How do I Interpret my DNA Results?
Once you’ve taken the DNA test and uploaded the result you can begin your analysis. However, unless you’re a fairly experienced family history researcher it can be daunting. I know I’m increasingly being asked to guide people with their DNA matching or do this on their behalf.
The best starting point is to analyse the degree to which you share common chromosomes with your DNA matches. This is expressed in centiMorgans (cM). Every person’s DNA is approximately 7400 cM in total. Since a person inherits roughly half of their DNA from each parent, you should inherit about 3700 cM from each. However, DNA is not passed down in a regular, predictable manner, so the amount you inherit from each parent will vary. This variation increases as you move back through the generations. The expected DNA match for a given relationship between two people is therefore expressed as a band of possible cM’s rather than a single figure. Which makes the whole process a lot trickier than it seems.
As the cM match between two people gets increasingly smaller, it becomes impossible to determine whether this is actually shared DNA or whether the match is simply a coincidence. There are, after all, only so many different DNA sequences to go around.
By the time you look at your 4th Great Grandparents – from whom we nominally inherit 115cM – the uneven way in which DNA is passed down through the generations means that in actually the contribution from a significant number of them will no longer be reliably detectable. This is why Autosomol testing is only of use for matches which are separated by up to 5 to 7 generations.
If you’d like to know more about DNA and where it may be taking us from a genealogical standpoint my reading recommendation is ‘Tracing your Ancestors using DNA – a guide for Family Historians’ published by Pen&Sword Books. This is a series of essays written by experts in the field and edited by Graham S. Holton. Very knowledgeable, very comprehensive, but also easy to follow.