DNA Testing, What Do My Origin Percentages Mean?

This week I got an email back from FamilyTreeDNA that they had finished processing one of our family’s DNA tests.  I logged in and checked out the results, the list of relatives and the origins map.  The origins map can be exciting, but sometimes a little confusing.  How do I explain to my family what those origin percentages really mean?

DNA is like a recipe for how a person’s body looks and functions.  Every person has DNA from his parents, exactly half from each parent.  But going up a generation, this doesn’t mean that each person gets a quarter of their DNA from each grandparent.  DNA gets randomly mixed up, so that there is a possibility a person could have no DNA from one grandparent and half his DNA from the other grandparent on that side of the family, though the average is indeed a quarter.

I found a great explanation of what this means written by The Genetic Genealogist, Blaine Bettinger.  He is the source of those awesome DNA statistics tables I wrote about here.  In his article Problems with AncestryDNA’s Genetic Ethnicity Prediction? he explains that everyone has two family trees, a genealogical and a genetic tree.  Here is an excerpt:

Your Genealogical Tree is the tree containing ALL of your ancestors. However, only a tiny subset of these individuals actually (randomly) contributed DNA to the genome that you walk around with today. These ancestors are the only individuals in your Genetic Tree. It has been estimated, for example, that at 10 generations, only about 10-12% of ancestors in your Genealogical Tree are actually in your Genetic Tree!

Accordingly, even if a decent percentage of your ancestors at 10 generations originated in the British Isles, there is possibility that your DNA – and thus your Genetic Ethnicity Prediction – could include very little or absolutely no British Isles ancestry, simply because of the rules of genetics.

(source: http://thegeneticgenealogist.com/2012/06/19/problems-with-ancestrydnas-genetic-ethnicity-prediction/ : accessed 30 Sep 2016)

Near the end of his article Q&A: Everyone Has Two Family Trees – A Genealogical Tree and a Genetic Tree he has a diagram showing just parts of the genetic tree being passed down to a person.

Due to the nature of the Genealogical versus the Genetic Family Tree, entire populations, ancestors, and ethnicities are regularly lost entirely from your DNA!

(source : http://thegeneticgenealogist.com/2009/11/10/qa-everyone-has-two-family-trees-a-genealogical-tree-and-a-genetic-tree/ : accessed 30 Sep 2016)

Okay, so we know that a DNA origins test looks at only the genetic family tree.  This contains a small portion of all a person’s ancestors.  Only bits of information from those people combined to make the recipe for you. 

So what does it mean to have a certain percent of your DNA from the “British Isles” or “Scandinavia”?

There is a good article about how percentages are calculated on Roberta Estes’s DNAeXplained blog called Determining Ethnicity Percentages.  Each family tree testing company has its own formula to figure this out, and the formulas are evolving.  But in general, first the testing company uses information about the DNA of people from various populations around the world.  Often testing companies use the DNA test results of people who are considered native to where they currently live.  Family Tree DNA uses information from academic papers.  Each company makes its formula.  Then that information is compared against the tester’s DNA. The test looks for chunks of DNA that the company’s formula figures are most likely found in certain locations.  Then you get assigned your percentages.

In general, according to the University College London (UCL) Molecular and Cultural Evolution Lab site on  Understanding genetic ancestry testing,

Ethnic/geographical assignments have some validity at a large scale. For example in Latin Americans it is usually possible to distinguish with confidence sections of an individual’s genome that are of sub-Saharan African, European and Native American origin. However, testing companies will often assign national labels to genetic clusters, whereas gene variant frequencies tend to change smoothly across borders. Thus, French people may be assigned a large percentage of “British” ancestry. Normandy and Kent are genetically similar, as you would expect from history and geography, so it is not easy to distinguish English from French based on DNA alone. . . .

The estimates will also change over time as additional reference populations are added and as the algorithms are adjusted or improved.

(source : http://www.ucl.ac.uk/mace-lab/debunking/understanding-testing : accessed 30 Sep 2016)

As one possible path to improvement, genetic researchers seem excited about the future potential for using more ancient DNA.  This is mentioned at the end of the lecture Inferring Human History using DNA by Garrett Hellenthal (see time 32:45).  Also the end of the lecture  Ancestry testing using DNA: the pros and cons by Professor Mark Thomas (see time 33:50; in general he disparages other types of genetic tests, but not the type of test we are talking about here).

So comparing a person’s DNA with that of other people, the testing company assigns parts of that DNA to different origin locations.  It seems that origin locations generally show where people with DNA like the tester live relatively recently in time.  Origin percentages distinguishing continents are generally pretty good.  But right now origin percentages distinguishing between areas like the British Isles and Scandinavia are considered less accurate, so look at them but be a little skeptical.  As time goes on, with more sample individuals and more research, the percentages will get even better.




Posted in DNA

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