Endogamy and DNA Testing Tips

Diahan Southard

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How does your DNA testing company correct for endogamy on DNA matches? You should know about Ancestry’s algorithm; MyHeritage’s rule for Ashkenazi Jewish ancestry; and these tips for segment size and shared matches.

Endogamy is more than just a set of first cousins getting married. Endogamy is the practice of marrying within the same group of people for several generations. Endogamy can be the result of cultural preferences or geographic barriers. Some well-known endogamous populations include Jewish, French-Canadian and Acadian. (If you’re new to this concept, you may want to read this introduction to endogamy.)

Endogamy DNA Testing Jewish ancestry family tree history IO.pngGenetically, what this means is that instead of only sharing DNA with the relatively few people in the world with whom you share a recent common ancestor, you share DNA with hundreds of people who are a wider part of your population. This means that those from endogamous communities will often share more DNA with each other than we would expect given their relationship. This can make it tricky to understand how to interpret genetic relationships and apply DNA techniques in your situation.

3 Tips for Endogamy and DNA

Here are three keys to more successfully using DNA in researching your ancestor who belonged to an endogamous community.

1. Know how your DNA testing company handles endogamy.

Most of the time, the DNA you share with others in your community (simply because you are part of the community) is shared in relatively small pieces. We measure DNA in something called a centimorgan (cM). In general, pieces bigger than 20 cM are considered “real,” meaning they share DNA because of a shared recent common ancestor. Some companies are better than others and trying to differentiate between “real” and “fake” shared DNA before they present you with results.

AncestryDNA longest shared segment Timber algorithm 11.pngFor example, AncestryDNA applies something they call the Timber algorithm. Timber looks for small segments that are shared by lots of people. These segments are often “fake” matching segments, and so the algorithm downplays them. This means that even if you have endogamy, the amount of shared DNA you see at Ancestry won’t be inflated, but will stay within the normal range for each given relationship.

When MyHeritage DNA calculates matching, they have a special rule for those who have Ashkenazi Jewish ancestry showing up in their ethnicity results. They require a larger initial piece of DNA to be shared between two people who have at least 50% Ashkenazi Jewish ancestry before they are considered a true match. While helpful, it doesn’t go as far as Ancestry’s tool to reduce the amount of total shared DNA in the Jewish population, nor does it address other endogamous communities.

2. Looking at DNA segment size can be very helpful.

At all the testing companies, you can see the size of the biggest piece of DNA you share with your match. For endogamous populations, this is often a key indicator of whether a match is “real” or not. For example, you may see a match who shares 134 cM, (enough to be a 3rd cousin). But the longest block of shared DNA is only 17 cM.

That means that you share many small pieces of DNA that together accumulate this nice number, but you are unlikely to be true 3rd cousins. In general, you want to look for matches sharing segments larger than 15—or even 20—when you know you are researching an endogamous community.

3. Be cautious with the Shared Matches tool.

Shared Matches AncestryDNA 11.pngThe Shared Matches (or In Common With) tool is arguably the most powerful tool in a genetic genealogist’s arsenal. It allows you to use one DNA match to gather a group of people who are likely also related to that match and to you in a similar way. This is because of the principle that people who share DNA likely share a recent common ancestor.

However, when you try this on a match from an endogamous population, they are going to share DNA with people who do not share a recent common ancestor, but who share multiple, more distant common ancestors. That means the match groups you create will not be effective in helping you identify your shared ancestor.

DNA & Endogamy Course!

If you have endogamy on your family tree (the practice of repeat intermarriage within a small community), it makes analysis of your DNA matches challenging. We have developed a Course that teaches strategies for helping you address endogamy more productively.

Learn more about the DNA & Endogamy Course!

Or maybe you’d like to learn more about your ethnicity results. If so, take a look at our free guide on DNA ethnicity estimates!

Take me to the free guide!
<a href="https://www.yourdnaguide.com/author/guideyourdnaguide-com" target="_self">Diahan Southard</a>

Diahan Southard

As founder and CEO of Your DNA Guide, Diahan Southard has been teaching people how to find family history answers in their DNA for several years, and she's been in the genetic genealogy field since its infancy. Diahan teaches internationally, writes for popular magazines, consults with leading testing companies, is author of Your DNA Guide–The Book, and producer of Your DNA Guide–the Academy, an online learning experience.

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3 Comments

  1. Jose Dimauro

    Endogamy may be more widespread that we think, mainly of course in older populations. Both, my paternal and maternal families are, as far as I can tell (1600’s) from a small town in Sicily. I "match" to many many people with that geographical background without being able to determine how. These articles about endogamy are sending me to a new way of looking to my matches. Thank you!

    Reply
  2. Roger Grondin

    I am looking forward to the course.

    Reply
  3. Joanne E. Fletcher

    My DNA results match my pedigree. Both show endogamy from 1700’s backwards. E.g; I have multiple Royal Stewart linages. Can’t wait for this course and how we can use Endogamy within genealogy.

    Reply

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