Understanding AncestryDNA match of 1,866 cM if not Half Sibling?

Question

I have somewhat recently started using Ancestry.com, so just a fair warning that I'm still learning how everything works!

That being said, I am the child of my biological mother, who I know, and an anonymous sperm donor, who I do not.

The other day I got a match saying "Close family - first cousin." I messaged the guy since I assumed he'd also be a son of this sperm donor.

This is where the story gets confusing.

I'm 22 and he is 24, we're from the same hometown, and he looks a fair amount like me. Our DNA shows that we have 1866 centiMorgans of DNA in common across 64 DNA segments. So almost 100& certainly half-siblings.

But when I reached out to him, he said he had no idea why our DNA was showing us as related. When he asked his parents, they confirmed that they are his biological parents and that they did not use a sperm donor, nor was his father a donor.

His dad is Creole, but my ancestry didn't show any of that heritage. My biological donor also self-reported his ancestry, which did not include Creole at all. I know he is not related to my maternal side since my mom hasn't had any other kids and it doesn't seem likely he's a long-lost brother of my 67-year-old mom (assuming he's actually an uncle).

I'm stumped. It doesn't seem possible for this guy to be my half-brother. But our DNA match is incredibly strong and we're from the same city.

Does anyone have any idea what is going on?

Answer 1

I can't answer your question, but I can suggest something to try:

First, you should determine whether he is related to you on your father's side or your mother's side. Here's one way to do that:

1. Find someone on AncestryDNA who is related to you through your mother. If you can't find someone, have your mother or a close relative of hers tested.

2. If this third person is related to your potential half-brother, you know he's related to you through your mother and is not a link to the sperm donor.

3. If this third person is not related to your potential half-brother, you know he is related to you through the sperm donor (or a close relative of his).

An alternative way of finding out which side your potential half-brother is on would be to:

1. Download your DNA from Ancestry.com and upload it to GEDmatch (instructions).
2. Ask your potential half-brother to do the same.
3. Compare your DNA there. If there is a strong X-match, he is related to you through your mother.

GEDmatch is free, but you should be aware it has less stringent privacy protection than other sites.