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I'm new at the DNA testing but here are my results. After receiving my DNA results in January 2016 my first match was with a known cousin who is also on Ancestry with a score of 959 centimorgans at 41 segments. That's all the information from the site but I thought this looks very reliable since she is a known first cousin.

My second match came a day later with a male individual with a shared score of 1775 centimorgans at 46 segments and the predicted relationship is HALF SIBLING. Since they are adopted this would make him my half brother, same father but different mothers.

When I consulted and asked Ancestry DNA about the results they said their test was very reliable with those high scores. Another option they suggested was to take a relationship test but half brother testing still may or may not be confirmed with this test and show the same result. The documentation is limited and there is only hear-say information about the possible relationship between the couple.

Should I take these results as reliable or does anyone recommend a test that would be more conclusive? There are no parents still living.

If I can rule out everything except half-brother would you still recommend either GEDMatch or FTDNA? If yes, which would be better?

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  • Hi, welcome to G&FH.SE! You've tested on Ancestry -- have you transferred your results to FTDNA or GedMatch?
    – Jan Murphy ♦
    Commented Mar 11, 2017 at 19:49
  • Thank you for the help. Im going to look into transferring the results.
    – skilax50
    Commented Mar 13, 2017 at 2:22
  • "After transferring, you can unlock all Family Finder features, which include the Chromosome Browser, myOrigins, and ancientOrigins for only $19." familytreedna.com/learn/imports/autosomal-transfer/…
    – Jan Murphy ♦
    Commented Mar 13, 2017 at 2:32
  • Diahan Southard's Guide on how to upload your Ancestry results to GEDmatch: yourdnaguide.com/upload-to-gedmatch
    – Jan Murphy ♦
    Commented Mar 13, 2017 at 2:34
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    Do some of your own research as to who was where, when, and look at pictures. Ancestry.com DNA matched me with first cousins on both sides of my family (I am adopted), and the pictures and commonalities speak for themselves (such as light eyes, dark curly hair, females over 5'10" and twins in the family). Through this, I found a brother, many other cousins, and aunts. Through the putative brother, I was able to confirm that our father and my mother worked together. It all came together through stories, and helped prove that Ancestry.com DNA was spot on with the relationship definitions.
    – Christine
    Commented Oct 8, 2017 at 15:38

6 Answers 6

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The result should be reliable in that it shows a close relationship. As shown on the Autosomal DNA statistics page on the ISOGG Wiki, the relationship is almost certainly one of these:

  • one of you is half-sibling of the other
  • one of you is aunt or uncle of the other
  • one of you is grandparent of the other
  • one of you is double-first-cousin of the other

Some of these can probably be eliminated by your particular circumstances, such as your ages. Any remaining possible relationships may be hard to distinguish from any DNA results of the two of you.

You should be able to distinguish double-first-cousin from the other possible relationships by further analysis of your existing DNA results. This is because double cousins share DNA on both maternal and paternal sides. To see this, you could both upload your existing result data to GedMatch.com (a free service), and do a one-to-one graphic match. If that shows any significant "full match" (both sides) segments, then it is likely a double cousin relationship, otherwise likely not.

New DNA testing is unlikely to help much except in certain situations. If you are both male, a Y-chromosome STR test could confirm or refute that you both descend in the male line from a somewhat recent (several hundred years) male ancestor. If either of you have living known relatives, it might be possible to gain insight into the relationship by having them do autosomal DNA tests and comparing their results to the existing results that you two have received.

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  • Thank you . I first looked at this page before writing my question and can rule out everything but half sibling! I'm still going to try to upload the other data to another site for comparison. Thanks again.
    – skilax50
    Commented Mar 13, 2017 at 2:26
  • The great answer!
    – George Gaál
    Commented Mar 21, 2017 at 12:12
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Just a note that you should not automatically jump to the conclusion that you and your probably-half-sibling must share a father instead of a mother; young unmarried pregnant women were often pressured to give away their babies and never speak of it again. Your DNA results should be able to show whether you two specifically share an X chromosome match or not.

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I had a similar situation with DNA suggesting a half sibling. When looking at shared matches, my half sibling and I shared the same relatives On both of my father’s parents side. No doubt he is my father’s son. I have met my half sibling and, after the initial shock, it has been a blessing for me.

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If possible, you can double-check Ancestry DNA with two great services. They are: GEDMATCH and FTDNA. But firstly you need to get "raw" files of your and your "half-brother" test. Then you can upload these files to these services and get another report. More interesting that GEDMATCH can show what parcitular DNA code you both share. And more - if you have alive siblings and another relatives, you can construct DNA of your parents with advanced features of GEDMATCH. Second service, FTDNA, is commercial organization, which make competitive DNA tests. But the very important thing that it allows so-called "autosomal" transfer from Ancestry DNA for free!!! So after transfer you will be able to use this service like if you ordered test directly from FTDNA. And get matches from it's database too.

FTDNA also allows to order Y-DNA tests. They can help to answer the question do you share the same father.

Please understand me correctly. I am sure that 1775 centimorgans is reliable result. Besides Ancestry is not the leader in DNA testing. And it's report is not very informative. But these numbers say only that your "half-brother" is related to you and don't say in which way. I am sure that he is not your child or one of your parents. Because the numbers will be different. And with such result we can't tell for sure if he is your nephew, or uncle or maybe - full brother. Despite the fact usually siblings have 2150-3050 cM shared DNA. We need to take into account that DNA inheritance is statistical process.

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Go to your shared matches, if they are all on one side of the family you are almost definitely half-siblings.

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    Looking at shared matches would indeed provide useful information, although it would not necessarily prove half-sibling status -- it still could be an uncle or nephew. William, I suggest editing your answer or justifying why you think it would mean half-siblings.
    – Ellen Spertus
    Commented Jul 16, 2018 at 17:12
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I tested on Ancestry a year ago and got the report that said my brother could be a full sibling or half sibling, among other possibilities that really don’t apply. When I checked into shared matches, I found some on either side of my family. Same result on GEDmatch. I can’t figure out any other way these matches make sense, so I am assuming we are full siblings. Sounds like I have company in trying to pin this down!

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