What can DNA and ethnicity measurements tell me?

Most of the online genealogy sites now offer a different way to find out about your family – DNA tests.  Often, they will come back with an estimation of your ethnicity to tell you where your far-distant ancestors hailed from.  But how accurate are they, and what can they tell you about your family history?

For people who have not been able to find out about their family history, due to adoption or illegitimacy, a DNA test may well provide a lot of information that was impossible to find otherwise.  There have been a lot of cases now of adopted children discovering cousins after a DNA test, which have enabled them to trace their birth families.  The DNA tests offered by most of the mainstream providers will provide a list of matches and an estimate of how closely they are linked to you, and that information can be relied upon.

However, the ethnicity estimates are best guesses, and sometimes a little vague.  These identical triplets, for example, should have identical DNA as they are genetically exactly the same, and yet their results were calculated to be different by the algorithm used to analyse them.

The problem is, that the companies attempt to tell you where your most distant ancestors came from, relies on identifying bits of your DNA with current populations in those locations.  That process should refine over time as more and more people get their DNA tested and provide background information to the system.  At the moment there are big gaps in the information and so best guesses are used.

You also have to understand how populations have moved over time.  Many people with a Scottish heritage, for example, are going to find a lot of Irish in their ethnicity assessment, because the Scots originally came from Ireland.  Vikings came from Scandinavia originally, but did you know that Normans had a Viking heritage?  So if you have Norman ancestors, would you say they came from France, because that’s where the Normans were. or from Scandinavia, because that’s where their ancestors were?  How far back do you go? In terms of a DNA test, your French ancestry may well show up as Scandinavian.

The ethinicity estimates are only a guess, and using fairly large areas too.  This is bound to become more refined over time.  The possibilities now for much more detailed DNA analysis are much more advanced than they were just a few years ago.  Things are bound to become more sophisticated.

In my own family, my mother tested 85% British, which isn’t surprising as I have found no foreigners in her family tree until you get right back to the 14th century.  I learned that actually, we all have two genealogies, our family tree, which tells us who we are descended from, and a genetic family tree… and it was a surprise to me to learn that not all people in our researched family tree may be in our genetic family tree.  This took a while to understand.  Each person inherits 50% of their DNA from each parent, but if you have siblings, you may not share the same DNA (unless you are identical siblings like twins or triplets, in which case you will share the same DNA).  The 50% you inherit is randomly selected from the 100% of each parents’ DNA.  So you may have brown hair and your sibling may have red hair, and another blonde hair, simply because you’ve inherited different bits of DNA.

But the result of this is that there is 50% of each parent’s DNA which passes to you, and 50% of each parent’s DNA does not pass to you.  This means that 50% of the DNA they inherited from their parents fails to reach you.  So you may know that your great great great grandfather was Theophilus Earwaker, and know all about his famed musical prowess, but you may not have inherited any of his DNA, even though he is in your family tree.  The further back you go, the more likely it is that you have none of an ancestor’s remaining DNA.

The mathematics of family history is pretty paradoxical, and well illustrated by the traditional mathematical problem involving a chess board and grains of rice.  I’ve heard it told that a ruler in India asked someone how they wanted to be rewarded for their work and they asked for a grain of rice on the first square of the board, and two on the second, four on the third, eight on the fourth, doubling up every square. Someone who hasn’t come across that puzzle before may think that is a fairly modest demand… but it is not.  The puzzle is first recorded in 1256 by Ibn Khalikan according to Wikipedia.  The result is that the worker would receive more grain than even the ruler might have in his possession – 18,446,744,073,709,551,615 grains.  Not to mention the pain of having to count so many!  This is a lot more than most people would guess would result from this calculation.

So it is with family trees.  In each generation back, you have twice as many ancestors in that generation as the generation before: two parents, four grandparents, eight great grandparents, 16 great great grandparents and so on.  Much like the rice on the chessboard, unfeasibly large numbers begin to appear fairly quickly, until – in the UK – you have more ancestors than the country had population at the time.  For example, if you allow 30 years per generation (roughly) and go back 22 generations, you should have 8,388,608 ancestors in just that generation… when the total population for the UK in 1330 was fewer than 3 million people.  Of course, you will have a lot of people many times over in your tree, cousins marrying will mean their ancestry merges and the same happens if sisters marry and are both your ancestor, which accounts for the discrepancy.  I have found families where all of five brothers and sisters appear on my tree.

The fact that makes my head feel as though my brain is exploding, though, is that you have twice as many ancestors as your parents, and your children will have twice as many as you.  They must, as you have joined your ancestry with your partner’s ancestry to make a new person.  So although they will only have half your DNA and half your partner’s DNA, they will still have double the ancestors you have and so on down the line.  It’s no wonder that some DNA has to be discarded in every generation!

DNA testing can be very useful if you have brickwalls in your family tree, or unanswered questions where you have multiple possibilities.  If you appear as a link to someone who is sure about their family tree, it may help to confirm or deny a postulated connection.  In my own case, I found a cousin who had no idea where her great great grandmother had come from, but her surname was our only shared name, and I was sure that she was a younger sister of my own great grandmother.  To maximize your chances of finding links with other people, you may want to consider uploading your data to GEDmatch, which allows you to compare your DNA with people who have used other providers.  There are instructions on how to do that here.  Some of the mainstream companies have also started to offer you the chance to upload your data and compare your DNA with their customers. There’s a useful list on the International Society of Genetic Genealogy Wiki here.


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