This post has been updated.
The What Are the Odds? tool (WATO) at DNAPainter.com has revolutionized how we identify unknown parents, grandparents, and even great grandparents of someone who has taken a genealogy DNA test. It lets you analyze multiple DNA matches at the same time to see where the tester fits best into the family tree of the matches. (You can watch a video introduction here.)
Best of all, you can use autosomal DNA matches from any of the DNA matching services. Even so, there are some quirks you should be aware of, because each company has its own unique matching algorithms.
AncestryDNA has the largest database by far, with more than 20 million people tested. Chances are, most of the matches you use in a WATO analysis will come from there. They use an algorithm called Timber that down-weights some segments that they think are less informative. As a result, the same match at Ancestry might share a bit less than at the other companies. AncestryDNA also reports the “unweighted” amount of shared DNA, before Timber is applied. For WATO, you can use the “weighted” centimorgan amount that’s reported on the main match page rather than the “unweighted” amount. After all, they invented Timber to correct for unreliable segments, and the underlying data for WATO came from AncestryDNA in the first place.
23andMe has the next largest database, with more than 12 million kits sold. You will often find useful matches there who haven’t tested elsewhere. They have two quirks that might impact a WATO analysis. First, they include segments on the X chromosome in their DNA totals. If you are able to see the chromosome browser for a match, subtract those X segments out before you use the shared DNA amount in WATO. If you can’t see the chromosome browser and can only see the percent DNA shared, multiply that percent by 74.5 and use that number in WATO.
(UPDATE: As of August 2021, adjustments at FTDNA are no longer necessary. This paragraph has been left intact for posterity, but you can ignore it and use FTDNA totals as is.) FamilyTreeDNA includes tiny segments in their total that are usually false positives. It’s essential that you subtract out all of the segments less than 7 cM before you use an FTDNA match in WATO. If you don’t, the tool could give you misleading results.
Finally, GEDmatch has two different matching algorithms: a quick one for the one-to-many match list and a more precise one for one-to-one comparisons with a given match. Use the one-to-one values in WATO.
A quick and easy way to correct your match totals is with the Individual Match Filter tool, also at DNAPainter.com. Simply copy the match data into the tool and it will do the rest. It even has instructions for how to obtain the data. Remember, you only really need to correct the totals for matches from 23andMe (in cases of X segments) and, especially, at FamilyTreeDNA.
The table below summarizes the main matching sites, their quirks, and any adjustments needed to use WATO.
Updates to This Post
29 March 2022 — Noted that adjustments are no longer necessary for FTDNA matches.