DNA, whose genetic data is located in the 23 pairs of chromosomes residing within the cells’ nucleus.

Mitochondrial DNA is actually circular in shape, made of over 16,500 pairs of molecules called nucleotides, which help produce energy for each cell.

A child will inherit mtDNA only from their mother, as mtDNA is passed down through a mother’s egg.

Therefore, both men and women carry their mother’s mtDNA, but only women will pass that mtDNA down to the next generation.

This is different than Y-DNA- which is exclusive to males and passed down from their fathers- and means that both women AND men can complete an mtDNA test.

mtDNA Benefits and Challenges

Mitochondrial DNA testing offers several important benefits to genealogy researchers.

There is very little recombination or changing, of mtDNA as it passes down through the generations.

This allows a researcher to use mtDNA results to trace a matrilineal line from mother, grandmother, great-grandmother, and so on.

When reviewing mtDNA cousin matches, a significant mismatch can rule out a relationship on that matrilineal line.

Adoptees may benefit from completing an mtDNA test, as the results can point them towards their biological mother’s family.

Mitochondrial DNA also provides clues to ethnic and geographical origins by virtue of its unchanging nature over hundreds of years.

For example, mtDNA may reveal if a maternal line has Native American, European, or African roots. 

There are existing mtDNA haplogroups, similar to Y-DNA haplogroups, which can help trace ancestral origins to a particular branch of the mitochondrial genetic family tree.

However, keep in mind that the unchanging nature of mtDNA also presents a challenge when attempting to locate a shared ancestor within a particular genealogical timeframe, as all descendants on a maternal line will share the same, nearly identical mtDNA.

A cousin match can indicate evidence of a shared maternal ancestor- but it cannot identify which woman in that shared ancestral line is the common ancestor.

An mtDNA test can reveal that two people are maternally related, but cannot discern their relationship, so those with matching mtDNA may be mother and daughter, sisters, aunt and niece, or very distant cousins.

This is a great opportunity to utilize both mtDNA and autosomal DNA testing results to narrow down the pool of potential cousin matches.

With mtDNA results in hand, you can determine which of your autosomal matches are on the correct matrilineal line, and start to locate ancestors within a more recent genealogical timeframe.

Additionally, you can collaborate with your autosomal/mtDNA cousin matches to build a shared family tree and try to determine a common maternal ancestor.

But remember, in cultures where women changed their names at marriage and were more difficult to locate in records, having a detailed family tree for those female ancestors is essential.

Without traditional genealogical evidence to provide additional clues, mtDNA results can be difficult to incorporate into your research.

Choosing the Right Test-Taker

When selecting a test-taker, it is crucial to understand the inheritance pattern of mitochondrial DNA.

Remember, a mother passes mtDNA to all of her children, but only her daughters can pass that mtDNA to the next generation.

Take a look at your family tree, and highlight all the people in that tree who could have inherited mtDNA from your target ancestor or those that share the same mtDNA from their mother, grandmother, and so on.

If you come across a generation that only produced sons, you may need to move back a generation or more in order to find a potential, living candidate.

Remember to look at the family trees of sisters, aunts, grandmothers, and female cousins through that maternal line.

Also remember, living male descendants carry mtDNA- they will not pass it on to the next generation, but they can certainly be a test taker!

mtDNA Testing

Remember when we learned that mitochondrial DNA is circular in shape?

Testing companies have broken that circle, or genome, into three different sections, giving each section a different name:

  1. Hypervariable control region 1 (HVR1)
  2. Hypervariable control region 2 (HVR2) – Both of these regions are more likely to change from one person to the next unless those people are closely related. This makes HVR1 and HVR2 ideal for genealogical testing.
  3. Coding Region (CR) – The largest area of the genome, the coding region changes much less frequently.

There are two different ways to test these mtDNA regions:

mtDNA Sequencing

This type of test can sequence either all or a portion of the mtDNA genome. Sequenced sections consist of series of the four different nucleotide base pairs, identified by the letters A, C, G, and T.

These nucleotides make up all DNA, including autosomal DNA. Of the sequencing tests, there are low-resolution tests, which sequence only the HVR1 and HVR2 sections, and high-resolution tests, which sequence all 3 sections.

High-resolution tests are also referred to as “Full Sequence” testing and are recommended over low-resolution tests for genealogical purposes.

SNP Testing

This type of test is also done with Y-DNA. SNP’s, or single nucleotide polymorphisms, are pieces of DNA that can vary from person to person, but those who are closely related should have the same SNP’s at every location tested.

SNP testing will test hundreds or thousands of SNP’s around the mtDNA circle, rather than examine the entire genome.

How Does Testing Work

After mtDNA is tested using one of the above methods, it is compared to a reference sequence, and any differences between the test-taker and the reference sequence are provided in a list.

The two reference sequences commonly used today are called the revised Cambridge Reference Sequence (rCRS) and the Reconstructed Sapiens Reference Sequence (RSRS).

Both of these reference sequences are used by Family Tree DNA to compare mtDNA testing results.

Those differences, sometimes called “mutations,” can then be used to determine a test-taker’s haplogroup, and to locate cousin matches who share the same mutations.

One issue with mtDNA test results is heteroplasmy – or when a test-taker has more than one mtDNA sequence residing in a cell.

The presence of heteroplasmy can skew test results, and indicate that a person is not a close maternal match, when in fact they are.

The resources listed below can help you learn more about this phenomenon, and provide more details about the mtDNA testing process.

Understanding Test Results

Mitochondrial DNA test results are compared to a reference sequence, and the resulting differences are provided in a chart or list.

For example, a full-sequence mtDNA test will examine all of the 16,500-plus nucleotide base pairs.

If, at position 263, the test-taker has a G instead of the A found in the reference sequence, the result will appear in the chart as either “263G” or “A263G,”- indicating that the “G” did not match the “A” found in the reference sequence.

Most types of mitochondrial DNA testing will provide information about the test-taker’s haplogroup and ancestral origins, regardless of whether all or part of the mtDNA genome was tested.

Additionally, there are several websites where you can learn more about your haplogroup determination and connect with others in that haplogroup. 

See footnotes for a list of helpful mtDNA resources.

Because mtDNA test results can provide a very long list of “mutations,” testing databases do not offer the ability to directly compare your test results with others. Instead, a genetic distance value is assigned to your potential matches.

If you do a full sequence test and have a genetic distance of “1” with another test-taker, then either you or the test taker have only one “mutation” that the other does not have, and this could indicate a potential cousin match.

Because of the unchanging nature of mtDNA over time, it is recommended that those who are looking for more reliable cousin matching select a test that sequences the entire mtDNA genome, rather than a portion.

An exact or very close cousin match through a full-sequence test is likely to be related within a more recent timeframe- within the last 500 years or so.

This can greatly aid researchers in locating a shared ancestor on family trees.

Where to Buy

Not all DNA testing companies provide mtDNA testing, and those that do, provide different types of testing.

Be sure to do your research so you can select the testing company that meets your needs.

23andMe

23andMe provides a Maternal Haplogroup report as part of their DNA kit bundle.

However, the company does not do a full-sequence test and does not provide cousin matching.

This is a good option for those who just want some starting information on their maternal ancestral origins. 

Living DNA

Similar to 23andMe, Living DNA provides a “Motherline” report that provides information about your maternal haplogroup, but does not provide a full-sequence test or matching.

Living DNA, like 23andMe, also provides an interactive map that offers clues to ancestral migration patterns.

Family Tree DNA (FTDNA)

Family Tree DNA is the choice for those that want the full mtDNA package- full-sequence testing, matching ability, and haplogroup information.

Family Tree DNA currently has two testing options:

mtDNA Plus

This test examines only the HVR1 and HVR2 regions, and identifies basic haplogroup and migration paths. 

mtFull Sequence

This test is recommended for genealogical research purposes; it examines the entire mtDNA genome, identifies haplogroup and migration paths, and provides more refined results for your research.

A new feature is the “mtDNA Journey Video,” a personalized video based on your haplogroup results, which animates your maternal line’s unique migration path through the generations.

Conclusion

Don’t let the complex nature of mitochondrial DNA testing sway you from learning more about this powerful genealogical research tool.

With education and practice, anyone can learn how to use mtDNA test results to their advantage.

Testing your maternal-line DNA may provide the breakthrough you need to leap over that brick wall in your family tree research!

External Resources and Footnotes

There are lots of resources available to help you learn more about mtDNA!

Books

Genetic Genealogy in Practice, Blaine T. Bettinger and Debbie Parker Wayne (National Genealogical Society, 2016)- this workbook devotes a whole chapter on mtDNA

The Family Tree Guide to DNA Testing and Genetic Genealogy, Blaine T. Bettinger (Family Tree Books, 2016) – Bettinger’s book also goes in-depth on mtDNA testing.

Websites

ISOGG Wiki: The International Society of Genetic Genealogy’s Wiki page is a must-bookmark site for a treasure trove of genetic genealogy resources!

Phylo Tree: This site provides a comprehensive tree of over 5,400 mitochondrial DNA haplogroups, and is a great resource for research.

James Lick’s mtHap Haplogroup Analysis tool: similar to GEDMatch, this great resource allows you to upload your raw DNA data from 23andMe and other testing sites, in order to confirm haplogroup information, or to obtain an updated haplogroup, if your testing company used an older version of the mtDNA haplotree.

Misc: Additionally, mtDNA has been used by forensic scientists, genealogists, and investigators to help identify remains- an online search will provide numerous examples of mtDNA investigation in action.

A notable example is the identification of King Richard III’s remains, discovered under a parking lot in England- you can read more about this fascinating story on the University of Leicester’s website.

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