Genealogical Ponderings

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  1. Demystifying DNA 4: Autosomal DNA, Ancestry DNA and Family Finder tests



    In my series of posts about Demystifying DNA testing for use in family history this is probably the type of testing most people want to hear about: autosomal DNA testing. In other words, the tests used by Ancestry, 23 and Me, My Heritage, Family Tree DNA (and soon Living DNA) to find matches with close relatives, matches where the common ancestor is only 2-5 generations ago. Autosomal DNA testing is also known as a Family Finder test at Family Tree DNA and you may also find it described as “close cousin” testing.

    This is the type of testing taken by the highest number of people and the type of testing most likely to generate matches to your own known family names.

    Before we get into where to test and how to use your matches we need to look at the science some more to ensure we will get the most from the data. See the Introduction post for the general background to DNA.


    Autosomal DNA: The “Sciencey” Bit


    Remember from the first post in this series the image that showed your autosomal DNA? Here is it again:


    Human karyotype


    Ignore the Xs and Ys. We’ve talked about Y-DNA before and I’ll come onto X-DNA in a later post. Right now we are interested in the numbers 1 to 22: our 22 pairs of autosomal chromosomes.

    For each pair, one chromosome came from our father and one from our mother, in its entirety. So, for example, looking at Chromosome 1, perhaps the left hand chromosome came from our father and the right hand side from our mother.

    It is how we inherit autosomal DNA that makes it such a powerful tool for use with family history research.


    The Inheritance of Autosomal DNA


    The creation of eggs and sperm, in which DNA is passed from parent to child, occurs by a process called meiosis. During meiosis each chromosome is duplicated resulting in four copies of each chromosome, two paternal copies (in blue below) and two maternal copies (purple). DNA is then exchanged between the four copies, a process called recombination, which essentially mixes up the paternal and maternal DNA. Only one of the four chromosomes survives to be passed on in the egg or sperm. You don’t need to worry about the detail, the important part is that the two of each chromosome a parent has are mixed up so that a child receives a combination of both.


    Recombination (used with permission of


    Let’s take this a step further with an example: the descent of DNA from a couple, John and Mary. We will just look at one chromosome pair but the same principle applies to all 22 chromosome pairs. In all cases we will assume that the left chromosome came from the father and the right from the mother. So, John’s blue chromosome is from his father and the purple from his mother, and so on.


    Autosomal DNA descent – 1 generation


    John and Mary have two children: Thomas and Sarah. Each child gets 50% of the DNA from their father and 50% from their mother, but in different combinations. Thomas gets two sections of blue from John and two sections of purple. Sarah gets the top half of the blue and the bottom half of the purple. They have each inherited DNA from their father but they have not inherited exactly the same DNA. The way in which they inherit from Mary is also different from one another. On average siblings share about 50% of their DNA but there is a range, as we will discuss further later.

    Next we look at the case where Thomas and Sarah and have children of their own:


    Autosomal DNA descent – 2 generations


    Here we see that Robert’s paternal chromosome (the one on the left) is a combination of the two chromosomes for Thomas. He has some sections, or segments, of DNA from both of John’s chromosomes, and some from both of Mary’s chromosomes.

    Robert’s cousin Elizabeth, also has segments of DNA from all four of John and Mary’s chromosomes but Robert’s and Elizabeth’s DNA are different from one another. On average first cousins share 25% of their DNA.

    There’s a really important point to note here. A grandchild CANNOT inherit DNA from his grandparent that was not passed from grandparent to parent. Look back to the example above. Robert cannot have the top portion of the purple chromosome of John’s, because John did not pass it to Thomas. Likewise, Elizabeth cannot have the bottom section of the blue chromosome, because John did not pass it to Sarah.

    We have started to talk about the percentage of DNA you receive from different ancestors. The further back in time you go, the less DNA you share with your ancestors on average:


    Percentage of autosomal DNA shared on average with ancestors


    In fact, the amount you share with your distant ancestors eventually becomes so small that there is a chance that you will not share any DNA at all. This is the second important point: You do not inherit autosomal DNA from every one of your ancestors. Whilst all of your ancestors are included in your genealogical family tree, even if some of them are yet to be identified, not all of your ancestors are included on what we call your genetic family tree. The genetic family tree is shown below, DNA is only shared with those ancestors shaded in grey:


    The GENETIC family tree (used with permission of


    This could potentially be very frustrating if your aim was to find links to one of your great x 4 grandparents shown in white above. However, remember, the process of recombination is different each time. You may not have inherited DNA from that particular ancestor but your siblings, aunts and uncles etc may well have done. This is why it is always worth testing as many family members can you can.


    Uses of Autosomal DNA


    The primary use of autosomal DNA is for finding connections with those descended from common ancestors in recent generations, your close cousins.

    Potential uses of this type of testing include:

    • Confirming your family history research carried out so far using traditional research techniques
    • Expanding your family tree by connecting to those with whom you share DNA
    • Finding the answer to a particular problem or breaking down a brick wall
    • Use by adoptees searching for birth relatives (particularly powerful in combination with Y-DNA or mtDNA testing, as discussed in earlier blog posts).


    Who can take a test?


    Both males and females can take autosomal DNA tests and will find cousins in the same way.


    The Data


    So how do the tests work? When we talked about Y-DNA and mtDNA tests we talked about the raw data or comparing the raw data to reference standards. When your autosomal DNA is analysed data is collected at around 600,000-700,000 SNPs or positions. Below is a short extract from my Family Finder test results at Family Tree DNA. The entire spreadsheet contains 708,093 rows of data.


    Short extract of autosomal DNA data


    Rather than compare raw data the commercial companies do the data crunching for us using matching algorithms. Rather than look at the data in the form above we are presented with a lists of matches. Here’s an example, taken from the Ancestry website:


    Ancestry DNA matches


    You can see that there are a range of relationships assigned to my matches. In fact the top three matches are my father, my brother and my uncle. We have talked about the fact that on average you share about 50% of your DNA with each parent, and will share about 50% with a sibling. The average amounts of DNA shared with some of your other likely living relatives are shown in the table below:


    Amount of autosomal DNA shared on average with living relatives (from the ISOGG website)


    Don’t worry too much about what a cM is at this stage, just think of it as an amount of DNA. For each of the matches above you can click for more detail, for example my predicted third cousin match looks like this:


    Finding amount of shared DNA for an Ancestry match


    Clicking on the little “i” gives you the actual numbers. You can see here that Ancestry’s calculations have given me this match as a third cousin. If we look at the table above we can see that 147cM sits somewhere between a second and third cousin. In fact, I know this to be my second cousin once removed. As you can imagine there is a range for each relationship and the above table simplifies things considerably. If you want to look at things in more detail I suggest using this tool, which was developed using actual data from known DNA matches (click on the image for a larger version):


    Calculating relationships (used with permission of


    Even better than this though, there is now an interactive version of the Shared cM Project, where you can type in your result and see the most likely relationships. For my 147cM match (again, click the image for a larger version):


    Possible relationships with a shared match of 147cM


    If we ignore the half relationships for now for simplicity, we can see that the known 2C1R (second cousin once removed) sits nicely in the middle of the possible relationships. You can try this for any of your matches.

    Incidentally, if you get confused about second cousins, cousins once removed etc there is some useful information on Wikipedia and the following chart is also useful:


    Explaining cousin relationships


    Investigating matches


    Once you have discovered some matches to your DNA data the next step is to start to work out how they connect to your family tree. They may have a family tree uploaded themselves and you may see familiar names. You may feel sure you know where there is a link and either need to work on your tree or theirs to bring the two together.

    The ease with which you can link to your connections will depend on how extensive the research is so far by both of you and how distant the proposed relationship.

    Depending on where you live in the world your matches at the various websites will look very different. The majority of those who have tested are still based in the US so you would expect US testers to have more close matches in their results. For example, a second cousin shares a common great grandparent. I think most of us are comfortable with our family history research back to this point. There’s a fair chance we identified most, if not all of our second cousins and if we haven’t, it would not be too onerous a task. Personally I have 5 first cousins and 25 second cousins, you may have more.

    Unfortunately I do not have any matches on any of the websites that are second or even third cousin matches (apart from people I have had tested). All of my matches are around fourth cousins or more distant relationships.

    So, let’s look at the likelihood of being to identify where a fourth cousin match fits into your family tree. A fourth cousin shares one set of your great x 3 grandparents with you, but you have 16 sets of great x 3 grandparents! Let’s say each couple between then and now has had an average of two children. Starting at the most recent generations that means my mother and father have one sibling each (my aunts or uncles). If they both have two children I will have 4 first cousins altogether. Are you with me so far? Building this up gives us the following figures:



    These are just figures based on easy to calculate assumptions. If we factor in studies of population change, two separate pieces of research suggest we could have on average 940 or 1572 fourth cousins (for more information visit the ISOGG website). In my own research I’ve identified 5 first cousins, 25 second cousins, 34 third cousins and 55 fourth cousins. I feel I have some way to go!

    So if you, like me, only have more distant cousin matches you need to come up with some clever strategies to focus your efforts appropriately.


    Testing other relatives


    One of the tools offered by Ancestry, Family Tree DNA, 23 and Me AND My Heritage is the ability to view matches “in common with” another match. I have tested my father and by looking at those in common with him I can be confident that these are probably* matches on my paternal side.

    You can narrow this down even more. I mentioned my second cousin once removed earlier. He is descended from the set of my father’s great grandparents that I am particularly interested in at the moment. As we have 4 sets of great grandparents, any matches that match both my father AND this cousin can be assumed to be probably linked to only one small subsection of my family tree. This is powerful stuff!

    The potential for different scenarios here is endless.

    * WARNING: This approach only works if your family tree is straightforward. You may be related to a match in more than one way or your parents may even be related to one another. All it takes is for one marriage of first or second cousins way back in time to completely complicate your genetic family tree.


    Analyse the data in more detail


    All discussion so far has looked at just the amount of DNA we share with our matches. However, remember our discussion about how DNA is inherited. If we can start to build up a picture of where in our DNA we have particular matches we begin to develop new powers!

    Although Ancestry has by far the greatest number of testers it is the only one of the big companies offering DNA matches with autosomal DNA that does not offer a chromosome browser.

    The image below shows the chromosomes on which a match occurs between my father and his second cousin. Assuming no complications in the family tree, this is DNA that can only have come from one set of my father’s great grandparents.


    Chromosome browser showing the match between second cousins (My Heritage)


    As you test more and more relatives and identify more and more matches with known connections through traditional research you can build upon this. In fact, you can begin to work out which parts of each chromosome came from each ancestor. Think of the potential for breaking down brick walls.

    The complicating factor is that we have two of each chromosome. If my father and his second cousin have a match on a chromosome with a third person in the same area as their match with each other does it mean that all three are related? Not necessarily. This is more easily demonstrated with an image. In the image below we are comparing the DNA on a single chromosome, let’s say chromosome 5. Each tester has two copies of the chromosome, one paternal (P) and one maternal (M).


    The comparison of DNA between matches on the same chromosome


    Brian and David are related on their maternal chromosomes (in blue). Both match to Adam in the same position. However, the data is different. Brian and Adam match on their paternal sides (in red), David and Adam match each other on David’s paternal side but on Adam’s maternal side (in green). So Brian is related to David, Brian is related to Adam and David is related to Adam but they are not all related to one another from the same ancestors. Are you still with me?


    This is called triangulation: we look at the match, as above, and when we bring in a third person we check to see whether they all match in the same location on the same chromosome with the same data. (It is highly unlikely we will have exactly the same data on both chromosomes for a length sufficient to be considered a match).

    Here is an example. This is the date from my father and his second cousin again but now with data added in from a third individual. Here the lady in question matches both individuals on chromosome 7 but there is an area of overlap indicating that this data all matches and is therefore on the same side for both my father and his second cousin:


    Chromosome browser showing a triangulated segment on chromosome 7


    In Summary


    This is only an introduction to autosomal DNA testing, to give you a flavour of what can be achieved. There is much more to add and many tools and external websites that can be used to look at the data and matches in more detail.

    If you want to learn more, I am pleased to announce that I will be running a four week online course, titled Demystifying DNA for Family Historians, for Pharos Teaching and Tutoring in 2019. More details may be found HERE.


    The Testing Companies

    Autosomal tests are available at all of the big 5 DNA companies:

    Ancestry DNA:

    Offers matches and amount of shared DNA but gives no chromosome data. Data can be downloaded for upload elsewhere.

    Family Tree DNA, 23andMe and My Heritage:

    All three offer matches, amount of shared DNA and chromosome data. Each has a chromosome browser to examine data in more detail. Data can be downloaded for upload elsewhere.

    Family Tree DNA and My Heritage offer free upload of data from other companies (though you will have to pay to use all of the tools available).

    Living DNA:

    To date Living DNA has focused on providing estimates of ethnicity and its selling point is that is provides a more detailed breakdown for those with UK heritage than other companies. Data can be downloaded for upload elsewhere.

    Matches are coming soon and uploads of data from other companies are accepted.


    2 thoughts on “Demystifying DNA 4: Autosomal DNA, Ancestry DNA and Family Finder tests

    1. Ken Hunt

      Thank you for this well-presented blog. With your permission I would like to share it with my local FHS DNA support group members.
      Just one thing you might clear up for me in your example of triangulation. You write “David matches Adam on his paternal side but on Adam’s maternal side (in green). ” Did you mean that it looks like there’s a match but because the segments are different David and Adam don’t match each other?
      Thank you once again.
      Best regards, Ken

      1. Karen Cummings Post author

        Ken, thank you for your kind words. I’m more than happy for you to share the article, just so long as you credit me as author and include a link to the original post.

        With regard to David and Adam. I will tweak the sentence to make it a little more clear. David and Adam ARE related to each other but David’s father is in some way related to Adam’s mother. As it is David’s mother who is related to Brian’s mother, David, Adam and Brian are not all related to one another through the same part of the family. Does that make more sense? Karen


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