DNA Research #DNA Re: Conflicting Results from 2 DNA Sites #dna


RichardWerbin
 

I am reluctant to say this, but AncestryDNA is probably correct and
ftDNA is probably wrong.

Many people who are Jewish have tested at ftDNA and for a long time it
was thought that ftDNA did a better job of estimating cousin # for
Ashkenazi descendants. This is no longer the case in my experience
comparing results at ftDNA with gedmatch and AncestryDNA and talking
to ftDNA support on the phone.

The problem is endogamy. There were many close cousin marriages in
Eastern Europe. The result is that matches are often coming >from
multiple lines of descent and >from very distant relatives. This means
that a 100 cM total common DNA may be coming >from 20 small segments of
less than 5cM each. The results are misleading.

At gedmatch you can choose to set a minimum segment size to include in
a 1-1 comparison. Their default is 7 cM. Some people recommend a 10 cM
minimum for Ashkenaz. The only way I can match total cM between ftDNA
and gedmatch for some cousin matches is if I set the gedmatch minimum
to 3 cM or less.

If you view a match in the ftDNA chromosome browser, the default
threshold is 5 cM and you have a choice of 1, 5, 7, 10 cM.

I just got off the phone with ftDNA support who confirmed that Family
Finder match list is showing total cM based on a 1 cM threshold. It is
much too small if you are >from an endogamous Ashkenazi family.

The result is that they are seriously overcounting the total cM in the
match list because they are adding in a large number of very small
segments in some cases. Always check the total in their chromosome
browser and set the threshold to at least 5 cM.

Yes, sometimes when ftDNA says 3rd cousin, the real relationship is
often something like 5th - 10th cousin. Make sure you check the size
of the largest common segment at ftDNA. Double check at AncestryDNA
and gedmatch. AncestryDNA does not tell you the size of the largest
common segment. You can see that at ftDNA and gedmatch.

Richard Werbin

---Original Message---
From: "Pat Fuller" <patbfuller@...>
Date: Tue, 28 May 2019 17:46:26 -0700

I match someone at the 2nd-4th cousin level with 119 shared cM on
Family Tree DNA. But on Ancestry DNA, our match is at the 4th-6th
cousin level with only 26.5 shared cM. I have never received such
discrepant results >from 2 DNA sites before. I am in touch with the
person I match, so I know it's the same person. Could someone please
explain? Could there be a mistake? How can we determine which one is
correct?

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