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.


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 🙂

Test with Ancestry first and then transfer?

Economically, it seems the best strategy to test with AncestryDNA first and then transfer your data over to Family Tree DNA and My Heritage for free.

However, this is not necessarily always the best approach, as shown in this blog post from Roberta Estes:
Good advice from an expert 🙂

Rule #1: Read the fine print

I’ve had a number of enquiries regarding the security of getting a DNA test done and also of uploading DNA data to third-party sites. This is a matter of concern for many people and is too big to tackle in a single post. I will plan on giving a presentation about security at one of our future WAGS DNA meetings.

For now, I would like to draw your attention to recent discussions that arose out of the My Heritage offer for free DNA uploading. There are two excellent blogs – which are excellent to subscribe to for all their entries – that have given very useful overviews to the security issue in the past couple of weeks.

Rule Number One: READ THE FINE PRINT of anything you ‘consent’ to.

The first article is Robert Estes, DNA Explained, which is a very long and thorough post about the importance of reading the fine print in the terms and conditions and also in the consent information. The main issue she addresses is the fact that pharmaceutical companies make profits from your DNA which can be sold on – in anonymous data.

The second is Judy Russell, Legal Genealogist, who addresses My Heritage specifically but it includes raising awareness of security issues in general.

And always refer back to Rule Number One!

Here’s a chart Roberta has made that compares the companies, which shows that if you are really concerned about security and sale of your DNA data, then Family Tree DNA and Gedmatch are by far and away the safest and do not sell your DNA.


* – Both 23andMe and Ancestry appear to utilize all clients DNA for anonymized distribution, but not for identified distribution without an individual opt-in.

*1 – According to the 23andMe Privacy Policy, although you can opt in to the higher level of research testing where your identity is not removed, you cannot opt out of the anonymized level of DNA sharing/sale. Please review current 23andMe documentation before making a decision.

*2 – Can Opt in or Opt out.

*3 – Can opt out of non-anonymized sales, but not anonymized sales. Please verify utilizing the current Ancestry documents before making a decision.

*4 – DNA.land indicates that you can withdraw consent, but does not say anything about deleting your DNA file.

*5 – DNA.Land states in their consent agreement that they will not provide identified DNA information without first contacting you.

*6 – At 23andMe, deleting DNA from data base closes account.

*7 – Automatically opted in for anonymized sales/sharing, but must opt in for identified DNA sharing.

*8 – 23andMe has been and continues to experience significant difficulties and at this point are not considered a viable genetic genealogy option by many, or stated another way, they would be the last choice of the main three testing companies.

*9 – All legal action must be brought in Tel Aviv, Israel, individually, and not as a class action suit, according to item 9 in the DNA Terms of Use document.

*10 – Website in Chinese, information through an automated English translator, so the information provided here is necessarily incomplete and may not be entirely accurate.