I'm currently working on no fewer than three such mysteries in my own DNA results. One is definitely an adoptee (the person mentioned the circumstances of his adoption in his AncestryDNA profile); the other two appear very strongly to be descended from extramarital affairs.
(Aside: if the 450+ cM match Sally is talking about is on AncestryDNA, I have an extremely hard time believing that is purely the endogamy rather than, as Steve diplomatically put it, a "non-paternity event." Maybe something weird could happen with 450 cM in FamilytTreeDNA or MyHeritageDNA, but people in the 400-500 cM range on Ancestry have been genuine close cousins in every case I've seen.)
It's worth at least double-checking that the person has actually logged into Ancestry over the time period you've been contacting him. I suspect that lots of people take a peek at their ethnicity results, and then never log in again. If you think that it's likely that he's deliberately ignoring you, I'm not sure what to tell you, but here's how my similar situation shook out:
I noticed my match to "K" right away when I got my AncestryDNA results back in October 2019. I poked around his Ancestry tree and noted the following:
Undeterred, a couple of weeks later, I messaged his daughter. I checked her profile every few weeks and realized that probably, she wasn't deliberately ignoring me, she just never logs in to Ancestry.
So finally, after waiting several months, I also messaged his granddaughter. Five or six weeks passed, and finally, she replied! After a couple of messages dancing around my main point, I finally blurted it out: "has your grandfather ever mentioned that he might be adopted, or that there are other unusual circumstances around his birth, like one or both of the parents who raised him not being his biological parent?" And I explained why I was wondering - the lack of obvious Jewish people in his tree, as well as a clear pattern of matches to people in my family.
She called me 15 minutes later.
The story of what had happened went something like this:
K's daughter was actually the one in the family who tested first on AncestryDNA, three or four years ago. She was startled to see her ethnicity come back as 25% Jewish, and confronted her father about it, who swore up and down that he had no Jewish heritage and that he had no idea how such a thing could be possible. Unsatisfied by this explanation, she insisted that K test also, to see which parent might have contributed the Jewish piece of her ethnicity. So K tests (his wife had died a few years before, and so couldn't be tested herself) and sure enough, his own test came back 50% Jewish. But K's family are not genealogists, and it didn't occur to them to dig deeper to figure out why AncestryDNA thought K was half Jewish. To them, it was just an odd little curiosity.
After I spoke on the phone with K's granddaughter, she shared his matches with me, and it could not be more obvious 1) that K is definitely a close-ish relative - he has crazily strong matches to EVERYONE in my family who has tested (virtually all of the 20+ cousins I've turned up so far on that service); 2) that K's biological father is not the man who raised him: he is the biological child of his mother, and a man in my family. And his daughter and granddaughter are now obsessed with cracking the mystery! (They are worried enough about dropping this bombshell on K that they are keeping all of this a secret from him unless I can produce a conclusive identification of his biological father in my family. Right now I am in the process of contacting the children of my 3-4 prime suspects and seeing if they are willing to test to figure out if K is their half-brother. K is most likely my second cousin twice removed.)
So if you want to try to solve your mystery without intruding on your match's privacy, there are a couple of things to check first:
-First, if you haven't already, contact the three of your cousins who have tested on AncestryDNA and ask them to share their results with you. That way, you might be able to confirm that this mystery person is definitely a relative, and maybe the additional data from those matches will be enough to confirm that you've likely identified the exact link to your family.
-Second, go through your shared matches with the mystery person to see if you can turn up anyone else who's also likely to be in their family. Maybe they're not interested in talking to you, but perhaps one of their siblings or cousins will be!
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