Using Longest Segment in Endogamous DNA Analysis

When you're from an endogamous population, you tend to share a lot of small segments with others from that population--which causes relationship predictions that are significantly closer than they are in actuality.  For example, someone who's predicted to be a second cousin may actually be your eighth cousin in 12 ways.  So in such populations, the amount of shared DNA is not necessarily indicative of the actual relationship.  So what can be done?

I don't really have over 2100 fourth cousins--let alone that many who have tested at Ancestry

Many of my known relatives have tested.  So let's take a look at both the amounts of shared DNA I have with each of these individuals as well as the longest segment I share with each.

Name	Actual Relationship	Shared cM	Longest Segment	Comments
Sonia	Grandmother	1325	219
Sid	Uncle	1959	193
Eddie	Uncle	1849	143
Ruth	Great Aunt	1004	96
Karen	1C1R	461	44
Sandy (Karen's 2C)	2C1R	164	22
Scott	1C1R	686	61
George	1C2R	308	40
Mitzi (George's 1C)	1C2R	354	46
Ina (Mitzi's & George's 1/2 1C)	Half 1C2R	209	73
Ken	2C2R	175	57
Myron (2nd cousin to Ken; 1/2 second cousin to Sue)	2C2R	108	16
Marilyn (1st cousin to Myron; 2nd cousin to Ken; 1/2 second cousin to Sue)	2C2R	104	21
Sara (Myron's niece)	3C1R	89	16
Sue (1/2 second cousin to Ken, Marilyn and Myron)	Half 2C2R	104	21
Beth	2C1R	232	45	2 AJ grandparents
Dave (Beth's nephew)	3C	281	57	1 AJ grandparent
Liz (Beth's niece; Dave's 1C)	3C	110	19	1 AJ grandparent
Elise	3C1R	142	27
Judith	3C1R	193	29
Jonathan (Judith's son)	4C	182	29
Esther (Judith's sister)	3C1R	166	29
Pat	2C1R	118	23
Ben	3C, 4C	101	13	Has 2 Supkoff great grandparents
Howard	4C	41	9	50% AJ

Table Showing Actual & Genetic Relationships Between Myself and Known Relatives

Most of these known relatives--even someone as distant as my fourth cousin Jonathan--share at least one reasonably large segment with me; some share more than one.  So looking at the longest segment one has in common with one's matches can be a good strategy to identify people who may actually be relatively closely related.  Twenty or more cM seems to be a decent benchmark to warrant further investigation.

However, there are a few people here who wouldn't have been found using this strategy--people like Ben, Howard, Myron and Sara.  All happen to be related on my mother's side--so I took a look at how much each shares with my mother's brother Eddie and their first cousin Karen--who not only are one generation closer to each person than I am, but who also would have inherited different DNA than my mother did:

Name	Actual Relationship	Shared cM	Longest Segment	Comments
Myron	2C1R	231	49
Sara	3C	161	23
Ben	2C1R, 3C1R	146	17
Howard	3C1R	86	12	50% AJ

Table Showing Actual & Genetic Relationships Between Uncle Eddie and Known Relatives

Eddie shares a significant segment with Myron and a reasonably-sized one with Sara--enough to make me start investigating if I hadn't known them before.  Ben and Howard are a bit better but not much.  How about if I look at Karen?

Name	Actual Relationship	Shared cM	Longest Segment	Comments
Myron	2C1R	161	29
Sara	3C	125	20
Ben	2C1R, 3C1R	149	30
Howard	3C1R	130	43	50% AJ

Table Showing Actual & Genetic Relationships Between Uncle Eddie and Known Relatives

My mother's first cousin Karen has large shared segments with these 4 individuals--in particular Howard, with whom Eddie & I didn't share much at all.

So while looking at largest shared segments is a reasonable strategy for identifying relatives, there may be some actual relatives who are missed; however that can be tempered by also looking at longest segments shared with known relatives.

In an upcoming post I'll discuss how contacting matches with long segments in common worked.

Note:  I'm on Twitter.  Follow me (@larasgenealogy).

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