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I was adopted at birth and know very little identifying information about my birth parents.

I recently took a DNA test on Ancestry.com and found a match that was a possible 1st/2nd cousin match.

After much research, I was able to get in touch with this individual. After explaining the circumstances of my birth, he feels with all of his heart that he is my half-brother.

Currently, neither of us have any immediate relatives who've taken DNA tests.

According to Ancestry.com, we share 611 Centimorgans across 38 DNA segments. I don't think we're brothers because from all I can tell, there's just not enough shared DNA.

He's going to gedmatch today and upload his information. I would like to know. All the circumstances and timing makes sense that he would be my half-brother, but the DNA is saying different.

Anyone have any ideas?

I've read that have sibling can have some weird shared DNA.

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    Reviewing genealogy.stackexchange.com/a/12305/104 might give you some guidance – user104 Jan 19 '17 at 17:14
  • Could he be not your half-brother, but half-nephew? And the best to reveal truth - to find some immediate relatives who also will agree to make DNA test. For example, parents of your half-brother. Also please take into account that tables on ISOGG site are for regular situations. If there were such things like inbreeding or marriages between cousins, DNA test will show very pecular results. But they can be explained too!!!! – George Gaál Jan 19 '17 at 22:53
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The post ColeValleyGirl pointed out may give useful background; in particular, the table on the referenced ISOGG page gives guidance on average DNA for various relationships. It suggests that the relation in question likely falls in either the 12.5% or 6.25% groups of relationships. I think it's unlikely to be in the half-sibling range (centered on 25%). There's a graph showing the likely ranges of total-Cm on Gliesian LLC.

For people sharing 611 cMs, Gliesian LLC highlights the likely possible full and half relationships. There are quite a few possibilities since your match amount happens to fall towards the extremes of the expected ranges of two different degrees of relationship.

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Entering your shared 611 Centimorgans into the Shared cM Project tool at DNA Painter does not suggest a half sibling relationship.

From the results presented there a relationship nearer to a first or second cousin is suggested but there are many possibilities, as you will see by clicking the link above.

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My brother did an Ancestry DNA test in January/18. I just did the same test with Ancestry in March/18. He showed as "Extreme Confidence" as a 1st cousin.

I contacted Ancestry and inquired about why my brother would show as my possible 1st cousin. They did a walk through with me, and we share 1,621 Centimorgans >80 Segments. He is actually a half sibling. I also have 2nd cousins listed there, and I can canfirm they are in fact my cousins, children on both sides of my parents. Their numbers match more closely to the numbers you have posted: 611 Centimorgans across 38 DNA segments.

Based on the facts, I believe he is your second or third cousin.

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