Could a first/second cousin DNA match actually be my half brother?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

Yes. Most likely. My half brother showed up as my 1st cousin. My Aunt who is my fathers 1/2 sibling showed up as my 2nd cousin. Most likely he IS your brother.