Match/no match for second cousins- NPE probability?

+5 votes
269 views
A question:  Persons A, B, and C are supposed second cousins to each other based on traditional genealogy, with a combination of well sourced and unsourced but detailed family trees.  On GEDMatch/FTDNA, persons A and B match each other but neither one matches C.  (Also add to this person D, a third cousin to the whole group, who is a DNA match for A and B, but not C.)  How confident can I be that there was an NPE somewhere between person C and the MRCAs of A and B (great grandparents)?  Seems like it, but is it even remotely possible that the relationship is correct but there's a lack of match just due to the quirks of probability?  I looked at the Genetic Genealogist's shared cM project tables and it really looks like I should expect at least some matching at this level of relatedness.
in Genealogy Help by Lisa Hazard G2G6 Pilot (263k points)
I should mention that A and B tested through FTDNA, C and D tested through Ancestry, and all four are on GEDMatch.
Is one or more of them males?

A member of my surname project was stuck! He knew the name of his 3rd great grandfather, but could not get any info about his parents.

Then a father and son who had taken the Y-111 test at FTDNA JOINED OUR PROJECT.

Their surname matched his GGG grandfather’s middle name. Quite a coincidence! Al had taken the Y37 test which matched father and son 36/37. All three also matched with 8 zeros. This is certainly strong proof that they have a common ancestry.

We found a man whose first and last name matched the first and middle name of Al’s GGG grandfather with a last name of Winters. The middle name of Al’s GGGGF was Lippincott, the same as the last name of the father & son.

We have a good idea who the mother of his GGGGM was.

The mother of TLW was 12 and and his father was 17 at TLW’s birth in 1808.

Y DNA played a valuable part in proving an NPE IN BREAKING DOWN THE BRICK WALL.

Perhaps your situation may have a similar outcome. Keep an open mind and think outside the box.
Thanks!  D is male but his mother is the next link in his connection to the MRCA, and A, B and C are female, so Y DNA wouldn't help in this particular case.  The complete lack of an auDNA match to three supposed cousins at the second/third cousin level tells me that C probably has some surprises in her tree.
Just to leave no stone unturned, have you tried lowering the threshold to match with C? That's usually not a good idea, but if she actually matches the others in the same area of  the same chromosome but with only 5 or 6 cM it would be interesting.
Thanks, good suggestion!  I just used the multiple kit comparison tool for all four people, and then the 2D chromosome browser with C as the lead person so I could see how everyone matched her. A and B each have some matches with C in the 3-6 cM range, but none of the segments are in common with each other.  She doesn't even have tiny matches with the third cousin, D.

1 Answer

+12 votes
 
Best answer
According to an article by the Genetic Genealogist, the odds of second cousins sharing NO DNA is essentially nil: https://thegeneticgenealogist.com/2016/10/03/second-cousins-or-closer-that-dont-share-dna/

You're more likely to be struck by lightning TWICE than share no DNA with your second cousin. If A, B and D all have DNA matches but no one matches with C, then you have your answer.
by Jessica Key G2G6 Pilot (310k points)
selected by Lisa Hazard
Thanks!  I hadn't seen that particular article, but it confirms what I suspected.

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