Your match list is the most genealogically valuable component of your DNA test. It gives you a list of people with whom you share varying amounts of genetic material.
To take full advantage of the match list, you'll need to first understand endogamy, a cultural tradition which creates significant challenges for Jewish genetic genealogy. You'll then need to sort your matches into categories: small segment matches, known matches, and unknown matches. Finally, you'll need to investigate how your unknown matches are related to you. In the end, you should be able to expand your family tree with some of your unknown matches.
Endogamy is the practice of marriage between related persons within a group over several generations. The present population of Ashkenazi Jews, a 2014 study posited, descends from a group of about 350 people who lived 600-800 years ago in the German Rhineland. In the following centuries up to around 100 years ago, Ashkenazim married almost exclusively within the faith. This means that Ashkenazim who are not related within the past several generations often still share DNA from a common ancestor in earlier generations. Ashkenazi testers must sort through their match lists to determine who is a match from a recent generation and which matches share DNA from so long ago as to be considered not genealogically relevant.
Resources on endogamy
To differentiate genealogically relevant matches from distant matches, you’ll need to look at the amount of DNA you share with someone (measured in centimorgans) and the size of the DNA segments you share with them.
Centimorgans (cM) are a unit of measurement for the frequency of genetic recombination. You share, on average, 3400 cM with each parent, 2550 cM with each sibling, 1700 cM with each grandparent and aunt and uncle, and 850 cM with each first cousin and great-grandparent, to name a few closer relationships. These numbers are averages, as recombination is random, and you could inherit more or less of the expected amount from any particular ancestor, which will affect your matches with your siblings, cousins, etc.
Most Jewish genealogists disregard matches who share less than about 100 cM, if the tester is 100% or close to 100% Ashkenazi descent. If your ancestry is mixed, the threshold for considering matches to be related in recent generations is much lower. You will need to keep in mind that non-endogamous autosomal matching is very different.
How to view centimorgans: Your test company should tell you the number of centimorgans you share with a match as a part of your match list. Enter this number into the Shared cM tool on the website DNAPainter to see the possible relationships you might have with your match.
Segment size also plays a role in distinguishing distant matches from legitimate matches. The total number of centimorgans you share with a match are split up into different segments of DNA across different chromosomes. If you share a relatively large number of total centimorgans with a match, but you find that the centimorgans are split up into very small segments across chromosomes, you likely share multiple distant common ancestors with this match rather than a recent traceable common ancestor. If you share only one long segment with a match, you may not have a traceable relationship either. The most genealogically relevant matches share multiple relatively long segments of DNA with you.
How to view segment size: Your testing company will tell you the number of segments you have in common with your matches as a part of the match list. You can view the size and locations of your shared segments by using a chromosome browser, which is a tool offered by 23andMe, MyHeritage, and FamilyTreeDNA for visualizing shared segments on each chromosome. The chromosome browser can be used to check several of your matches simultaneously to see if they match at the same locations. Your matches would also have to match each other to conclude the shared segments come from the same common ancestral couple. Ancestry does not provide a chromosome browser, but uploading your data file to Gedmatch, MyHeritage, or FamilyTreeDNA will allow you to use the tools at those sites.
You might start by looking for people with whom you share at least one segment greater than 20 cM and a few others greater than15 cM, as these matches are likely related to you within a genealogically relevant timeline. However, people below this threshold may also be related to you.
Another good method for weeding out distant matches is to find the ratio of the number of centimorgans you share with someone to the number of segments you share with them. If the ratio is over 10, the person is likely a genealogically relevant match. If it’s under 10, they are likely too distant to be worth pursuing. For example, if you and a match share 54 centimorgans over 4 segments, it is worth investigating your relationship. However, if you share 54 centimorgans over 10 segments, it might not be worthwhile.
Resources for centimorgans, segments, and identifying false matches
Once you determine a match is genealogically relevant, you may need to continue working to figure out how the person is related to you. Testing companies will suggest what your relationship is to a match, but these suggestions are not always accurate. This is because you share the same amounts of DNA with people who are related to you in different ways.
Enter the number of centimorgans you share with a match into the Shared cM tool on DNAPainter to see the possible relationships you might have with a match. Then, use your knowledge of your family tree and your match’s tree to determine which relationship type makes sense for that person.
You may see people in your match list with names you don’t recognize. If the centimorgans and segment size suggest the person is a genealogically relevant match, you should work on building your family tree and your match’s tree through traditional documentary-based genealogical research to figure out how that match is related to you. If you haven’t built your tree back to second great-grandparents, you won’t recognize your third cousins, who descend from those same second great-grandparents, if they come up as matches.
Identifying paternal and maternal matches
To help figure out whether a match is related to you on your maternal or paternal side, try using your test company's 'shared matches' or “In Common With” tool or GEDMatch's 'people who match both' tool. With these tools, you can enter a match with a known relationship to you, like your maternal first cousin, for example, and get a list of which of your matches are related to both you and that maternal first cousin. This list is effectively a list of all of your maternal matches. You can produce a list of paternal matches using the same method. Be aware that in endogamous populations you will have a lot of matches who are related to you through both of your parents, so this technique may or may not isolate which side of your tree to look at.
Matches on multiple lines
Often, Ashkenazi testers find several '3rd cousin' matches. While some of these may be true 3rd cousins, most 3rd cousin matches are related to Ashkenazim through more distant ancestors on multiple lines. A 3rd cousin match may really be your 5th cousin on one line and your 6th cousin on another, for example. When the centimorgans associated with these 5th and 6th cousin matches are added together, their sum is the same as the number of centimorgans you would share with a true 3rd cousin. Pay close attention to the length of your shared segments with 3rd cousin matches; if you share many small segments rather than a few longer segments, this is a sign your 3rd cousin match may really be related to you more distantly on multiple lines.
Missing matches
It is also possible to not match with a genuine relative. Since each parent passes down a random 50% of their DNA to a child, and that child passes down a random 50% of their DNA to the next generation, some genetic material gets lost with each new generation. So, DNA that you could have shared with a relative may not have been passed down to you. This situation mainly occurs with more distant genetic cousins. You are of course still related to these people, but your genetic tree and your genealogical tree are not the same thing. This is another reason why building a document-based family tree is important to understanding your DNA results.
Resources for figuring out how someone is related to you
Many people who choose to take DNA tests are adoptees looking to learn more about their biological ancestry. Adoptees can benefit from the advice and resources throughout this guide but may also want to view resources tailored to adoptees. Although the genealogical paper trail is often limited for adoptees, it is still a good idea to combine genetic genealogy with traditional genealogy when researching adoption. In general, adoptees or those with unexpected parentage will need to do research on their matches’ trees in order to make up for the lack of their own paper trails. See our guidance for researching adoptions through documentary genealogy here.
Resources for adoptees