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My brother and I are in our seventies and recently found out we are full siblings through DNA testing. We do not know who our father was.

We have DNA matched with a possible 3rd cousin.

I have traced the cousin's 8 pairs of GGGrandparents and none of them match our 4 pairs of GGGrandparents on our mother's side, and our two families have no connections.

Does this mean that this 3rd cousin is from our father's family?

I have also looked at GGGGrandparents with same results.

Both my brother and I did an Autosomal DNA test with Ancestry. I have already uploaded both our raw DNA to GEDmatch and FTDNA. However, the person our DNA matched with as a 3rd cousin on Ancestry does not seem to have uploaded to GEDmatch, so I would not be able to give you his data.

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  • I don't know who my paternal gf is by name or nation, but an auDNA test with ftdna.com did say he (and thus I too) have Ashkenazi genes. Over time, the percentage has been lowered from 31% to 20% because of (I assume) a better way of fine testing the test's results. However I did find on ftdna some 3rd cousins (3 people in one family) who shared my gf's genes. I met them by traveling to both coasts of the USA. I found this far more satisfying because the man of the three persons looked like my father from his mouth down through his chin.(Dad never even thought he had any living relatives.) Thi
    – R B Berg-Burnett
    Aug 9, 2017 at 22:57
  • The remainder of the comment above was: "This relative was 90 at the time and has since died. No one in that family has any idea where the specific connection between us is, but the year-long letters between us and the continuing connection between me and the youngest of the three is an enormous satisfaction."
    – PolyGeo ♦
    Aug 9, 2017 at 23:30

2 Answers 2

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Unfortunately it means nothing. I will explain.

Firstly, the grade of relationship shown by test may differ, because the spread has statistical nature. More - "3rd cousin" may be false-positive result because small identical segments of DNA from your parents and from parents of tested person glued together resulting big IBS segment. So real relation is more distant as stated.

Second thing is possibility of non-parent event. As we look at more generations, such event will be more common. So please take all results with a grain of salt

I will be glad to help you and advise the next steps if you provide more information. All I said doesn't mean that DNA tests fail. The information they provided is very precious but it needs very complex processing (and critical approach too!!!) to get real clues and great results.

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  • Thank you. Any help would be much appreciated. What information do you need?
    – Pat
    Aug 6, 2017 at 18:54
  • Please write more where did you get the test (what lab). I would like to see the amount of shared DNA (in cM) between all listed persons. The best thing you could make - upload your autosomal raw data to gedmatch service and give me the kits numbers. This service is free and could help in finding other possible relatives. But the drawback that everybody will see your e-mail address and see your match list.
    – George Gaál
    Aug 6, 2017 at 19:42
  • To George Gaal, I am confused about whether I am using the comments and answers correctly on this site. I'm not sure that you will be able to help me. Both my brother and I did an Autosomal DNA test with Ancestry. I have already uploaded both our raw DNA to GEDmatch and FTDNA. However, the person our DNA matched with as a 3rd cousin on Ancestry does not seem to have uploaded to GEDmatch, so I would not be able to give you his data.
    – Pat
    Aug 6, 2017 at 22:04
  • Hi Pat. My suggested next steps would be: a) write to this person and ask them politely if they'd mind uploading to Gedmatch; b) see if you can find a reference person on your maternal side (perhaps a first cousin?) to compare with. As George explains, it's all rather opaque, and you don't yet have any proof that the match is on your paternal side. But what Ancestry calls a third cousin is generally quite significant so it's probably worth pursuing!
    – Jonny Perl
    Aug 8, 2017 at 21:46
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As George noted above, simply because your paper trail doesn't correspond with the match you've found does not in itself prove that the match is from your paternal side. But Ancestry's '3rd cousin' indicator does constitute quite a high number of centimorgans, so it's worth pursuing further along with any other significant matches. Do you have any 'shared matches' on Ancestry with this person?

I would politely ask this match (and any other significant ones) if they can upload to gedmatch and/or familytreedna, who also do a free autosomal transfer.

The most important way to progress with deciphering your DNA results is to get people to test where you know the connection. Ideally these people will be at least one degree away from you (an uncle or first cousin would be ideal; failing that even a 2nd cousin could still be helpful). Having these tests to compare will help you to place your matches.

Since you don't know who your father was, having a known person (e.g. a first cousin) on your maternal side test would be of particular help, since if a match is also related to a maternal cousin, they are almost certainly from that side and not from your unknown paternal side. Failing this, perhaps you can use your existing research to identify some people who have already tested as maternal matches. Any known relationships will potentially help you.

The process of tracing and identifying matches is quite hard work, but you will find there are many helpful people willing to share their knowledge, and perhaps even help you interpret your results.

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