• Andy Robson

Making Sense OF DNA Matching


Your DNA matches can be an incredible tool to help you make new discoveries about your family history, break down genealogy brick walls, connect with new relatives or dig deeper into research you’ve already done.


DNA matches are measured in centiMorgans (cM). If you did a DNA comparison with yourself, the match would be around 7,400cM. The number of cM’s for a match then becomes progressively smaller as the relationship to yourself becomes further away.


Each of our parents passes around half of their DNA to us. So, our father, for example, has around 7,400cM worth of matches in his total genetic make-up. Around 3,700cM of this will come down to us. But around 3,700cM will not. This is why a parent will get far more hits on their DNA matches than you will. The additional matches are being made to the half of your parent’s DNA that has not been passed down to you.


If fact, it’s even more complicated than this! DNA is not passed by any kind of strict rule. Instead, it is a somewhat random process. So, we don’t actually get 50% of our DNA from each parent. Rather we might get 48% from one and 52% from the other. As we move back through the generations, this effect is amplified, some DNA being preserved, other DNA being reduced or even eliminated.


It has been calculated that if you go back to your 4 x Great Grandparents (all 64 of them), around half of them will either not have contributed anything to your DNA at all or have contributed so little that it cannot be detected!


And then, of course, you might have multiple relationships in your ancestry. A 3rd cousin in one path through your Tree might also be a 2nd cousin through another path. This means that this person makes 2 contributions to your DNA rather than one, and so their relationship to you looks a lot stronger than it actually is.


All this means that DNA matching is not a simple matter of yes/no, black/white decisions. Rather they are might-be’s/possibles, in other words, shades of grey.


This is why a given genetic relationship is indicated by a band of possible cM values rather than a definite one. So, for example, a 2nd cousin relationship could be indicated by anything from 43 to 504cM. And that’s a quite massive range. Usually an ‘average’ value is also provided. But it must be realised that this is just a guideline, it can’t be taken as an absolute value for that relationship.


These bands and averages are constantly being ‘tweaked’ based on real-world values and are therefore getting better all the time. However, the science simply isn’t there to allow the definition of matches to be as accurate as we’d all like it to be. It is important to realise that the number of matches you obtain for your DNA test is a matter of luck. In other words, it is simply chance how many people who are genetically related to you have taken a DNA test. No test, no match!


And the same goes for the quality of your matches. Read a lot of pieces about DNA matching and you’d think you should have a host of first and second cousins fighting for your attention. I’m afraid not. You might get lucky, but most people I deal with have nothing better than a couple of 2nd-3rd cousin matches and a lot of 4th-6th cousins.


Ancestry publish the following Table as a guidance for the accuracy of your matches based on the level of cM. In other words, how do I know that this person matched to me is genuinely a recent genetic relation? Or is the DNA ‘match’ simply a matter of coincidence so that the relationship to you, if there is one, is much further back in time than is being suggested.



To complicate matters, the DNA we share with our matches could either be a single, long strand. Or multiple short strands. Obviously, there is far less chance of a long strand of, say, 10cM being a ‘coincidence’ than 10 short strands of 1cM. So, we are talking about a band of ‘acceptable’ matches rather than a hard cut-off.


However, as a general rule I wouldn’t consider any match less than 30cM for the purposes of finding lost ancestors.


The level of match, 4th-6th cousin, for example, provided by Ancestry or another DNA matching site is also just a guideline. In fact, a given level of cM can indicate a whole host of genetic relationships.


So, for example, a match of 100cM could realistically indicate any of the following relationships:


1st cousin, 3-times removed (1C3R)

2nd cousin, once removed (2C1R)

2nd cousin, twice removed (2C2R)

2nd cousin, 3-times removed (2C3R)

3rd cousin (3C)

3rd cousin, one removed (3C1R)


And that’s without considering half relations!


After establishing your DNA matches and giving guides to what the genetic relationship might be, Ancestry will try and match your Tree to that of your matches’ Trees.


This is an extremely powerful feature both for confirming your research and for confirming that your relationship ‘on paper’ really exists.


To make a match, Ancestry must find the same individual in both your own and your match’s Trees. And it has very clearly defined algorithms for confirming that it is the same person. This is an important point to remember. The two Trees might be very closely related, but if Ancestry can’t find a matching person, then it doesn’t make a Tree match.


There have been many times when I’ve slightly extended a sibling’s family on a Client’ Tree. Or taken the match’s Tree back a generation or two. And suddenly the Tree match becomes detectable.


However, Ancestry can only match what it can see. If your matches have only very small Trees, or perhaps no Trees at all, then Ancestry can’t define a Tree match no matter how strong the DNA evidence is.


So, once again, it all comes down to luck. If you have very few ‘Tree matches’ among your DNA matches, it might have absolutely nothing to do with the extent of your Tree or the validity of your research. It might purely be that not many of your genetic relations have created a significant Tree. So, the Tree match can’t be made.


And it should always be borne in mind that your match’s Tree might not actually be correct. Sadly, from personal experience, that is the case depressingly often.


The book I always recommend if you’d like to know more about DNA in Genealogy is ‘Tracing your Ancestors using DNA – a guide for Family Historians’ published by Pen and Sword Books. This is a series of essays written by experts in the field and edited by Graham S Holton.