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Many people find relationships with other people that they only share a small amount of DNA with, e.g., 10 cM or less.

They then, after researching the other person's tree, find a common ancestor and a part of their trees that they both share.

Can the DNA match then be used as evidence in concluding that the two of them share DNA that was passed down from that common ancestor?

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    I hate to sound like a DNA sceptic but - assuming that the paper is accepted as showing that the 2 have a common ancestor, what is the value of concluding that the matching DNA comes from the common ancestor? – AdrianB38 Jul 29 at 10:04
  • @Adrian - can you expand that into an answer? – lkessler Jul 29 at 14:07
  • done, as requested. – AdrianB38 Jul 29 at 19:28
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Applying the Genealogical Proof Standard (https://en.m.wikipedia.org/wiki/Genealogical_Proof_Standard) seems appropriate in cases like this. You have a relatively weak piece of evidence in the form of a small DNA match, and you have a possible explanation in the form of family trees which indicate a common ancestor.

The first element of the standard, "reasonably exhaustive research" bears on this.

First, the trees need to be evaluated to see whether (and how strongly) the evidence in them supports the conclusion that the two people have the common ancestor as the trees indicate.

Then, other potential evidence needs to be considered and evaluated in order to decide whether it supports or refutes the conclusion, following the GPS elements of "analysis and correlation of the collected information" and "resolution of any conflicting evidence". In the case of small DNA matches, that evidence is probably DNA information from more possible descendants of the proposed common ancestor. Generating that evidence might involve looking at existing DNA test results for some individuals and asking other people to take DNA tests whose results will be relevant.

It's possible that the small DNA match plus the matching trees will indeed lead to a correct conclusion that the proposed common ancestor is the source of the shared DNA. Or new evidence might fail to support the conclusion, or suggest that it is wrong.

To summarize, yes a small DNA match is evidence in favor of such a conclusion, but by itself probably isn't sufficent to justify such a conclusion.

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I'll go out a limb here and claim that no, it generally can't be used as evidence. Diahan Southard (among others) suggests that the probabilities for a 7 cM match are about 80% IBC, 20% IBD. To use such a match as evidence woud require overcoming those probabilities, somehow (I should note that it depends on where the match was observed as, e.g., Ancestry's Timber algorithm may skew those probabilities). I can think of at least two ways to overcome the odds - (1) multiple-segment matching, and (2) triangulation. (1) would involve having enough matching small segments that at least one of them is statistically likely to be IBD. I assume this isn't an option, since we're talking about a single segment here, and you'd need about 15 or so matching 7 cM segments to get over 90% certainty that at least one was IBD. (2) would involve more than one other descendant of the common ancestor, and a chromosome browser. You'd need to verify that you match this specific segment with multiple descendants of the common ancestor, and also that you don't match it with anyone who isn't a descendant of said ancestor (that second part is a bitch, as it's hard to prove a negative - also bear in mind that the common ancestor for a small segment could be arbitrarily far back, so you could never know whether you're seeing false positives or not).

I agree that small-segment matches are useful in identifying candidates for paper documentation, but I would claim they don't constitute evidence themselves.

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Personally I am sure that this doesn't answer the question but it seems important.

Assume that the purpose of genealogy is to construct family relationships and potentially add further information about the lives of those people.

As I understand the question, the two people have found sufficient evidence in "paper-based" sources that they have a common ancestor. Also they have identified that they share a small amount of DNA. Note that the identification of the common ancestor does not depend on the DNA (though it might have been a spur for investigation).

The question appears to be whether the independently proven common ancestor and the DNA match can be used to deduce that the matching DNA came from the common ancestor?

I would like to query the value of concluding that the matching DNA comes from the common ancestor. As far as the two researchers go, the "paper" evidence has already demonstrated the relationship. Deciding exactly where that matching DNA comes from would therefore seem to only have relevance if a third researcher appears.

If the third matches both the first and second researchers, there is no immediate reason to assume that the third matches the first and second on the same bit of DNA, so no reason to assume that the 3rd shares the same common ancestor.

If the matching bits of DNA can be identified and the 3rd matches both 1st and 2nd on the same bits of DNA as the 1st and 2nd match, then several possibilities hold.

  1. The 3rd matches both by chance. (I have no idea of the probabilities of that).

  2. The 3rd matches both by descent from the previously identified common ancestor.

  3. The 3rd matches both by descent from an ancestor of the previously identified common ancestor and it is purely by chance that the common DNA has mutated in the 3rd to match. Again, I have no idea of the probabilities.

As I said, this does not answer the original question - rather it queries the value of answering it, especially when only the 2 researchers are involved.

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  • Should anyone feel like editting my answer to take forward the arguments in it, please feel free to do so. – AdrianB38 Jul 29 at 19:28

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