DNA Testing, What Do My Origin Percentages Mean?

This week I received an email from FamilyTreeDNA saying 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?

Which ancestors passed their DNA to me?

DNA is like a recipe for how a person’s body looks and functions.  Every person receives DNA from his parents.  For autosomal DNA, the most common type of DNA tested, a child receives exactly half from each parent.  Going back a generation, a person receives about a quarter of his autosomal DNA from each grandparent.  But it does not have to be exactly 25%; nature shuffles pieces of DNA from each grandparent when forming the half that each parent contributes.  The author’s own results are: 30%, 20%, 27%, and 23% autosomal DNA from each grandparent.  There is even a very small probability that a person could have no autosomal DNA from one grandparent and half from another grandparent, although the average is indeed a quarter.  Going back to the great-grandparents, a person receives about 12.5% from each of his grandparents.  But again, nature shuffles the DNA, so some could contribute more or less.  With all this shuffling, eventually some ancestors don’t contribute to a person’s autosomal DNA at all.

I found a great explanation of what this means written by The Genetic Genealogist, Blaine Bettinger.  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.

“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)

A DNA origins test looks at only the genetic family tree.  This contains a small fraction of all a person’s ancestors.  Only bits of information from those people combined to make the recipe for you. 

How are origin percentages generated?

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).

The testing company compares your DNA to the DNA of people from various locations around the world and assigns sections of your DNA to those origin locations.  The resulting origin percentages are pretty accurate in terms of which continents your DNA came from.  But right now origin percentages are less accurate distinguishing between areas within a continent, like the British Isles versus Scandinavia.  As time goes on, with more sample individuals and more research, origin percentages will improve.

 

Posted in DNA

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