Centimorgans vs. Generations

Question from Barbara:
In the comments section of your post about Gedmatch Generations, you stated that the amount of shared DNA is more important than the generation difference suggested by Gedmatch. I am a complete newbie to this whole DNA thingy.  I have done DNA testing with a couple of the companies.  I also have a GED match number.  So I know the names of matches… but I really do not know how to determine the amount of shared DNA.

Image source: Lisa Louise Cooke, who has some excellent DNA Quick Reference Guides at her website.

Reply:

As seen in Lisa Louise Cooke’s summary, all of the companies give you a total amount of shared DNA and also how many segments of DNA that is added up from. In Ancestry, as shown in my screenshots below, you can find the total shared DNA by clicking on the little “i” icon on the match’s page.

Your DNA is measured in “centimorgans”, written as cMs. Just like a piece of string can be measured in centimeters (cms), so you can imagine your DNA as a long string of code, labelled with a four-letter alphabet A, G, C, T, that can be measured in ‘centimorgans’. Your own string of DNA will be unique to you, with the sequences of that four-letter DNA alphabet coming from the DNA signatures of your ancestors. When someone else shares a segment of that ‘signature’ that is identical to yours, then you know you both inherited that bit from the same ancestor. That shared bit of DNA signature is measured in centimorgans (cMs) – the more cMs you share with someone, the nearer to you both is the common ancestor you share.

That total amount of cMs that you share with your match, will be the sum of adding up all the little bits of that shared signature that occurs on each of the 23 chromosomes you both inherited from your parents. That is, the amount of DNA you share with your match doesn’t just occur in one place somewhere on one of those 23 chromosomes – it is added up from lots of little bits of shared ‘signature’. That is why most of the companies also tell you how many ‘segments’ of DNA you share with your match.

For example, with my Uncle Trevor, I share a total of 1662cMs, which is the total from adding up all the of shared DNA on 64 separate bits, called ‘segments’, on all of my chromosomes.​ Uncle Trevor and I share the same grandparents.

However, with my fourth cousin once removed, Jack, I only share 8.7cMs, which is on just two places somewhere on those 23 chromosomes.​ The common ancestors that Jack shares with me are my GGGG-grandparents!

Now, if I didn’t know how I was related to Uncle Trevor or Cousin Jack, I would use that total amount of shared DNA to give me some idea about how we are related. I would use this chart from Blaine Bettinger to find out how many cMs I share with my match, compare it to the chart and then try and figure out how we might be related.

With the large amount of shared DNA of 1662cMs, the possible relationships are quite narrow, but with shared DNA of just 8cMs, the range of possible relationships becomes…well, a LOT 😮 And this will present greater challenges in trying to find our connection.

The problem with the generations amount given on Gedmatch is that there is no ‘one-size-fits-all’ answer for DNA – there may be a variety of possibilities that depends on a variety of factors such as the comparative ages of the matches (for example, is your match a generation older or younger than you?).

This range of possible relationships, based on the total amount of shared DNA, is your starting point for investigating how you are related to your match. Your research now involves using the traditional genealogical tools of comparing trees and engaging in searching for archival connections. You are then on your way to becoming a DNA Detective 🙂