Scientific American on DNA ‘accuracy’

This is an excellent summary of the value of DNA testing as it pertains to health reporting, family history research and ‘ethnicity’.

How Accurate Are Online DNA Tests?

I agree with all that Adam Rutherford has to say in this article, in particular his assessment of the value of ‘ethnicity’ predictions:

But to say that you are 20 percent Irish, 4 percent Native American or 12 percent Scandinavian is fun, trivial and has very little scientific meaning. We all have thousands of ancestors, and our family trees become matted webs as we go back in time, which means that before long, our ancestors become everyone’s ancestors.

However, I disagree with his final comment that “DNA will tell you little about your culture, history and identity”. I have helped many people who have questions about their identity based on their search for their family connections, particularly those who do not know who were their biological parents. Also for Aboriginal Australians whose recent ancestors were removed from their Country as very young children and never knew where they came from, identifying ancestors and locating Country is tremendously important for creating as sense of kinship and identity. So knowing where you come from, which can be helped through DNA testing, is very key to self-identity.

Half or Full Siblings

Half or full siblings? Another interesting and informative blog post from Judy Russell about how to tell if siblings share the same parents or just one parent.

Half or Full…

I recently had just such a case of borderline 2018cMs, but found that, if the expected family tree is known, checking the shared matches of the two siblings will generally show if one of their parents is different, because they will have matches in common on one parent’s side but not on the other.


Racial Equality and DNA Testing

This opinion piece from Kenan Malik at The Observer, presents an interesting point of view regarding the way DNA testing can be abused to suit a racial agenda that is not in the true cause of racial equality. Although the article highlights some very valid points about the misuse of DNA testing, I don’t agree with the author’s dismissal of DNA testing as ‘pseudo-science’.

Racial equality once meant tearing down barriers, not doing a DNA test

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 🙂

DNA Down Under

The Unlock the Past event DNA DownUnder, a DNA-themed roadshow starring American DNA guru Blaine Bettinger, is now uploaded on its own webpage: DNA DownUnder.

There will events in all the major capitals, plus a three-day conference in Sydney. Presenters include Louise Coakley, Kerry Farmer, Michelle Patient, Helen Smith, and Cate Pearce (that’s me 🙂 ).

BRISBANE – Wed 14 Aug 2019
PERTH – Sat 17 Aug 2019
ADELAIDE – Mon 19 Aug 2019
MELBOURNE – Thu 22 Aug 2019
CANBERRA – Sat 24 Aug 2019
SYDNEY – Mon-Wed 26-28 Aug 2019 an extended in-depth program in Sydney

Check out the website and register your interest now 🙂