Endogamy and DNA

I really enjoyed the presentation Paul Woodbury of Legacy Tree Genealogists did about Endogamy last year at the SCGS Jamboree so I am delighted to present this guest post from him on that topic.

Personally I struggle with the endogamy in my mother’s family tree. My jewish grandad was an only child so there are not that many close cousins on his side. However due to endogamy my maternal aunts have hundreds of 2nd-3rd/4th cousin matches. No wonder I spend more time working with the easier Norwegian DNA from my Dad!

Endogamy and DNA By Paul Woodbury

Autosomal DNA testing is a valuable resource for genealogists seeking to overcome recent brick walls in their family history, particularly in instances where traditional historical research is limited or unavailing. At Legacy Tree Genealogists, we frequently use autosomal DNA test results to answer questions regarding adoption, unknown paternity, or ancestors who are difficult to trace. However, there are some factors that can complicate the use of autosomal DNA in tracing ancestors. One of those factors, which is what we will be discussing in the article, is Endogamy.

Endogamy is the custom of marrying only within the limits of a local community, clan, or tribe over the course of many generations. The reasons for this genetic isolation could be cultural or religious (as with Ashkenazi Jews and Low-German Mennonites) or geographic (as with island and tribal populations). Members of endogamous populations may descend from a limited pool of “founder” ancestors who represented the initial genetic makeup of their population. After many generations and hundreds of years of isolation from outside pedigrees, genetic profiles of population members can easily be distinguished from the DNA of outside populations. However, this can also cause pedigree collapse.

Pedigree collapse occurs when two closely related individuals produce offspring. As a result, the number of unique individuals occupying locations in a pedigree decreases or “collapses”. For example, while most people have eight unique great-grandparents, a child of two first cousins will only have six unique great-grandparents. They will also have inherited a larger portion of their DNA from the ancestors held in common between their parents. This makes it more difficult to determine exact relationships between genetic matches.

Here we explore three keys for dealing with endogamy in autosomal DNA test results: 1) Exploring the exact amounts of shared DNA between relatives; 2) Testing multiple relatives; and 3) Using direct line tests to narrow results.

1) Exploring Exact Amounts of DNA

Runs of Homozygosity

In order to accurately evaluate DNA test results in endogamous populations, consider exactly how much DNA two individuals share in common. Most genetic matches will only share DNA segments on one copy of their chromosomes – either maternal or paternal. However, if both of an individual’s parents are from the same endogamous population, or are known close relatives to each other, then they may have a “Run of Homozygosity” (a region of their DNA where the maternal copy is identical to the paternal copy). In these cases, the subject is a genetic match to themselves. If another genetic cousin overlaps in this same region of DNA, then the amount of DNA that person shares in common with the test subject should be doubled for that particular segment since they match the maternal copy and the paternal copy.

None of the DNA testing companies report total amounts of shared DNA which take into account runs of homozygosity. However, these regions can be discovered through analysis at Gedmatch.com and through David Pike’s Utilities, and might be used to confirm and refine the total amounts of shared DNA between two individuals. However, the researcher must have access to test results for both the subject and the genetic match in order to perform these comparisons.

Applying Different Centimorgan Thresholds

In endogamous populations, much of the population shares extremely small segments of DNA from many distant ancestors. If these small segments of DNA are included as part of the total shared DNA between two individuals, it can skew the estimates for how closely they might actually be related. At
Family Tree DNA, all segments larger than 1 cM (centimorgan) are included as part of the total shared DNA. At Ancestry, all segments larger than 5 cMs are included; however, some larger segments may be excluded based on their matching algorithms. At 23andMe all segments larger than 5 cMs are included as part of the total. Depending on the nature of the endogamous population, it may be beneficial to recalculate the total amounts of shared DNA between matches through comparison at Gedmatch and through application of higher centimorgan thresholds. For example an individual may share 120 centimorgans at Family Tree DNA, but when segments smaller than 7 cMs are excluded, this total may drop to 60 or 70 and may be more representative of the nature of their closest relationship. The appropriate threshold to apply in any given case will depend on the amount of endogamy within a population and whether the test subject is a full member of that population or has recent admixture from outside populations. Consider calculating several totals using different thresholds to give a better indication and overview of the shared DNA.

2) Testing Multiple Relatives

Since different descendants of an ancestral couple inherit different segments of DNA, it is always a good idea to invite additional relatives to test when exploring a brick wall problem. Other relatives will have different segments and will therefore have different matches as well. When dealing with endogamous populations it is particularly important to test as many known relatives as possible. When dealing with a case of unknown parentage or adoption, collaborate with genetic cousins and work with them to test more of their known relatives, focusing first on the older generations of their living family. Even if you know that specific relatives may be related through multiple family lines, test them anyway; their test results will greatly assist in filtering and organizing your matches.

Create Complete Trees

In endogamous populations it is quite likely that close relatives will be related through multiple family lines. Therefore, it is essential to have trees that are as complete and far back as possible for each relative tested. Since shared segments of DNA may (and often do) come from more distant relatives in endogamous populations, consider extending family trees 7 generations or more. This will not only permit researchers to identify the multiple ancestors shared in common with the relative, but will also aid in the exploration of the multiple relationships that the relative has with other relatives and genetic cousins.

In some cases, it may not be possible to build extensive family trees for relatives. Record loss or limited availability may prevent the construction of detailed multi-generation family trees. In these cases, consider documenting and focusing on the ancestral origins rather than ancestral couples. For example, in Ashkenazi Jewish research, try to identify the ancestral villages in Eastern Europe for genetic matches. This may help to tie brick wall ancestors to specific localities and regions.

Obtain Shared Segment Data for All Relatives

Though each of the DNA testing companies offers information regarding relatives that a subject holds “in common with” genetic cousins (also known as ICW), in order to be useful in the investigations of endogamy, researchers must also have access to the actual test results of other tested relatives. It is not enough to know that a genetic cousin matches a known relative. In an endogamous population we can expect that most members of the population match many other members of the population. We also need to know how much DNA they share with that relative. Some people may not be willing to share their specific results, but if you explain what you are trying to do and offer to share your findings, many people will be happy to help.

Exploring Averages and Assigning Segments

Testing multiple relatives and obtaining access to the test results for multiple relatives allows researchers to evaluate relationships between a match and a larger group of known relatives. By calculating the amounts of DNA shared between a genetic cousin and multiple relatives, it is possible to more accurately estimate the nature of relationships. Knowing that a subject shares 200 cMs with a genetic cousin, and that three first cousins of the subject share 69, 139, and 150 cMs (average of 139 cMs) with the same genetic cousin might lead us to estimate that the match is related at the level of second cousins once removed (or some equivalent relationship rather than at the level of full second cousins). By averaging between multiple relatives, researchers can identify the most likely level of relationship.

Access to the test results of relatives also permits the investigation of unique shared segments of DNA which could be representative of how much DNA a match might have shared with the common ancestor of multiple known relatives. Performing this type of research with multiple close relatives can also aid in chromosome mapping, which in turn can identify the segments of DNA that were inherited from a specific brick wall ancestor. Assigning segments of DNA to particular ancestral lines and sifting through the relationships between multiple family members enables researchers to identify and focus on the most pertinent matches to a particular research question.

3) Using Direct Line Tests to Narrow Results

Autosomal DNA analysis in endogamous populations can get messy quickly, so testing additional relatives with direct line tests (Y-DNA and mtDNA) and encouraging genetic cousins to do the same can help to narrow the possible relationships and can give increased structure to the investigation. Wherever possible, attempt to perform DNA testing of specific relatives in order to determine the Y-DNA signatures of each great-grandfather and the mtDNA signatures of each great-grandparent. Encourage genetic cousins to do the same for their own family. Direct line tests can serve as anchors and guides for interpretation of autosomal DNA test results and can identify family groups within a single pedigree that are likely related to each other.

 
Paul Woodbury is a Senior Genealogist with Legacy Tree Genealogists, a genealogy research firm with extensive expertise in genetic genealogy and DNA analysis. To learn more about Legacy Tree services and its research team, visit the Legacy Tree website at https://www.legacytree.com

3 thoughts on “Endogamy and DNA

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  1. In cases where only one grandparent is Jewish, can that be used to do a rough phase of DNA results? Can we expect that all of the AJ segments will appear only on one copy of every pair of chromosomes?

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