Working with WATO: Centimorgan Adjustments

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.

 

The Quirks

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.

No adjustments are needed for matches at either MyHeritage or Living DNA.  Just use those shared DNA amounts as reported.

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.

 

The Fix

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.

 

Summary

The table below summarizes the main matching sites, their quirks, and any adjustments needed to use WATO.

* Only available for matches who have granted permission

 

 

3 thoughts on “Working with WATO: Centimorgan Adjustments”

  1. Good morning! I have a question. I put in the Target and the question….added some information, but how do I add to what I’ve written?
    Here’s the scenario: Person, the target was adopted, I was adopted but know my family history. The Target is a1-2 cousin with 485 cm to me. How do I add his family history as much as we know, to my maternal history where are matches come from? I’ve tried starting with his biological grandparents but need to add my history, to get to identify his father. Father comes from my biological maternal side as the Target matches all my 5 1/2 siblings and my cousins from my matches.
    Thanks, Rita W Neher

    1. A 485-cM match is probably a half first cousin or a first cousin once removed. I’d probably start the WATO tree with your maternal great grandparents. You’ll have to use shared matches to figure out which set of GGPs to use. If you need more help, I recommend that you join the dedicated Facebook group here: https://www.facebook.com/groups/WhatAretheOdds

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