A False AncestryDNA Shared Ancestor Hint Analyzed

In this Blog, I would like to look at a Shared Ancestor Hint at AncestryDNA that appears to be a false DNA match. Deborah matches myself and my 3 siblings in the 6 cM range according to AncestryDNA using their own method which is supposed to take out false matches. Deborah’s match also results in a Shared Ancestor Hint (SAH) with myself and each of my 3 siblings:


This SAH looks fairly straightforward. It shows we have colonial Massachusetts ancestors that lived in Plymouth. Here is my phased paternal match with Deborah at Gedmatch:


These results were also consistent with the visual phasing I did between 2 of my siblings:


The above Chromosome 13 map shows that the match with Deborah was in my Hartley grandfather’s region to the right of each blue bar. The first bar shows Sharon’s DNA, the second shows mine and the third is my sister Heidi’s.

This was initially exciting. I now had a chance to identify specific DNA and assign that DNA to a specific set of colonial ancestors. I contacted Deborah and found out that both her parents were tested also and  those results were at Gedmatch. She affirmed that the match would be on her mother’s side (based on genealogy). I compared my kit with Deborah’s mother’s and got no match. That appeared to prove that the Ancestry Shared Ancestry Hint was incorrect. The DNA that Ancestry matched with Churchill and Burbank could not be right.

This lead me to believe that I matched on Deborah’s father’s side. I checked that match at DNA which came up a blank. I wrote back to Deborah to say that this looked like my first confirmed Identical By Chance match.

Just to make sure, I compared Deborah’s family and my family:


What this shows is that neither I nor any of my siblings match Deborah’s parents. That is shown by the blank pink areas in the upper right and the lower left portions of the chart. Again, that means our match is not real. Yet, Deborah shows up as having a Shared Ancestor Hint for me and my 3 siblings. What is also interesting is that my sister Sharon has a 21.6 cM false match with Deborah. However, when I compare Sharon and Deborah ‘One to One’ at Gedmatch, I get a more reasonable result:


I suppose Sharon and Deborah share smaller segments. I checked again with a 5 cM threshold and indeed Sharon and Deborah share 2 more segments between 5 and 7 cM.

Any False Triangulation?

One other way to check Deborah’s match with my family is to see if she triangulates. I compared Deborah and Sharon in the Utility at Gedmatch that shows others that match both Sharon and Deborah. Here is the results of those matches at Chromosome 13 as shown in a browser:


Deborah and Sharon’s match is the blue top right one. Here there is clearly no indication of triangulation with Sharon and Deborah.  I would expect many other matches lining up below the top right blue match bar if there was any triangulated matches. This is further indication of a false match.

Summary and Conclusions

  • Ancestry is wise in not supplying me with a Chromosome Browser as it would prove some of their Shared Ancestor Hints as false.
  • A corollary would be, if you don’t want to prove AnceteryDNA Shared Ancestor Hints wrong, don’t use Gedmatch to compare your results
  • As others have noted, it is not enough to match someone through your phased kit. You also have to match the other person’s phased kit (or one of their parents) to be a real match.
  • This analysis applies to a relatively small match. This Shared Ancestry Hint was also at or near the bottom of the AncestryDNA list.
  • Be wary of the smaller matches. Focus on the larger ones unless you have good analysis such as triangulation to verify a smaller match.


3 Replies to “A False AncestryDNA Shared Ancestor Hint Analyzed”

  1. Your Article is thoughtful, but I am left wondering:

    1. Since Ancestry DNA matches are based on two things: DNA and Family Trees, could one of the trees have a bad paper trail?

    2. Since Deborah and Sharon match, could one be adopted and not know it?

    I would like to see more information from the Family Comparison Chart. are the categories across the top Kit numbers? If so, could you replace the numbers with first names?

    1. Hi Bill,

      There is always the chance that one of the trees could be faulty. However, I trust my tree and trust Deborah’s tree in this case. Neither Sharon nor Deborah are adopted. This would be really clear from the DNA that we do have. I erased the kit numbers from the top row. They are in the same order going across as they are going down. So the greyed out diagonal boxes represent each person’s match with themselves.


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