Case studies in DNA analysis: falsifying hypotheses about the MRCA

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Surname/tag: dna
Profile manager: Barry Smith private message [send private message]
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I commented in March 2021 on a post on G2G, writing:

I have lots of 7th, 8th, and 9th cousins on Wikitree that I share DNA with. But in none of those cases do I trust that the segment I share came from the ancestor I have identified. I have been proven wrong too many times...

I give here a couple of examples of how I have been proved wrong. The examples are rather different, although common to both is that the the "proof" that I was wrong came from careful analysis of shared matches between close family members.

Example 1

Here is the part of the output of a comparison at GEDmatch between my mother's kit and one of her matches, let's call her "Alice"

Mom with Alice

They share an additional segment, with total shared DNA 50.7cM, suggesting they are around third cousins or so.

Examining the trees, a pair of common ancestors jumps out: Barnabas Eldredge and Theodosia Wadsworth. My Mom and Alice are 4th cousins once removed through this connection. So far so good.

Here's an image from a match to my mother over at MyHeritage. Let's call her "Barb".

Mom with Barb

Barb's tree shows she is also a descendant of Barnabas Eldredge and Theodosia Wadsworth. We also can see that there is a healthy overlap between the matching segments of my Mom and Alice and of my Mom and Barb on chromosome 4. The two matching segments both cover the region from position 35.9 million to 49.1 million.

Images: 2


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