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.
This subset of genealogists are any who have endogamy in their family history. Endogamy is any community who tends to marry within a defined culture or location. Jewish and Acadian are good examples of 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. 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. Where it gets tricky is when you have your DNA tested, 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.
But I digress. The above situation is not endogamy. 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.
At the outset, doing genealogy when you have endogamy might seem a bit easier. Afterall, 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.
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 companies provide this information except AncestryDNA. 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.
For some guidelines on how much DNA should be shared between people in an endogamous community, check out pages 22-29 in the Shared cM Project PDF https://thegeneticgenealogist.com/wp-content/uploads/2017/08/Shared_cM_Project_2017.pdf.
Originally published on February 2019 on the genealogygems.com.