In my shared matches on AncestryDNA I am used to seeing matches to the children of my 2nd cousins that are less than the match I am to their parents.

However, when examining my sister's DNA I see what seems to be a surprising result where she shares more DNA with the child of a 2nd cousin than she does with our 2nd cousin.

I'll refer to our female 2nd cousin as 2C and to her son (our 2nd cousin once removed) as 2C1R.

  • I have 169 centimorgans shared across 10 DNA segments with 2C, and 104 centimorgans shared across 6 DNA segments with 2C's son 2C1R
  • my sister has 68 centimorgans shared across 6 DNA segments with 2C, and 94 centimorgans shared across 6 DNA segments with 2C's son 2C1R

Is it possible to explain what I think seems to be an anomaly?

  • 1
    Interesting. Are you from an endogamous group? Is it possible that 2C1R's father is related to you? Are you and your sister full siblings? – Ellen Spertus Aug 7 at 21:36
  • @EllenSpertus We have English/Welsh/Scottish ancestry so I think we are unlikely to be considered endogamous. My sister and I are full siblings with 2,626 centimorgans shared across 79 DNA segments, and our DNA seems to so far strongly support my paper tree of just over 10K people. I do not know 2C1R or 2C1R's father (and have only met 2C once or twice when we were children) but I have no reason to think that we might be related to 2C1R's father. – PolyGeo Aug 7 at 21:44
  • Is it possible to upload all three to GEDmatch and see more closely what's going on? I feel like Ancestry's algorithms should be preventing what you're seeing... but I honestly don't understand their algorithms. And it seems like too much of a difference for identical by chance. – Leah Worster Aug 8 at 3:37
  • @LeahWorster I would love to have all three available on GEDmatch (where mine already is) but I do not know 2C1R and my sister currently does not have the same interest in genealogy that I do so I don't want to pressure her to upload her DNA there. For now I can live with just knowing that its not easily explained in the absence of GEDmatch. – PolyGeo Aug 8 at 3:43

The anomaly is your sister's 68 cM in 6 segments with your 2C as it appears too low.

Since you're on Ancestry, there's not a lot you can do to figure this out. Try comparing the sets of Shared Matches of you versus 2C, your sister vs 2C, you versus 2C1R, and your sister versus 2C1R. See if that sheds any light on the problem.

There could be several reasons:

  1. You and/or your sister are matching a segment with the 2C1R that was inherited from his other parent.

  2. Your sister and the 2C1R have some "identical by chance" markers which make the segments appear longer than they truly are by descent.

  3. The Ancestry algorithm is not fully reporting the shared cMs between your sister and the 2C, due to no-calls or some other anomaly.

Uploading them to a site with a chromosome browser would tell you.

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