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.)
Genetically, 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.
For 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. However, MyHeritage does have a few distinct benefits of its own. You can transfer your results from another testing company into MyHeritage without having tested at MyHeritage. And MyHeritage offers a 14-day free trial subscription, which includes access to all of their advanced DNA tools. If you haven’t taken advantage of those features, spend a few minutes checking them out!
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.
The 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 (or strongly suspect) endogamy on your family tree, take our Endogamy & DNA Course. Developed and tested by our expert DNA team, this Course teaches straightforward strategies for addressing endogamy more productively. We’ll help you tease apart whether you’re looking at endogamy, multiple relationships…or maybe some of each. We’ll help you set reasonable goals for your DNA analysis. And we’ll share the nuances in how the various testing companies adjust (or not) for endogamy, so you can understand what you’re seeing in your AncestryDNA, 23andMe, MyHeritage, or Family Tree DNA results.