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Concepts – Endogamy and DNA Segments

Roberta Estes

Members of endogamous populations intermarry for generations, creating many segments that match, especially at small centiMorgan levels. These matching segments occur because they are members of the same population – not because they are genealogically related in a recent or genealogical time-frame.

Said another way, endogamous people are all related to each other in some way because they descend from a small original population whose descendants continued to intermarry without introducing people outside of the community into the genetic line. In other words, the DNA segments of the original population simply keep getting passed around, because there are no new segments being introduced.

If you only have 10 segments at a specific genetic location to begin with, in the original population – then the descendants of those original people can only have some combination of the DNA of those original people until another person is introduced into the mix.

Examples of endogamous populations are Ashkenazi Jews, Native Americans, Acadians, Mennonite, Amish and so forth.

If you have some family lines from an endogamous population, you’ll match with many members of that group. If you are fully endogamous, you will have significantly more matches than people from non-endogamous groups.

I suggest that you read my article, Concepts: The Faces of Endogamy to set the stage for this article.

In this article, I want to provide you with a visual example of what endogamy looks like in a chromosome browser. It doesn’t matter which vendor you use so long as you can drop the cM count to 1, so I’m using FamilyTreeDNA for this example.

I’ve used three people as examples:

For all testers, I selected their closest match above 200 cM total plus the following 4 for a total of 5 people to compare in the chromosome browser. I have only shown chromosomes 1-8 because I’m trying to convey the concept, not exact details of each chromosome, and 8 chromosomes fit into one screen shot.

If you’re not familiar with the terminology, you can read about cM, centiMorgans, in the article “Concepts – CentiMorgans, SNPs, and Pickin’Crab.”

Let’s take a look at our 3 examples, one at a time.

Non-Endogamous European Individual

The tester is non-endogamous. Four of the 5 individuals are known family members, although none were target tested by the tester.

The tester’s matches at 1 cM are shown below:

Note that the grey hashed regions are regions not reported, so no one matches there.

Below, the same 5 matches shown at 7 cM where roughly half of the matches will be identical by chance. Identical by descent segments include identical by population. You can read about the various types of “identical by” segments in the article, “Concepts – Identical by…Descent, State, Population and Chance”.

Ashkenazi Jewish Individual

The tester, along with both of their parents have tested. None of the matches are known or identified relatives.

Even though none of these individuals can be identified, two are related on both sides, maternal and paternal, of the person who tested.

In the chromosome browser, at 1cM, we see the following:

At 7cM, the following:

Native American Individual

The tester is 15/16 Native from the Sioux tribe. It’s unlikely that their matches are entirely Native, meaning they are not entirely endogamous. None of the matches are known or identified family members.

At 1 cM shown below:

At 7 cM, below:

Side by Side

I’ve placed the three 1 cM charts side by side with the non-endogamous to the left, the Jewish in the center and the Native, at right.

It’s easy to see that the Jewish tester has more 1 cM segments than the non-endogamous tester, and the Native tester more than both of the others.

Summary Comparison Chart

The chart below shows the difference in total number of segments, number of segments between 1 and 6.99 cM, and number of segments at 7 cM or larger. I downloaded these results into a spreadsheet and counted the rows.

Total Segments Total segments at 1 – 6.99 cM Total at 7 or > cM % 7 or >
Non-Endogamous 98 70 28 29
Jewish 168 139 29 17
Native American 310 295 15 5

You’ll note that the non-endogamous individual only has 58% of the number of total segments compared to the Jewish individual, and 32% compared to the Native American individual. The Jewish individual has 54% of the number of segments that the Native person has.

I was initially surprised by the magnitude of this difference, but after thinking about it, I realized that the Native people have been endogamous for a lot longer in the Americas than the Ashkenazi Jewish people in Europe. At least 12,000 years compared to roughly 2000 years, or approximately (at least) 6 times longer. Furthermore, the Native people in the Americans were entirely isolated until the 1400s, with no possibility of outside admixture. Isolation lasted even longer in the tribes that were not coastal, such as the Sioux in the Dakotas.

Note that the Jewish person and non-endogamous person have almost as many 7cM segments as each other, but the Native person has roughly half as many when compared to the other two. That means that because I made my selection starting point based on total cM, and the Native person has a LOT more 1-6.99 cM segments than the others, at that level, there are fewer strong segment matches for the Native individual.

The Native person’s percentage of 7 cM or greater segments is a much smaller percentage of the total segments.

As a percentage, the 7 or greater cM segments are 29% of the non-endogamous person’s total, 17% of the Jewish person’s, but only 5% of the Native person’s total.

Endogamy not only makes a difference when comparing results, but the specific endogamous population along with their history, how heavily endogamous they are, and how long they have been endogamous appears to factor heavily into the comparison as well.

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