What is Endogamy? DNA Testing and Endogamy

Diahan Southard

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What is endogamy? This introduction to endogamy helps you understand what it is and how it can complicate genetic genealogy analysis. Learn a crucial tip for spotting endogamous relationships on your DNA match list.

What is endogamy?

As DNA analysis becomes more mainstream in genealogy research, there is one group of genealogists who may feel a little left behind, as DNA testing does not seem to produce the kinds of eureka movements for them as it does for others. These are the genealogists who have endogamy in their family history.


is the practice by a community of marrying within its defined culture or location.

Endogamy - is the practice by a community of marrying within its defined culture or location.

Jewish and Acadian are good examples of historically endogamous communities. Endogamy is more than just a cousin marriage in your family tree. While those can be tricky to reconcile with your DNA results, they can often be fairly straightforward, especially if you already know about the relationship.

Intermarriage and DNA

For example, let’s say your mom’s sister Susie married your dad’s brother Bob. Normally, your mom’s sister’s children would be your first cousins, and your dad’s brother’s children would be your first cousins. That relationship would bring a certain amount of DNA with it. But in our example, you are first cousins on your mom’s side and first cousins on your dad’s side. So you will actually share two times the amount of DNA that first cousins share. This is fine, as long as you understand that.

Click to read this related article.

Click to read this related article.

But things get tricky when you have your DNA tested and the company has no way of knowing what you know about your family. So when your double first cousin is tested, the company may call them your half-sibling, as double first cousins share about the same amount of DNA as half siblings. When this situation happens in the generation before us, it is pretty easy to catch. But if the same scenario was true between a set of your great-grandparents, the relationships between you and your second cousins will be affected in your DNA, and could be trickier to figure out. You CAN figure it out, though. Our two-part article series on pedigree collapse includes how to adjust your shared DNA calculations for a known instance.

But true endogamy is the above situation repeated over and over and over again in your family history so that in general, you share too much DNA with all of your cousins, given their actual genealogical relationships.

How endogamy complicates DNA test results

At the outset, doing genealogy when you have endogamy might seem a bit easier. After all, where others have to find 8 sets of great grandparents, people who have endogamy often have fewer, maybe just five or six great grandparents to document, because the same couple appears on multiple lines.

However, the DNA part of endogamy research is much much more difficult. Many times you will share DNA with others in your endogamous community not because you share a single recent common ancestor, but because you likely share multiple more distant common ancestors.

How to spot endogamous relationships

One way to help you tell the difference between someone who is closely related and someone who shares DNA because of endogamy is to pay attention not just to the total amount of DNA shared, or your relationship prediction, but to look at the size of the pieces of shared DNA. All of our testing companies provide this information. Here’s a screenshot of a father-daughter genetic relationship described at MyHeritage, showing the total amount of shared DNA, how many segments are shared, and the length of the largest segment:

shared DNA longest segment.jpg

Looking at the size of the biggest piece of DNA, you want that piece to be larger than 20 cM. If the biggest piece is smaller than that, it is unlikely you are related recently, and you can chalk up all that shared DNA to the tight-knit nature of your ancestral community, and not waste your time looking for a common ancestor.

We cover this strategy (and others!) in our Endogamy and 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.

Check out Endogamy & DNA Course

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<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.


  1. pcaverly

    pages 22-29 in the Shared cM Project – possible broken link.

  2. Sunny Jane Morton

    Thanks for the heads-up, pcaverly! We’ve fixed the link.

  3. Susan

    This has been helpful. When my friend’s dna test comes back, I’ll surely need to remember it. His grandmother and two of her sisters married 3 brothers in a local family. Instead of all their children being first cousins along, or even double first cousins, I guess, maybe they’d be triple first cousins? I haven’t even started looking at who the brothers or those 3 sisters married. Can’t wait to see how the dna on those 3 lines looks. I see it being a busy summer!

    • Diahan

      I am so glad you found this helpful! The children of those three couples will still just be double first cousins, as you will always only be comparing two people who will have two first cousin relationships. And yes, it looks like it will be a fun and busy summer!

  4. Carol

    This begins with a quest to discover who of my ancestors or their siblings left Scotland. I know who are my 3rd and 4x great grandparents. US history speaks that many Scots emigrated to early American colonies. As I sort matches, I find significant matches in the American South. Most of them are low matches, which is expected. They might be ignored as ‘false negatives’. However, I administer or have viewing rights to the kits of a number of my relatives.
    When my paternal half sister and I have African American matches with a 20 cM significance, we embark on a personal journey of reckoning. When others of my folk match persons both White and Black with trees that match, I seek guidance in this journey of discovery. I believe endogamy within both Scots and slaves and between slave owners and their slaves is involved in the resiliency of DNA through over a century (perhaps 2) of separation.

  5. Mary Innes

    One of the articles state that ‘Ancestry’ doesn’t show the biggest piece of DNA se we can check it is over 20 cms – is there any other way (other than pay to get it redone by another company) that we can factor endogamy into our searches?

  6. Diahan Southard

    Mary, Thanks for asking that question! AncestryDNA DOES now show the biggest piece of DNA you share with your matches, and we’ve updated the article accordingly.

    • samantha jones

      I’m helping an adoptee who descends from parents from a very endogamous group. She has one match on ancestry where the longest shared segment is 74 cMs.

      In all, she shares 481.94 cMs across 37 segments with this match. But that one segment of 74 seems to indicate that they may share a common ancestor relatively recently, correct? This same adoptee has 9 other matches whose longest segment ranges from 53 to 58 cMs.

      Mind you, her match (who shares 74 cMs on one segment) is also an adoptee and also descends from the same very endogamous community.

      However, for this strongly endogamous population, those segments really stand out, right? Have you ever seen segments that large when the matches are distantly related due to endogamy?

  7. Helen

    All my paternal ancestors came from one small isolated village in north Wales, current population 196 people! Endogamy is a nightmare, everyone appears on everyone else’s DNA matches

  8. Jennifer Kepner

    I have a question that I am hoping someone might be able to help with.
    Does this kind of relationship produce higher amounts of cM in one segment? For instance, if a person matches another person with 453 cM, 11 segs, longest seg=114, could the longest seg of 114 be due to endogamy? Thanks for any help that anyone might be able to provide.

    • Diahan

      Not usually a higher amount in one segment. In fact, the opposite is true. In endogamy you can often share a total of 453 and the longest block is only 20 cM.
      I would wager that you have a relatively close relationship, given the total amount and the size of the biggest pice.

      • Danielle

        I too have been wondering the same thing as Jennifer Kepner. You said, endogamy does not “usually” account for a higher cM count in a longest segment. Am I wrong in taking that it possibly could ? Do you know of any reasons it would and or could influence the cM of the longest segment?

        • Diahan

          I have learned in science you just never say always. 😉 So no, I don’t know of any specific examples when the longest segment is significantly longer because of endogamy, but I am not comfortable saying always.

  9. TJ

    My daughter with my dad is 1933cm across 33 segments and longest 223cm. Her longest segment is longer than mine. I am bio daughter to my dad 3426cm across 39 segments with longest segment 212cm?

    • Diahan

      I wouldn’t be too concerned about those 9 cM. It is likely just an error in either the way the segment was originally measured, or, if you are at Ancestry, a difference in the way your longest segment was calculated verses hers.

  10. Roger Grondin

    My partner has three close matches 400-500 cM range. Two are brothers and the other is the brother’s 1st cousin. This is a Lebanese family, so endogamy is a known issue.

    In one brother with a match of 512cM and the largest shared match is 103cM
    In the other matches at 373cM with a 60cM shared section
    The cousin matches at 404 cM with the largest shared section at 42cM

    Can I say with any confidence that they are half fist cousins?

    • Diahan

      Which testing company are you at? If you are at Ancestry, they have an algorithm called TIMBER that does a really good job presenting you with the amount of DNA that is "real" so it is easier to trust the total amount of cMs.
      If you are at 23andMe or FTDNA or MyHeritage, take a look at the chromosome browser and count up all the DNA you share in pieces that are over 15 cMs.
      Also, how old are these people compared to your partner? If they are all about the same age, and if after looking at the chromosome browser you are still sharing in that same cM range, then YES, this looks like a half 1C relationship.

  11. Guest

    I am part of the Acadian community. A 1C1R showed up on my matches as sharing 13% dna and 912 cm with the longest segment 93 cm. Could all of this extra dna that we share be due to endogamy? This match’s mother is my 1C on my father’s side, and we are not double first cousins.

    • Diahan

      When the biggest piece is SO BIG, that is usually an indication of a close relationship. I would say that a 1C1R could likely be your actual relationship, but it is hard to tell without looking at the data and the shared matches.

      • guest

        Yes, I would like to look at the shared matches too. My match’s mom’s dna would be a big clue! However, she has not submitted her dna to Ancestry. I just wonder if all of that extra dna (13%) could be due to endogamy, or if she is actually a surprise half niece?

        • Diahan

          Such a good question. And really difficult to tell the difference. But one clue may be in her age. If she is much younger or older than you, that could support the 1C1R relationship.

  12. Anthony J Sanderell

    Knox County Ohio USA is full of it. And it is still going on to this day.


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