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Our Project’s recent experience illustrates how Y-STRs can be misleading in predicting the branch of a family to which a new participant belongs, even at the 700 marker level.

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The 18-year-old Acree DNA Project that I administer tested in its early years at Ancestry.com, but now tests at FTDNA and YSEQ. We were fortunate to discover during our first year that the ancestor of most Acrees was William Acree (c1710-c1767) of Hanover Co., Virginia, and that his descendants (61 of us currently in the Project) possess the rare, distinguishing microallele 13.2 at DYS385b. With the advent of Big-Y testing, we found that we also possess the slightly more widespread Y-SNP A2156, and that descendants of William’s son John uniquely possess the distinguishing Y-SNP A2155.

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FTDNA’s Y-STR matching of our latest Project participant, a descendant of William’s son William Jr., at the 111-marker level, shows incorrectly that his first four matches are not with other descendants of William Jr., but rather with descendants of John (including me), at a genetic distance of 3 steps His first match with a descendant of William Jr .is shown fifth, with a GD of 4. His other matches, including non-Acrees, show GDs of 5-8 steps.? A known Acree fourth cousin once-removed of?his (also a descendant of William Jr.), who appears among his autosomal (Family Finder) matches, comes in tenth on the list, with a GD of 8. The Big-Y’s 700-marker comparisons are also misleading: He has 4 differences with me and 7 with this fourth cousin.

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That’s certainly a misleading performance for Y-STRs. Fortunately, the Big-Y Block Tree (not the Big-Y match list) correctly groups our newcomer among William Jr.’s descendants, clearly apart from John’s descendants. This experience testifies once again to a consistently dependable performance for hierarchical Y-SNPs, in contrast to ambiguous Y-STR strings.

I'll take the bait.

Genetic distances can't be "misleading" or "mistaken" (unless there is an arithmetic error). ?They are an objective measurement, whether calculated using Y-STRs or Y-SNPs.

It is true that researchers can make mistakes or be misled by data, but that's true no matter what form the data take. When it comes to genetic genealogy it always pays to use multiple lines of evidence.

Vince Vizachero

Actually, this is really why BigY (and sequencing tests in general) are so important - because they remove this ambiguity of descent: as long as you have at least one SNP mutation that defines a branch in your family tree, you know exactly how the other branches relate to it.

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I would prefer to say Y-STR tests can be ambiguous rather than misleading, but their main problems (as I recently highlighted) are that STRs can mutate back to their original value ("back mutation") and that two different families can experience the same STR mutation ("convergent mutations"). In either of these cases, instead of adding to the genetic distance, a mutation can subtract from it.

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Rather than relying on genetic distance itself, it is often best to get hold of the original Y-STR results. In the days before BigY, we had to group people based on their Y-STR values, and we would look for key combinations of values that formed tight clusters of closely related people, e.g. DYS492=13 predicts R-U106 with 97% accuracy, while a high probability of being R-DF98 comes from a DYF395S1=16-16 + DYS557=15 combination. We could often form smaller groups than this, and I have spend days of my life trying to merge people's STR results within R-DF98 into sub-groups before BigY testing really took off. But these groups weren't foolproof, and BigY testing has proved some and disproved others. This is a real driver behind our encouragement for anyone joining our project to upgrade to BigY - without it we can only guess approximately at where people fit in with others.

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- Iain