I have a match on Ancestry and it says we don’t share any matches.  How could that be?


I see this question come up several times a month in the various Facebook genealogy groups.  Well, there are four scenarios that can give rise to this situation, so let’s go through each of them.

1.      Ancestry sets a threshold of 20 cM for displaying shared matches.  That is, the shared match would have to share at least 20 cM with you and 20 cM with your match to be displayed as a shared match.  So if you’re down in the weeds of your matches and looking to find shared matches with donors who only match you at 10 cM, it’s possible you might find some, but what’s also pretty likely is that the segment that you inherited from your common ancestor is smaller than 20 cM, and while there may be many donors that match both you and your match on that segment, none of them match you at 20 cM (and most probably don't match your match to that degree either).  See the below diagram that explains whether Ancestry will display match B as a shared match between you and Match A.
Only the outcomes shaded in green will result in Ancestry displaying Match B as a shared match between you and Match A.


2.      In our second scenario, you’re not down in the weeds (so you think).  Your match comes in at say 22 cM per Ancestry's matching algorithm.  However, the actual segment you share with your match might only really measure 17 or 18 cM.  What kind of double-talk is that?  Well, measuring cM is tricky.  When we compare two un-phased kits, often the apparent length of the match greatly exceeds the actual length of the matching segment due to a concept we refer to as fuzzy ends.  What happens is that the core of the segment is truly identical by descent (attributable to DNA inherited from a single common ancestor), but the match seems a bit longer on each end due to a couple cM of coincidental matching where your paternal copy of your chromosome matches your match’s paternal copy for a little bit, and then it matches his or her maternal copy a little, and then maybe your maternal copy matches his or her paternal copy, etc., and all of these tiny matching regions are purely coincidental.  They just make the match look bigger.  This brings us back to #1 because you really were in the weeds, but you just didn’t know it.  See the below diagram for a visual representation of what fuzzy ends are all about.


3.      A third possibility is that there is very low “match density” on the segment you share with your match due to the segment having been inherited from a recent immigrant to a place where DNA testing for genealogical purposes is popular.  So you and your match may have a solid IBD (identical by descent) match at 21 or 22 cM, but that shared segment was inherited from an ancestor who lived in a part of the world where very few people have tested their DNA (and most of that ancestor’s descendants also remained in that part of the world).  You will see this a lot, for example, when this segment was inherited from a recent immigrant from Germany or Lithuania or France.  If you do a Tier 1 “Segment Search” on GEDmatch, you will see that some segments just have very few matches, or even none, for this reason.

4.      Your common ancestor just didn’t have a big family.  While this is least likely to be the answer, it does happen.  Maybe your common ancestor only has ten thousand living descendants, of whom 10 inherited the segment that you also inherited (all the rest inherited segments elsewhere on their genomes or none at all), and of those, only two have tested with Ancestry (you and your match!).

Related Issues


Quite often, when I see this type of post, I also see one of the following two added statements (or variants thereof) that are worth addressing.


The first of these is “I’ve tested a paternal relative and a maternal relative, so shouldn’t they match one or the other?”  Well, no, not really.  I think because Y-DNA and mitochondrial DNA were popular before the emergence of commercial autosomal DNA testing, a lot of people don’t understand that there are more than two “sides” to a family tree when it comes to DNA matches.  Autosomal DNA can be inherited from any branch of your tree, not just your direct paternal or maternal lines.  So your match in question might be on your father's mothers' side, whereas the relatives you tested were on your father's father's side and your mother's father's side.  Therefore, there's no reason other than sheer coincidence that the match would match either of your tested relatives.


Sometimes the poster adds something like this:  “I’ve tested both my father and mother’s full sibling, and between the two, that should cover all my ancestors, so shouldn’t one of these two show up as a shared match at least?  If they're not related to either, how are they related to me?”  How to tackle this one is not obvious unless you've studied a bit of biology or genetics.

The answer is that DNA is inherited in discrete blocks and segments, and not as a mixture or "soup."  While your mother’s full sibling has the same ancestors as your mother, they will not inherit all the same segments of DNA from those ancestors.  So you might have a maternal match that your maternal aunt does not share with you because she did not inherit the segment that you share with your match.  You may have inherited, for example, a segment of DNA from your maternal grandfather across this span of chromosome, whereas your aunt may have inherited a segment from her mother (your maternal grandmother), or may have inherited paternal DNA but from the opposite paternal grandparent (Your DNA might be from your maternal grandfather’s father and hers from your maternal grandfather’s mother).  Therefore, your maternal aunt will not share DNA with this match while you do, even though the match is in fact maternal to you.  In fact, statistically speaking, this will be the case with almost half of your distant maternal matches (that they won’t also match your aunt).  This might be a fun calculation for a future post, but we had enough fun with math last week I think!

If you've read this far, I hope you'll check out Borland Genetics, the first DNA database dedicated to DNA reconstruction.  Click here to learn more:  www.borlandgenetics.com

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