I’ll call him “Eric.” He rides the top of my list at Ancestry DNA, and at 23andme is my closest match beyond those relatives I consider my immediate family. He is an adoptee, born out West in 1980. He tested to find clues to his parentage and to his ethnic background, and I was the first blood relative he’s ever “met.”
Ancestry estimated our relationship at 3rd cousins, and I encouraged him to test with 23andme, which posited 2nd to 3rd cousins (2.43% shared DNA across 6 segments.) More critically, 23andme revealed that my connection to Eric is through my father, with whom he is an estimated 1st to 2nd cousin, sharing 6.76% across 18 segments! (He and my sister share 3.3% across 7). A 1st or 2nd cousin of my dad, who is 79 years old? How in the world???
Common matches between Eric and my father allowed me to focus on my grandmother’s side as the link. My grandmother had one known sibling. (Well, maybe, two, as I’ve recently learned of a possible half-brother who’s only a few years older than I am. I’m not including him as a possibility for now.) Her half-sister M., with whom she shared a mother, was born in 1907. Full first cousins share, on average, 12.5% DNA, so my father and M.’s children would share about half that. In other words, those cousins would share with my father the approximate percentage of DNA that my father shares with Eric. The male cousins were too old to have been Eric’s father (and, if they were, Eric would share only about 3% with my father.) Thus, they are eliminated as Eric’s father or grandfather. And I can eliminate the female cousins on the ground that he does not share their haplogroup, which is H3.
Arriving at this point, I was momentarily stumped. And then I remembered that uncles and great-great-nephews also share roughly 6.25% DNA. (And constitute a relationship that makes more sense given Eric and my father’s relative ages.) This train of thought, I think — and without going into requisite detail — will ultimately lead us to the truth.