In this blog, I’ll write about false DNA matches: what they are; how to find them; examples – some from the Frazer DNA Project I am working on.
What Are False Autosomal DNA Matches?
False DNA matches are those that are not Identical By Descent (IBD). Perhaps you have heard the term. It basically means that the match is not from a person that is your ancestor. That sounds like defining something by what it is already. And it kind of is. A false match is also called Identical By State (IBS) or Identical By Chance (IBC). These are two different names for pretty much the same thing. It basically means that when the computer generated your match it wasn’t from an ancestor. ISOGG has a good article on the subject.
How Can I Tell If I Have a False Match?
There are several ways. I’ll list a few. I will give examples later in the Blog. The first list is more sure fire, but involves additional testing of parents or other relatives.
FINDING FALSE MATCHES BASED ON ADDITIONAL DNA TESTING
- If a person matches you but doesn’t match your mother or father’s DNA results, that is a false match. As you got all your DNA from your parents, this has to be a false match.
- Conversely, if you match someone else but don’t match their mother or father’s results, you have a false match.
- This is similar to the above. There is a way to phase your own results if you have had one or both of your parents tested. If you do not match on the phased (that is maternal or paternal) portion of your results, then it is most likely a false match.
- The last method has to do with chromosome mapping. I have written some about this in the past. If you have mapped your DNA to one grandparent, and the match is in the same area of your chromosome, from a different grandparent, then that has to be a false match. I’ll give an example later. There are 2 ways to do this mapping. One way is to test a lot of relatives and map their results to a common ancestor. Another way is if you have 2 siblings tested in addition to yourself, it is possible to figure out from which of your 4 grandparents your DNA came from. This method has been pioneered by Kathy Johnston.
WHAT IF I DON’T HAVE PARENTS OR OTHER RELATIVES TESTED?
- Testing parents is the best way. Then it is good to test other relatives. If that is not possible, then one may look at statistics. Many of the statistics are at the ISOGG article I mentioned above
- 15 cM or greater match – these are considered to be all good matches
- less than 5 cM – very few at this level will be considered true matches. ISOGG reports that about 85% of matches at this level are false. So it’s better not to go there.
- Triangulation – this is a way to determine true matches (or IBD). I have read that any match 5 cM or greater that triangulates will likely be a true match. In my experience only the larger cM matches tend to triangulate, so for me, this is a self-fulfilling prophecy. I won’t get into the triangulation aspect much in this blog.
My False Match With Deb
Deb was one of the first false matches that I was in touch with. I had thought that perhaps we had colonial ancestry. We shared many colonial ancestors including some of the Pilgrims from Plymouth, Massachusetts. She mentioned that she had her parents tested also. This would have been helpful to find out which side we were related on. However, I matched neither her mother’s nor father’s results. So it had to be a false match. Here is how we show to match at Ancestry:
It looks legit. It even says that Deb and I have a Shared Ancestor Hint. But in this case it is a bad hint. Another clue that this might be a false match is that the match is fairly low. At Ancestry, they use a filter and the match was only 6.0 cM. Here she is on my spreadsheet.
The matches in my spreadsheet are above the thresholds for FTDNA and Gedmatch. The lower number is phased to my father’s side, so one would think that the match would be good. However, my paternal phasing is based on a test with my mother. These phasings are not 100% accurate apparently. Deb also matches with my 2 sisters. In addition, she matches my two sisters on the X Chromosome. Apparently, these are all false matches. I have also read that many female X Chromosome matches are false. I suppose these are two examples. The bottom line is that I don’t match Deb’s parents and my sisters don’t. So these cannot be real matches.
Another False Ancestry Match
I have another example that I just thought of. I have another Shared Ancestor Hint. This one is on my mother’s side. It is based on an AncestryDNA match between Kay and myself. Kay also matches my sister Heidi but not my sister Sharon. So Heidi shows this same False Shared Ancestor Hint.
This match is down to 5.4 cM at Ancestry with their fancy filtering methodology. Unfortunately, Ancestry apparently doesn’t have the technology to check that even though my mother tested with AncestryDNA, my mother doesn’t also match Kay – at least not by DNA. However, Ancestry clearly shows that Kay and my hint’s line to me is through my mother. So this is a false match. Ancestry is wrong again. However, they do have a lot more money than I do.
Frazer False Matches
I have perhaps more experience with the Frazer side of my DNA than other DNA having worked on the Frazer DNA Project for a while. There are also false matches within that project. Here are a few false matches on Chromosome 7 between my two sisters and Frazer DNA Project Members. My sisters are SH and HHM.
HERE’S HOW I FIGURED OUT HOW THESE 2 FRAZER MATCHES WERE FALSE
Jane and MFA are in the Frazer DNA Project. In fact they have great matches elsewhere and even triangulate. So why am I calling these matches false? The main reason is the Chromosome Mapping I have done. This was done by a method I have described in previous posts. Three siblings are compared (my 2 sisters and me). Crossover points are determined. Here is what my Chromosome 7 looks like.
I have a cousin on my mom’s side who tested (in green). Her match at 56-75 with my 2 sisters and me ensures that the maternal side is on the top of the 3 DNA bars. This is because at that location (56-75), there is only one color that all the siblings share (green). That means blue and purple represent DNA from my paternal side. Blue is from our Hartley grandfather and purple in this case represents my Frazer grandmother. The numbers represent matches with relatives who I have had tested that are related to two of my four grandparents. In this case, the relatives matched my mother’s father (green) and my father’s father (blue).
SHARON’S FALSE FRAZER MATCH
My sister Sharon’s DNA is represented by the first horizontal bar. She has blue Hartley DNA from the beginning to point 129,000,000 (or 129 for short). At that point from 129 to the right end, the DNA from Sharon’s Frazer grandmother takes over. 129 is the crossover between where she gets her Hartley DNA to where she gets her Frazer DNA on Chromosome 7.
I have that Sharon matched Jane from the Frazer DNA Project from 98 to 107 for 7.6 cM. However, this cannot be a Frazer match as Sharon got all her DNA from the beginning of her paternal side to point 129 from her Hartley (non-Frazer) grandfather.
HEIDI’S FALSE FRAZER MATCH
Likewise, my sister’s match with MFA of the Frazer DNA Project is also false. Her bottom bar is all blue which means she has all [non-Frazer] Hartley DNA. There is no room for her to match MFA from the Frazer DNA Project from 130 to 135. In fact, Heidi has a match with her reference Hartley relative from 134 to 139. What the map shows above is that you cannot get DNA from 2 different paternal (or maternal) grandparents at the same location. It has to be either one or the other.
Interestingly, these false matches happened in the places where they could not have happened. If they were to have been real matches, they could’ve happened with me (Joel) as I have more purple area on my bar above. Or MFA could’ve had a true match with Sharon where she had some purple room, rather than with Heidi – which is a false match.
So Are False Matches Good Or Bad?
They are neither good nor bad. However, if you have a match that is false and you think it is true, then that could be bad. That would lead to wrong conclusions. Notice that in the above example, both the matches were just above the Gedmatch 7.0 cM cutoff. Just because a match is above the cutoff, doesn’t mean it is a real match. That level was chosen because there are probably more true matches than false matches at that level, but there are still a lot of false matches around 7 cM. Gedmatch and testing companies don’t generally like to filter out matches that could be true.
- It is good to be aware of (and wary of) false matches
- Just because a match is above a threshold doesn’t mean that it is a true match
- Matches below a threshold could be true also, but the odds are against that
- False matches do not triangulate
- False matches do not match either of your parents’ DNA
- Neither do they match either of your matches’ parents’ DNA
- False matches may match a phased kit of your own DNA as phasing a parent based on another parent’s testing is not 100% accurate
- If a match doesn’t match your paternally or maternally phased kit, it can be considered false
- A low match level means high likelihood of false matches; a high match level means a high likelihood of true matches
- At about 15 cM there should be no false matches
- Don’t blindly accept AncestryDNA Shared Ancestor Hints.