Endogamy and DNA Testing

Marc McDermott
First Published: | Updated: April 2, 2024

In cases throughout human history, we run into a common theme of social groupings and migration. From the time early hunter-gatherer groups walked around the known world, they remained in small tribes. Fast forward to just a few hundred years ago, and we lived in much the same way. Though not maintaining the hunter-gatherer lifestyle, in the sense that gene pools were limited, genetically, things tended to be quite “close”.

Even still today, there are areas through which we find these communities where small, tight-knit groups keep within themselves. This occurs through religious communities, geographically closed-off areas such as small islands and far far-out desolate areas or rural communities. In the United States, people joke about the southern folks often marrying their cousins, and, though the truth is held in this capacity, we fail to realize that it likely happened within our family trees. When our history involves communities such as these, we are dealing with endogamous populations.

Inbreeding is what people refer to when cousins marry, but, realistically, inbreeding is close immediate family like siblings parent/child. This is illegal and dangerous for whatever offspring might come from it. Endogamy is not inbreeding, and though it can involve genetic mutations, the most likely scenario is that it will inflate the amount of shared DNA between the cousins who come from those same genetic networks.

So, what is endogamy? Let me give you an example of how endogamy looks and how, in a common occurrence, it played out in my own family. In the 1880s, my Great Grandfather Peter left a small village on the Mosel river in Germany and came to America. Many people in his village had traveled across the pond to find a new life in America. Peter, just barely 16, was excited to see what life had to offer in a place called Wisconsin. His parents remained in Germany, so Peter came over with his Aunt, Uncle, and their daughter, his cousin, Catherine.

After a long journey across the pond, they settled in Wisconsin, and very suddenly, Catherine’s parents got ill and died. Peter and Catherine now found themselves in a new world, alone, young, and struggling. Peter went to farming and relied on the only person he knew how hehe and Catherine got married. Before the end of their lives, Peter and Catherine had eight children and were all raised on a chestnut tree farm.

Peter and Catherine’s fathers were both brothers. This means that Peter and Catherine were first cousins, sharing a set of grandparents. Their fathers each inherited about fifty percent of their DNA from their mother and the other part from the father. Peter’s father and Catherine’s father shared about fifty percent of their DNA. When their DNA was passed down to Peter and Catherine, it likely passed along about twelve and a half percent of shared DNA from their grandparents, which Peter and Catherine would share. Remember, Peter’s father and Catherine’s father would not have inherited the same segments of their DNA from both parents.

When Peter and Catherine have children, they may both pass along parts of the same twelve and a half percent of that DNA to their children. Though not dangerous nor uncommon throughout the scope of human history, the descendants of Peter and Catherine might share more DNA with each other because of this occurrence.

Let us say that John, Peter and Catherine’s oldest son, and Mary, their eldest daughter, both lived their life, had children, and eventually, their grandchildren took DNA tests. John’s grandchild and Mary’s grandchild would notice they share quite a bit of DNA with each other. They are both first cousins to each other, but because their grandparents are the children of first cousins, they have a sliver of their DNA that has come from the same source twice. Then again, it is also entirely possible that they might not share any inflated DNA if they inherited more DNA from their other parents and grandparents.

Because DNA is so imperfect, we cannot assume we inherited exactly fifty percent from each parent. That is not realistic.

Peter and Catherine’s story is quite common among Europeans whose generations came across the Atlantic together. This is an example of only one form of endogamy on a single part of the tree but let’s dive into other places and instances which really get messy.

As I said above, very tribal communities, remote communities, or geographically contained populations, we see this same type of event occur over more than just a single generation. Let us take Northern Ontario, for instance. During the beginning of the Hudson Bay Company, we historically see an influx of Irish and Scottish settlers who had children with the indigenous tribes of Northern Ontario. With many tribal communities, they may likely have mingled with other neighboring tribes, but often they married within their tribe or within a neighboring tribe who would have likely been of some relation.

The Irish and Scottish fur traders remained in Northern Ontario for generations and, even still today, their descendants are marrying into each other. When we look at a family tree of these individuals, we see the overlapping of common surnames until we take the lines back as far as the paper trails will allow us to go.

In Puerto Rico, because the island is geographically closed off, we notice the same thing within many family trees. Jewish communities remained closed off and marrying within themselves for hundreds of years throughout Europe, and this has caused Ashkenazi Jewish populations to be among the most endogamous groups.

If anyone from among common situations such as these, you are likely to see a huge amount of close DNA matches whom you might not even know. Due to the fact you and your matches appear to be so closely related, you might often think, “Oh no, who slept with who” but, instead, it is the DNA inflation from endogamy that is causing such high numbers.

Dealing with endogamy, it is often easiest to display the shared DNA through a chromosome map so you can find which segments are very repetitive. By doing this, you can pinpoint which parts of the DNA could have been inflated, and this makes it easier to infer actual relation instead of dealing with the cM amount that is usually inflated.

Other tricks and tips that I have heard are dividing the number of cms by the number of times you see a common ancestor in the pedigrees of two matches.

Either way, the numbers won’t be perfect because DNA is not perfect. However, it is important to understand that the numbers do not mean you are that closely related when dealing with endogamy. You have to look into the family tree to pinpoint the common ancestors and repeat the common ancestor(s). By doing this, you will understand that the relationship is because of endogamy.

Humans have been tribal; we have been religiously separated from each other; we have been in desolate mountain and valley villages. We have stuck with what we know and what we are comfortable with for many generations, and because of this, there are ample remnants of this within our genetic make-up. You cannot ever take your DNA matches at face value for more than just this reason. Approach each DNA match individually and do your best to understand the DNA world because even the best genetic genealogists get thrown for a loop when endogamy is alive and well.

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