Page 12 - O Mahony Journal 2025

• 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).

Sequencing 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.

• Understanding Test Results
Your mtDNA test results provide both your matrilineal haplogroup, meaning the ancient
migration path that your direct maternal ancestors took, and your closest matrilineal
matches.

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.

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.

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