Can Ancestry miss a strain from a close ancestor, and is a second test the solution?

+2 votes
I have a few second and third cousins related to my great grandmother on the paternal side who don’t show in my list of cousins on Ancestry. Is it that I took very little from her or maybe possible that a strain of my DNA was not reported by mistake? Is a second test on Ancestry a solution? I did an autosomal DNA with FTDNA but I could not recognize anyone on their list. I prefer Ancestry. Thanks.
in Genealogy Help by Living Angus G2G3 (3.5k points)
retagged by Lynda Crackett

2 Answers

+6 votes
Best answer

Mistakes are unlikely. If you are not seeing a match that you would expect to see then you may need to look for alternative explanations. These statistics on the ISOGG Wiki show the probability of a match showing for each level of relationship. Be aware that if there is a half relationship involved then the probabilities would be lower.

by Lynda Crackett G2G6 Pilot (685k points)
selected by Living Angus
Thank you so much. The alternative explanation is scary.

Just a note that, in the article Lynda linked, I would take the AncestryDNA claims of distant cousin detection with a sizable grain of salt, maybe two grains. Their phasing algorithm--which is genotype phasing, meaning that it is based on estimates of population models, not some massive database containing actual trio-phased data--is proprietary and has never been revealed. Meaning we can only take Ancestry's word for it.

The 23andMe numbers, on the other hand, were derived from an academic, peer-reviewed study led by a then-lead scientist for 23andMe, Brenna Henn. You can scroll down that page a bit to the section titled "How many cousins do we have?" to find one of the summary charts and a link to the published study at PLOS.

I personally consider Ancestry's success numbers at 4th cousins and beyond to be highly inflated and suspect. We have no way to view or substantiate their data, and they don't have to give them to us. That the 23andMe and FTDNA numbers are reasonably in sync while Ancestry's are off the chart should tell us something.

Harry, Thanks for the best answer star.
+4 votes
A second test with Ancestry won't change anything, unfortunately (unless there had been some paperwork mistake with one of your tests. Given that both your tests (FTDNA and Ancestry) match 100% on Gedmatch, this is not possible).

As Lynda said, you should investigate alternatives at this point. If you can find another cousin who should be related and have them test as well, that may help give you more information.
by John Trotter G2G6 Mach 4 (43.6k points)
Good catch on the two different GEDmatch kit numbers, John. That rules out any major error from either of those tests.
How can I find out that both my tests (FTDNA and Ancestry) match 100% on Gedmatch?
Harry, you can use the "one to one compare" feature of GEDMatch to compare your two tests. Clicking this link will take you there:

You will see that you have a full match on all of your chromosomes (with a couple missing pieces, I imagine from where the types of tests differ). But basically this proves that the exact same DNA was used in both tests.

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