Now What Online: DNA Done Right
Our expert answers an adoptee's question about using DNA to confirm biological relationships,
Q. I was adopted and I'm trying to prove my surname connection to a possible biological relative. I believe that I've found a male third cousin. Each of us is in a direct male line back to a common great-grandfather. Which Y-chromosome test(s) would be appropriate to prove the relationship?

A. Your situation is an ideal application of Y-DNA testing, especially since you've already developed a theory to test.

Y-chromosome tests come in a variety of "resolutions." Just as higher-resolution printers provide a clearer picture, higher-resolution tests provide greater accuracy. Resolution is measured in 'markers,' or locations on the Y-chromosome. The more markers the test evaluates, the higher its resolution.

In your situation, a basic 10- to 15-marker test would probably be sufficient to determine whether you're related to your suspected cousin. That's because you're simply looking for a haplotype match, rather than trying to estimate the number of generations back to a common ancestor. (Haplotype is the term for a set of Y-DNA test results.) But I'd suggest that you opt for a mid-range test of 23 to 25 markers. This will reduce the possibility of a false-positive match. Although it's extremely rare for two unrelated men to match closely on even a low-resolution test, it can happen if your haplotype should prove to be a very common one. (Like surnames, some haplotypes are more common than others.) Given the personal importance of this test, I'd take the extra precaution.

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