How to Understand Your Closest Autosomal DNA Test Matches
This article is based on a similar article from the January – March 2022 issue of NGS Magazine and is reprinted here with permission. We hope it helps you know how to better understand your autosomal DNA Test matches and what it means for you.
Autosomal DNA test results at the major genetic genealogy testing companies (23andMe, Ancestry, Family Tree DNA, LivingDNA, MyHeritage) include two main elements: ethnicity admixture estimates and genetic cousin match lists. Ethnicity estimates are a central focus of companies’ marketing efforts and a significant motivation for many testers (maybe you took a DNA test to find out your ethnicity). They can provide important context for genealogical investigation. However, genetic cousin match lists are the more useful element of autosomal DNA test results for solving family history mysteries and answering long-standing questions about a family tree.
Reviewing the closest genetic cousins in your autosomal DNA test matches list can help confirm the biological accuracy of close proposed genealogical relationships, provide a framework for interpreting more distant genetic relationships and guide future targeted testing efforts. Meanwhile, close genetic relationships to unknown relatives, lower than expected amounts of shared DNA with your known relatives, or a lack of genetic connections to your known tested relatives can signal the possibility of misattributed parentage – a topic we recently discussed here.
How do companies create and prioritize your autosomal DNA test match lists?
Your genetic cousin match lists are created by comparison of your DNA markers against the DNA markers of other customers in a company’s database. Autosomal DNA tests analyze several hundred thousand locations (known as SNPs) across each customer’s genome. For each location queried, the raw data of the test results report two base-pair values: one maternal and one paternal.
When you and someone else in a company’s database share the same marker values on at least one chromosome copy over several hundred consecutive SNPs, it is assumed you share a chunk or segment of DNA. The longer this chunk of DNA, the more likely it is that you and your genetic cousin inherited it from a recent common ancestor. A range of possible or likely relationships is estimated based on the size, position, and number of segments you and your match share.
DNA testing companies most often prioritize the order of genetic cousins in their lists based on the total number of centimorgans your genetic cousins share with you. Centimorgans are a measurement unit expressing the likelihood of recombination between two locations on a chromosome over a single generation.
As this likelihood of recombination is dependent on the segment’s location on a chromosome and several other factors, there is not a consistent conversion between the length and centimorgan value of a segment. Closer genetic relatives share higher total centimorgan amounts in more clearly defined ranges than more distant genetic relatives.
For example, parents and children will share around 3400 cM (with slight variation depending on the company). Siblings most often share between 2200 and 3020 cM, and if your genetic cousin shares between 2400 cM and 3100 cM, they are almost certainly a full sibling. Half-siblings, aunts, uncles, nieces, and nephews most often share between about 1340 and 2150 cM, and if your genetic cousin shares between 1550 and 1960 cM with you, they are almost certainly related as a half-sibling, aunt, uncle, nephew or niece.
Beyond these relationship levels, ranges of observed and expected amounts of shared DNA overlap more, making the relationship level more ambiguous based on the number of total shared cM between you and your matches. Genetic cousins sharing 600 cM with you might be related as a first cousin, half-first cousin, or first cousin once removed. As the number of shared cMs between you and your genetic cousins decreases, possible relationships increase. While a genetic cousin sharing 600 cM could be related as a first cousin or a first cousin once removed, the number of possibilities is limited to a relationship within four to five generational steps. Meanwhile, a genetic cousin sharing 40 cM could be related anywhere from second to distant cousins.
Each company provides broad relationship category estimates based on the total number of centimorgans you share with particular genetic cousins. Resources for more fine-tuned estimates are available through “The Shared cM Project” and DNA Painter’s “The Shared cM Project 4.0 tool v4.”
Blaine Bettinger, “The Shared cM Project,” The Genetic Genealogist (Blog), https://thegeneticgenealogist.com/: accessed November 2021; and,
Jonny Perl, DNA Painter, “The Shared cM Project 4.0 tool v4,” https://dnapainter.com/tools/sharedcmv4: accessed November 2021; and,
Jonny Perl, DNA Painter, “The Shared cM Project 4.0 tool v4 beta,” h https://dnapainter.com/tools/sharedcmv4-beta: accessed November 2021.
Evaluation of Closest Genetic Cousins (Over About 200 cM) Using Autosomal DNA Test Matches
If you are beginning to explore your genetic test results, consider starting by analyzing your closest genetic cousins, who share more than 200 centimorgans (about 2.7% at 23andMe). Individuals sharing higher than this amount of DNA are most often related at a level closer than third cousins. Therefore, it is likely that the nature of their relationship to you and your common ancestors’ identities can be determined.
For analysis of closest genetic cousins, consider the following questions:
Do you recognize any of your closest autosomal DNA test matches?
After taking a DNA test, you may be pleasantly surprised to find siblings, aunts, uncles, first cousins, and other close relatives who have already done DNA testing. Even if you do not personally know some of these closest matches, you may find them through a study of attached family trees, collaboration with genetic cousins, or researching the identity of your match and how you are related.
Do your matches have surnames or family trees that clarify their likely relationship?
Even if your match list does not include close recognizable genetic cousins, it might include individuals with surnames from different branches of your family tree, familiar names of distant collateral relatives, or individuals whose family trees aid in the identification of shared ancestors or surnames.
Do your close-known relatives share appropriate amounts of DNA?
Even if your known relatives appear in your match list, there is still more work to do. Do your known relatives or genetic cousins with common ancestors share appropriate amounts of DNA, given their proposed relationships?
Do your first cousins share amounts of DNA appropriate for first cousins, or could they be half-first cousins?
These questions might be answered by utilizing the tools and resources at DNA Painter and the Shared cM Project.
If your known relatives share amounts of DNA more typical of half relationships, you should perform additional research to determine if you, your genetic cousin, or both of you have a case of misattributed parentage somewhere in your ancestral line. You should also explore the possibility that a full relationship is still possible, but the amount of shared DNA is just low or high, given your proposed relationship.
Do you have too many close autosomal DNA test matches?
In some situations, you might have hundreds of close DNA matches sharing more than 200 cM. This can happen if your ancestors lived in an endogamous population where a population’s isolation due to language, culture, religion, or geography resulted in many generations of intermarriage. As a result, your genetic cousins may share multiple sets of common ancestors with you, or you may descend from the same common ancestors numerous times. Both scenarios can result in higher amounts of shared DNA than expected, given your closest genealogical relationship to a genetic cousin. Other situations where you might have many matches can occur when your ancestors were members of very large families. In these scenarios, it may be wise to focus on your matches sharing the most DNA and analyzing those relationships first.
Do you have no close matches?
Other times, test takers may not have any close genetic cousins sharing more than 200 cM. Alternatively, they may not have close matches from one of their proposed ancestral lines. If this describes your match list, more information is needed before you jump to a hasty conclusion of misattributed parentage. It may be that your other family members have not yet performed DNA testing either because you descend from a line of small families with few descendants or your ancestors came from a geographic area that is underrepresented in the database. To confirm the biological accuracy of the first few generations of your proposed genealogy, targeted testing or confirmation of previous testing of documented relatives from various ancestral lines may be necessary.
When the close-known relatives in your match list share appropriate amounts of DNA given their proposed relationships, the identities and relationships of these individuals may aid in efforts to interpret and understand the relationships to your more distant genetic cousins through shared match relationships. They might help isolate potentially pertinent genetic cousins from an ancestral line of interest in the context of a specific research question and objective. On the other hand, a lack of close matches raises possibilities for targeted testing of your known relatives.
Alternatively, reviewing your close genetic cousins may result in the discovery of surprise relationships. Perhaps you do not share DNA with close family members known to have tested. Maybe you have no strong genetic connections to cousins from a particular ancestral line. Given proposed relationships with known relatives, you may share less DNA than expected. Maybe you have close genetic cousins who have no known or documented relationship with you. Each of these situations can signal a case of misattributed parentage either for you or your DNA matches. If this is the case, explore this possibility with the guidance of our recent article.
If you have specific questions about your autosomal DNA test match results and would like to have a free consultation with one of our experts, you can fill out our contact form here.
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