DNA: Proactive strategies for engaging with your match list

Today’s post is the last in my three-part mini-series on practical ways to make the most of your DNA results.  The first post, concentrating on what you can do right away, aimed to encourage you to get to know the layout of your testing company’s site and what resources there are attached to your DNA results.  The second looked at ThruLines (Ancestry) and Theory of Family Relativity (MyHeritage).  Throughout, the importance of constant reference to your family trees is emphasised.  DNA works alongside the genealogy; it doesn’t replace it.

Those first two posts focused on using the information being offered to you on a plate and trying to relate it to your own research.  In this third post it’s time to get more proactive.  Perhaps there’s a gap in your tree or a brick wall, and you want to see if you can use DNA to solve it.  Or perhaps there’s a mystery match of a decent size and you want to find where this person connects with you on your tree.  In other words, you’re now coming to the DNA with a question.

Before going any further I’m going to tell you about the limitations of my own DNA testing capability.  I have no surviving parents, grandparents, aunts, uncles or cousins, and only one brother.  Already this means I’m depending on second cousins testing.  Now as it happens I do have an unusually large number of second cousins.  One great aunt alone was the matriarch of eleven children and about 48 grandchildren.  But although I’ve had contact with some, I’ve never met any of them and certainly couldn’t approach any to ask if they would oblige by spitting into a tube for DNA testing purposes to serve my whim.  You’d think that out of so many second cousins some of them might have taken a DNA test anyway.  Well yes, I know of three.  I make good use of the results of two of them, but the other’s results are private.

Whenever I read books or blogposts from respected DNA authorities I’m in awe of the number of family members for whom they have DNA results.  It’s plain that some buy DNA tests in bulk when they’re on offer and simply dole them out to family members, all of whom willingly oblige.  Add into this mix the fact that DNA testing is far more popular in the US than it is in Europe, and you might start to get an idea of the gap between the kind of results these DNA gurus work with and the paltry results available to me.  Apart from my brother and the two second cousins, my closest matches are at 3rd cousin once removed distance and then we’re down to 4th cousins and a lot of matches in the 30-40cM range.

What all of the above means is that I have to work a lot harder to get answers from my DNA.  It also means that with perseverence, it is possible.  This genuinely is a case where if I can do it, so can you.  In fact chances are your matchlist will be stronger than mine.

So with all that in mind, here are some proactive ways you can engage with the information available on your match list and in the attached trees, and use this to confirm your research and/or break down brick walls.

Filtering
You can use the filter bar at the top of your match list to home in on focused information.

At Ancestry you can home in on unviewed matches; matches with common ancestors; matches with public / private / linked / unlinked trees; people you have already messaged, added to a group or made notes for; you can filter by relationship or shared cM; by date of test; and you can search for matches by name or with specific surnames or specific birthplaces in their trees:

Ancestry's DNA matchlist filter bar

MyHeritage allows you to filter by tree details (Theory of Family Relativity; Smart Matches; shared surname or birthplace; has a tree); by proximity of relationship; by country and ethnicity.  You can also sort by segment information, full name, and in recent order of testing.  And you can search by name or ancestral surname:

My Heritage filter bar for DNA matches

I’ve had some success using surname filters to find a common ancestral line, and also using birthplace filters to try to home in on a likely geographical area within Northern Ireland for an ancestral line with records suggesting two conflicting places of origin

Building trees
If you have a decent match that you don’t recognise and they have the beginnings of a tree attached to their results, you can try working their tree back yourself.  What you consider a ‘decent’ match will very much depend on how many close or extended family members you have on your matchlist.  You may, for example, consider that anything less than 80cM isn’t worth your time.  I will do it for much lower matches.

By way of example of what is possible.  I’ve done this, and found my connection with:

  • A 73cM match with only seven surnames (no first names) on the tree.  Our connection is 3C1R
  • A 55cM match with only the name of my match and the name of the person managing the DNA test, who was her son to, work with. Our connection is 4C1R.
  • A 53cM match with six entries on a tree – two of them private, one ‘unknown’, one with only a first name (which was not entirely correct), and two who were known by names other than the ones given at birth (!)  Our connection is 4C.

It isn’t always possible, but it often is.

If you want more ideas on how to progress these ‘Quick & Dirty’ trees, the following video (18:58 mins) might help get you started: Blaine Bettinger: Building Quick & Dirty Trees to Identify Genetic Matches

Remember that if you have close matches and both your trees are well-developed but you can’t find your common ancestors, then either your tree or your match’s tree is wrong, doesn’t go back far enough or sufficiently wide (siblings, half siblings, etc) OR you have uncovered a misattributed parentage in one of your trees. See my previous post about unexpected test results.  I have also started to look wider in my own tree building, bringing more lines forward in the hope of ‘meeting’ ancestors of distant cousins who haven’t yet been able to find their way further back.

Clustering
‘Clustering’ is the term used for grouping your DNA matches into groups using the ‘Shared Matches’ tool.  The idea is that the resulting ‘clusters’ will represent the distinct lines of your own family tree.  Clustering was developed  in 2018 by Dana Leeds.  Her technique, which became known as ‘The Leeds Method‘ uses a spreadsheet and you can read all about it on her website.

In 2019 MyHeritage introduced an Autocluster tool, based on the same principles, but saving a lot of time by generating the clusters for you at the click of a button.

Since then Ancestry have introduced a colour grouping facility to their match list and a system has been developed for using these as a clustering tool.  You can see this in operation in the following video: Larry Jones: How to Cluster your DNA matches With Ancestry’s New DNA Matches

The idea is that, by pointing to a common ancestral line, clustering narrows down where you have to look for your connection to these matches.  You can then focus on each of these family ‘clusters’ as a whole – look for connections in trees, perhaps even build one ‘Quick and Dirty’ tree for each cluster rather than a separate one for each mystery match you want to explore.

A problem I have with all of these clustering techniques is that I don’t have enough close matches to be able to set the systems up.  Dana Leeds bases her method on first cousin matches.  Larry Jones’s system on Ancestry is based on second cousin matches.  I don’t even really get going until 3rd and 4th cousin matches.  Nevertheless, I do run the Autoclusters report on MyHeritage from time to time, and I do make use of the colour groupings on Ancestry.  I have eight colours: one for each great grandparent, and I add matches to these groups either when I have a confirmed match or when, based on shared matches, a connection looks likely.

Asking family members to test
Back in July I wrote about how our family members’ test results can help us in our research. In that post I wrote about the different ways our various family members’ DNA can help us to isolate the branches of our tree.  If you’re lucky enough to have relatives who will take a test for you, that post will help you work out who to ask.

Chromosome mapping
If you tested with 23andMe or if you tested / uploaded your DNA results to MyHeritage, FTDNA or GEDmatch, you’ll be able to view your matches in a chromosome browser.  This takes us into a whole new range of possibilities for working with our DNA, and I have another little mini-series of posts about this planned for the spring of 2021.

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My aim for this mini-series of posts about practical application of your DNA results has been to provide sufficient basic information to enable you to start to work with your DNA and then to be able to ask informed, focused questions as you need to.  And trust me – you WILL need to!  I hope you’ve found it helpful.

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Note
My posts about DNA are aimed at complete beginners and aim to provide information in manageable chunks, each post building on previous ones. Click [here] to read all of them in order, or to dip in and out as you wish. You’ll also find lots of resources, useful links and book recommendations.

DNA for genealogy: Where to test?

IMPORTANT: This is not a post about testing for paternity issues, etc.  The courts have very specific requirements for DNA testing to be used in legal hearings.  You can find out about that on the Get a DNA Test page of the government’s own website.

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Which companies offer autosomal DNA testing for genealogy?
Having spent the last few weeks introducing the topic of DNA testing for genealogy, the purpose of this post is to signpost you to the five main companies used for genetic genealogy testing by genealogists and family historians in the UK.  Other companies are available and if you come across them you can research and assess their benefits for yourself.  However from my knowledge, gained through personal experience and through membership of online discussion groups, these are the companies most genealogists currently use.  I have no connection with any of these companies other than as a user, and am receiving no benefit whatsoever for including any of them in this review.

If you’d like to take a DNA test to help with your family tree building, follow the links to each company, look at their websites including privacy statements and terms & conditions, and then make your decision.

You might also like to look at the following DNA Weekly Best Ancestry DNA Tests review, which is updated regularly.

I would also recommend joining an online discussion group, such as DNA Help for Genealogy on Facebook.  There, you’ll find people of all levels of understanding from complete beginner to advanced.  You can ask questions, including recommendations and preferences for the various testing companies as well as practical questions when your results are in.  Somewhere down the line you’ll find you can actually start to answer other people’s questions too. 🙂

Here are the five testing companies, linked to their websites:
Ancestry DNA
Family Tree DNA
Living DNA – UK based, partnered with FindMyPast
My Heritage DNA – my experience is that more European testers use this company
23 and Me

How do they differ?
I’ve put together this table showing features of each testing company that are considered important by genealogists.

Table showing features of five different DNA testing companies used by genealogists

Notes
When considering the differences between these five testing companies and the advantages or disadvantages of each it’s perhaps useful to bear in mind that two of the companies (Ancestry and MyHeritage) are primarily genealogy websites, providing tree-building, a huge number of record sets, and a DNA testing service that is increasingly dove-tailed into that.  One of the companies (Living DNA) has partnered exclusively with FindMyPast.  Together, these UK-focused companies have the potential to provide a similar ‘seamless’ service as for the previous two, with a lot of new developments in the pipeline.  The final two companies (Family Tree DNA or ‘FTDNA’ and 23andMe) are primarily DNA research and testing companies.  They have the facility for uploading or linking to family trees but have no record sets, etc that will help you to develop your trees.  However, their DNA features and tools are often more sophisticated.

Uploading to other sites: You will see that Ancestry and 23andMe do not permit uploads to their site, but the remaining three companies do.  Uploading will enable you to access the tester database but will not provide use of enhanced features of the test (e.g. Living’s 21 UK-based geographical origin locations feature).  Although uploading is free there will be a charge if you want to access additional tools.  If you’re looking for biological parents it will help you to have your data on all of these sites.

AutoCluster tool: This is a tool available on MyHeritage.  It groups together your DNA matches in colour-coded groups likely to be descended from the same common ancestor.

Ethnicities: A lot of people take a DNA test purely for the fun of seeing their ethnic origins breakdown.  Learning about your ethnicities is exciting but it’s only an estimate and still a work in progress.  From time to time as more people test or as algorithms are amended, your ethnicities estimate will change.

Chromosome browsers: This is a visual tool that enables you to see precisely where you and another person match.  You will be able to see which chromosomes, whereabouts on that chromosome, the length of segments and their start and end points.  It is really useful to have this information and once you’ve been able to allocate a segment to a specific common ancestor it will help with identifying whereabouts on your family tree new matches will connect.  I will do a post about this in autumn 2020.

Y-chromosomal and mitochondrial testing: This is covered in my previous blogpost on deep ancestral DNA testing.  You will see that of the companies included on the above table, only Family Tree DNA (FTDNA) offers testing for these types of DNA.  However, 23andMe and Living DNA provide Y-chromosome and mitochondrial haplogroups as part of the autosomal test.

Note about 23 and Me tests: Ancestry + Traits is the basic test, providing an insight into which of your traits (e.g. aversion to coriander, curly hair) can be traced to your ancestry.  Health + Ancestry test is more expensive, providing insights into your predisposition of developing certain health conditions.  It is not necessary for our family research purposes, but is there as an option should you want it.

GEDmatch: This is not a testing company, but a very useful website where you can upload your DNA test results regardless of which company you tested with.  It therefore provides you with a much wider pool of testers and potential DNA matches.  It is free to use although payment is required to access certain more advanced tools.  I will do a post about this in autumn 2020.

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This concludes my series of ‘introduction to DNA testing for genealogy’ posts.  I hope they have helped you to decide if DNA testing is for you and if so, what are the next steps you need to take to make it happen. Please note that every effort has been taken to ensure all the details provided are correct, but you should refer to the different companies’ websites before making any decisions.

We’ll now take a break from DNA testing but I do have more posts planned for the future that will help you to make practical use of your DNA results.  As mentioned above, these will include Chromosome browsers and GEDmatch, but a number of other DNA topics too.

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Edited August 2020
My posts about DNA are aimed at complete beginners and aim to provide information in manageable chunks, each post building on previous ones. Click [here] to read all of them in order, or to dip in and out as you wish. You’ll also find lots of resources and useful links

DNA: Asking other family members to test

So far in this introduction to DNA for genealogy we’ve looked at what autosomal DNA is and how we use it in genealogical research, and we’ve looked at the very important issue of potentially unexpected results and the ethical considerations flowing from that. It was essential to cover the latter two issues before progressing to today’s topic: asking our nearest and dearest if they would be prepared to take a DNA test to help with our research.

A quick review of how we use autosomal DNA for genealogy
As previously discussed, the point of establishing how much of your autosomal DNA you share with a second cousin, fourth cousin, third cousin once removed, etc is not the joy of knowing how much DNA you share with this former stranger.  The point is that by finding someone you match at this estimated level you are being guided to the number of generations you need to go back to find your Most Recent Common Ancestor (MRCA). If the amount of shared autosomal DNA, expressed as ‘centiMorgans’ (cM) suggests you are roughly 3rd cousins, then you would expect to find your MRCA at roughly great great grandparent level.  As mentioned in previous posts, it may be one generation closer or one generation further back, but it will be thereabouts.  Armed with that information, you look at both your family trees to locate the common ancestor.  Then, having found your MRCA, and ensured all research is correct to that point on both trees:

  • you know your tree is correct to that point
  • you know you have a biological link to these ancestors (there are no events which would cause a break in the biological link)
  • should you come across another DNA cousin who matches the two of you, you have a pretty good idea where to look for this new match.

It isn’t always as straightforward as this.  You or your new cousins might not have got as far back as that in your tree building.  As mentioned in my previous post on unexpected results, one of you might have no tree at all, as a result of adoption or unknown paternity.  There are also other issues that might complicate this which I’ll cover in a later post.  But for now, at this introductory level, we’ll stick with a basic scenario with everything going smoothly.  The important point is that, knowing where you and your new cousins share common ancestry, you can start to allocate them to a particular branch of your tree, and work on the basis that other testers who also match the three of you will also link to you on that branch.  Bearing in mind that we each have sixteen great great grandparents and 32 GGG grandparents, this will save you a lot of unneccesary work.

Why would we ask another family member to test?
When we ask a specific, known family member to test, we use their results in the same way: It helps us to sort more distant common matches into specific lines on our own tree.  However, since these are our close family members, and since every one of them will have inherited some DNA from our common ancestors that we didn’t, their test results will extend our ‘reach’.  Exactly how they will do this depends on their precise relationship to us.

Essentially the rule is:

  • Your direct line (parents, grandparents, etc) will produce stronger, better, DNA matches but limiting to an increasingly specific part of your tree with each older generation.
  • Your own siblings will have inherited much of the same DNA as you, but also a lot of different DNA.  Although their results will be of no use at all in guiding you to a specific part of your tree (because you have the same parents, grandparents, etc), the parts of their DNA you didn’t inherit will effectively provide you with more DNA matches.
  • Sibling of previous generations (your mother’s brother, your grandfather’s sister, etc) will combine the benefits of the first two categories, but the results will be a bit weaker than testing your direct line – which of course is often no longer possible.

Let’s look at each type of relationship in more detail.

Sibling
If you have one or more full siblings, you know that all of your DNA and all of their DNA comes from the same two people: your mother and father.  However although like you, your sibling will have received half their DNA from your mother and half from your father, unless you are identical twins they will not have received exactly the same DNA as you did.  (Key point: don’t bother asking your identical twin to test)

I can illustrate all this with reference to my own brother.

In my first post about autosomal DNA we looked at the Shared centiMorgan Project.  Click the image below to see it full size on Blaine T Bettinger’s website.

This shows that the average shared DNA with a full sibling is 2613cM, but it could be anything between 1613 and 3488.  My brother and I share 2616cM.  This is what proves we are full siblings.  What interests me, though, is all the bits of his DNA that I don’t share: they are the reason I asked him to take a test.  Since we are clearly full siblings, anyone who shares DNA with him is also my blood relative even if we haven’t inherited any of the same DNA.  Therefore I can use my brother’s DNA results and the trees of his matches as an extension of my own, to confirm and develop my own tree.  Their Most Recent Common Ancestors are my Most Recent Common Ancestors too.  It’s just that we haven’t inherited the same DNA from those ancestors.

If this is new to you you’ll be surprised at how many people can share a decent amount of DNA with one sibling and none at all with another.  After each other, the top matches for both my brother and me on Ancestry are:
A with whom I share 189cM but my brother shares only 102cM
B with whom I share 144cM but my brother shares 153cM
After them our next highest matches are completely different.
C with whom I share 51cM doesn’t show as a match to my brother.  In fact C is our 4th cousin two times over – we share two sets of 3xG grandparents.
Looking at our matches on MyHeritage, four of my brother’s top ten matches (all of them cM matches in the 50s) do not match me at all.

You can see how my brother’s results give me more information and more clues about my own ancestry.  If you have more than one sibling and they are all happy to take a DNA test to help with your research, so much the better!

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While full siblings give you extra ‘horizontal’ reach, every other close family member will help you in a different way: they help narrow down to one part of your tree where you and any DNA cousin you share actually match.  Siblings won’t do this because in terms of your direct lineage, everything is identical.

Parent
There are two advantages to having a parent’s DNA results:

  • First, since we inherit 50% of our DNA from each parent, it follows that we have only 50% of the DNA of each one.  This means that your parents’ DNA is closer to previous generations and will include the other 50% that didn’t pass to you.  You might show as a second cousin once removed to a match but your parent will be a full second cousin.  Because of all this, they will have more and better matches, with more shared centiMorgans.  As an example, one of my DNA cousins currently has 321 matches on Ancestry at 4th cousin or closer, whereas her mother has 511.  If her father were also still alive and willing to test, and supposing he had a similar number of matches, that would effectively transform their daughter’s 321 matches into about 1000 better ones.
  • Second, even if only one parent tests this will help you to narrow down any future match by 50%.  If you have your mother’s test results and your new DNA cousin doesn’t match your mother then your shared common ancestors are on your father’s side.  This will help save you a lot of time searching for your connection.

Grandparent
If you’re lucky enough to have a grandparent who is able and willing to take a DNA test for you the same applies as for your parents.  Not only will their results narrow down any matches to a specific quarter of your tree, but their DNA will be even closer to previous generations.  You may match another tester at fourth cousin level, but your grandparent will be a second cousin twice removed – a much stronger and clearer DNA match.

Key point: it always makes sense to test the oldest generation

Aunt or uncle
If you’re able to test a sibling of either parent this will enable you to narrow down any shared matches to one side of your tree or the other, just as your own parent’s test results would.  However, bearing in mind that siblings don’t inherit exactly the same DNA, your aunt or uncle would also extend the reach of your parent’s results horizontally, in the same way that your own sibling would for you.

Your aunt or uncle who is the half-sibling of your parent
This will have the same effect as a grandparent.  Since only one of your grandparents is the parent of your half-aunt or half-uncle, anyone matching the two of you has to be from that specific grandparent’s line.  However, for reasons outlined above, if you had the option to test your half-aunt/uncle or the actual grandparent who is that person’s parent, you should choose the grandparent.  This would provide the same information to help you narrow down matches to a specific quarter of your tree, but their match would be closer to past generations and therefore better.

Half sibling 
Your own half sibling’s test results will help in the same way as your shared parent’s results.  They will help you to narrow down a match to either your maternal or your paternal line.  However, for reasons outlined above, if a parent is still available and willing to test, their results will be better for you.

Cousin
Your cousin, being the child of your parent’s full sibling, will help you to narrow down shared matches to one side of your tree. On the one hand, your own parent will give you better information.  On the other hand, since your cousin’s parent’s DNA will not be identical to your own parent’s, they might extend your reach horizontally on this line, just as your own sibling would.  Even better, though, to test your actual aunt or uncle.

Slightly more distant relations, e.g. second cousin
In reality, unless you have a very close extended family, you are unlikely to pay for your second cousin to take a DNA test.  However, they might have tested of their own volition, and in the absence of any of the above family members, a second cousin’s results can be very helpful in narrowing a match down to a specific quarter of your tree.  You and they are the great grandchildren of the same couple, therefore any other tester who matches the two of you will probably be further back along that same line. 

Key point: Your own son or daughter’s test results will not help you
Your own child’s test results will not add anything to your DNA research, since they have inherited only 50% of your DNA and are obviously one generation further removed from all your ancestors.  On the other hand if they are interested in their ancestry, your own test results will help them to narrow down to one or other side of their tree, just as your own parents would help you.

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If all this was new to you I hope, by now, you have a basic understanding of how DNA testing can enhance your family research.  If you’re interested in taking a test, my next post will provide an overview of the five main testing companies.  After that, there will be more to say, but we’ll take a break from DNA for a while.

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Edited August 2020
My posts about DNA are aimed at complete beginners and aim to provide information in manageable chunks, each post building on previous ones. Click [here] to read all of them in order, or to dip in and out as you wish. You’ll also find lots of resources and useful links

Using DNA testing to develop your family research

Last September I wrote about deep ancestral DNA testing using Y-chromosomal and mitochondrial tests.  I said back then that I would write more about my experience of using DNA testing alongside traditional documentary research to develop my family tree.

Today I’m going to introduce the topic of autosomal DNA, including inheritance patterns and an overview of how we can use autosomal DNA testing in our family research.  The following four posts will look at:

  • unexpected results;
  • ethical issues, particularly flowing from unexpected results;
  • the benefits of asking certain other close family members to test for you;
  • different testing companies.

There’s a lot to say and if this is new to you it will seem complicated – it certainly did for me when I first started.  I found the best way to learn was to read to understand the basics, and then just do it!  So I’ll do my best to introduce it all in small chunks.  And alongside my own discussion of the topic, and my own experiences, I’ll offer links to websites, books and other resources. There are several online groups, including on Facebook, where you can ask questions if you’re stuck, and people are helpful.  After the initial introduction of some key points over the next few posts I’ll move back to other family history topics, interspersing with more DNA posts from time to time.  I do appreciate that this won’t interest everyone, but it’s a growing and important part of genealogical research these days.

Autosomal DNA
The DNA tests we see advertised for genealogists use a different type of DNA from the two types I wrote about in that previous post.  What they test is autosomal DNA (atDNA).  This comprises the twenty-two pairs of non-sex chromosomes within the nucleus of every cell.  There is also an additional pair of chromosomes within the nucleus, which are the sex chromosomes.  Females inherit one X chromosome from their mother and one from their father.  Males inherit just the one X chromosome from their mother and the Y chromosome from their father.  As we have seen, the Y chromosomal DNA requires a completely different test.  However, some atDNA tests do include testing of the X chromosome, and this can give additional information to help us to understand which of our lines to focus on when we have a match, but for now I’m focusing on the non-sex, autosomal chromosomes.

Building on what we covered in my previous DNA post, autosomal DNA differs from mitochondrial and Y-chromosomal DNA in the following ways:

  • Mitochondrial DNA is inherited by all babies from their mother but only her daughters pass it on.  Boys, therefore, receive this DNA but do not pass it on to their children.  Y-chromosomal DNA is passed on from the father only to his sons.  Daughters do not receive it at all.  By comparison, atDNA is passed on to every child.  There are no differences whatsoever based on the child’s sex.
  • Mitochondrial and Y-chromosomal DNA mutate (change) very, very slowly.  They are passed on largely unchanged.  This means that our mitochondrial DNA (and for males, Y DNA) can connect us to specific ancestors and their kin who lived many thousands of years ago, perhaps in the Middle East, perhaps in Africa.  By contrast, the atDNA changes with every successive generation.  I’ll say more about this below, since this is at the heart of how we use it in our genealogy research, but for now, just note it as a contrasting feature with these two other DNA types.
  • The operation of mitochondrial and Y-chromosomal DNA, passing largely unchanged from parent to child, means that when we follow it backwards we follow just one line: your mitochondrial DNA has passed to you from your mother, to her from her mother, to her from her mother, and so on, back through time.  The Y DNA has worked in the same way, from father to male child, right back through time.  Those types of DNA, then, can take us on a journey up a very narrow and specific part of our family tree: your mother, your maternal grandmother, just one of your 8 great grandparents, just one of your 16 great grandparents, just one of your 32 GGG grandparents, and so on.  (And the same for Y DNA for male inheritance.)  By contrast, autosomal testing provides a 360-degree coverage of all your atDNA inherited from all of your lines.  There is no difference for children of different sexes.
  • Experts tell us that at the present time atDNA testing is accurate only for five or so generations.  I have found connections further back than that which fit with the smaller amount of DNA and with my documented family tree, and that’s good enough for my purposes, but the experts say five generations or so.

Autosomal DNA inheritance
I said above that our autosomal DNA comprises twenty-two pairs of chromosomes.  One chromosome from each pair is inherited from our mother, the other from our father.  This means that we get half our autosomal DNA from our mother and half from our father.  Obviously, each of our parents also inherits half of their autosomal DNA from their mother and the other half from their father, and so on, back through time.

This might suggest that the inheritance of atDNA is very tidy, with progressively smaller, exact fractions from each of our ancestors: half from each parent, a quarter from each grandparent, an eighth from each great grandparent and so on.  But that is not the case.  The atDNA we receive from each parent will not be an exact 50-50% split of what they received from each of their parents.  On the other hand it isn’t entirely random either: there are parameters.

When we talk about amounts of atDNA we don’t usually refer to it in percentages.  There is a unit of measurement: the centiMorgan (cM).  One of the authorities on DNA testing for genetic genealogy is Blaine T. Bettinger.  Since around 2015 he has been investigating these parameters for centiMorgan inheritance through a research project known as the Shared CentiMorgan Project. It is the go-to document for calculating likely relationships based on DNA.  As I write this, his published results (Version 4) are up to date as of March 2020.  Click the following chart to see it full screen on Blaine’s own website.

These findings are based on submissions from almost 60,000 people who have tested their own autosomal DNA and have known and documented relationships with other testers who share some of their DNA.  Locate yourself at SELF on the chart, and from there look around the wide range of relationships with whom you might share atDNA.  You’ll see, for example, that the average amount of atDNA you share with a parent is 3483cM, but based on real test results from these 60,000 participants it could be as low as 2376 or as high as 3720cM.  The average you’ll share with a full sibling is 2613cM but it could be as low as 1613 and as high as 3488cM.  The average shared DNA with a great grandparent will be 887cM but it could be as low as 485 or as high as 1486cM.  Looking further afield, the average amount shared with your 4th cousin is 35cM but it could be as low as zero or as high as 139cM.

How can we use this information to develop our family trees?
By now you may be thinking:

  • ‘Why on earth would I want to know how much DNA I share with a 4th cousin?’
  • Or ‘My great grandparents are long dead.  I couldn’t access their DNA even if I wanted to.’
  • Or even ‘Yes, very nice.  And this has what, exactly, to do with developing my family tree?’

When we take a DNA for genealogy test and agree for the results to be included in a pool of testers we will be able to see which of the other testers share DNA with us.  Depending on which testing company you use, you will be told the name (or pseudonym) of your match; the amount of shared DNA in centiMorgans; the likely relationship you have with that person (based on the amount of shared cM); and you may possibly have information regarding the exact shared segments plus access to the other person’s tree.  You will also be able to see other testers who match both yourself and that other person.  The results are never displayed in a way that enables another person to see private information about your DNA, simply that you match with them at specific segments.

If my atDNA and another person’s atDNA is exactly the same at one or more places (segments) throughout the twenty-two chromosomes, then that means we have both inherited that part of our DNA from common ancestors.  The higher the amount of DNA we share (the centiMorgans), the closer our relationship is.  If, based on the shared cM, our suggested relationship is around 4th cousin, then we will be looking for a Most Recent Common Ancestor (MRCA) at around 3xG grandparent level.  We now shift to looking at our trees.  Assuming we have both done accurate research, if we both have the same ancestor named as 3xG grandparent (or thereabouts, e.g. it could be my 3xG grandparent and the other person’s 4xG grandparent) then we have found our match.  Based on this we now have the following:

  • a further piece of evidence that our documentary research is correct;
  • proof that there were no adoptions or unexpected paternity events along the way;
  • a new cousin who shares our interest in family history and DNA.  From here on, if you’re minded to, you might be able to share research and new discoveries (I have become great friends with some of my ‘new’ cousins, while for others the connection has been more focused and businesslike);
  • and something else that I think is rather wonderful: you now know that this little piece of you has come down through several generations, unchanged, from an ancestor whose name you have and whose life you have been researching.

Of course, it doesn’t always go as smoothly as that.  Your match might not have a tree – I have often taken what little information they have and worked their tree back to find our shared ancestor: the MRCA.  Your match might not even know who their parents are: I have now used DNA to help one person find their biological father and another to find a missing grandfather.  On the other hand, it may be that your match’s tree is more advanced than yours, and that this DNA connection will help you break through a brick wall and take your tree back a generation or two further.  But we can’t do any of this without other testers: our distant cousins living now, who have also tested and whose test results combined with our own are the key to unlocking information about our shared ancestral lines.

Find out more
Blaine’s excellent book, The Family Tree Guide to DNA Testing and Genetic Genealogy was my starting point in getting to grips with how this all worked.
You can read more about the Shared cM Project [here]
And download a PDF with (a LOT!) more information [here]
His website is perhaps of more use if you’re already familiar with DNA for genealogy and are looking for more information about specifics.
His YouTube channel is [here]

Edited August 2020
My posts about DNA are aimed at complete beginners and aim to provide information in manageable chunks, each post building on previous ones.  Click [here] to read all of them in order, or to dip in and out as you wish.  You’ll also find lots of resources and useful links