LESSON 7: atDNA Ancestral Origins Part 1
“Autosomal DNA is like box of chocolates … you never know what you will get.” Kitty Munson Cooper. If you are interested in a true life example please read it under the heading “My Sweet Valentine” below.
One of the first things folks want to know is where they came from. How much of me is Native American, how much is Portuguese? Which African tribe are my ancestors from? Is my Scandinavian Finnish or Norwegian or both? The hope of genetic genealogists is that some day we will be able to answer these questions with ever more precise locations and ethnic subdivisions but we aren’t quite there yet. In the meantime we are getting more data and thus more precise all the time. Testing for ancestral origins can use mtDNA, Y-DNA and/or atDNA. The first two will only give an indication of your strictly maternal or paternal lines’ ancestral origins. atDNA will look at all of the DNA across the SNPs tested to give you a composite of your origins.
atDNA ancestral breakdowns are done via special SNPs called AIMs or Ancestral Indicative Markers. These AIMs are markers that have a great deal of variation and are indicative of a place of origin or group of people. A series of AIM SNPs gives us an indication where someone’s genetic material flowed from for a particular segment of DNA. Ancestry and FTDNA also make use of AIMs (each with their own algorithms) so the results may be different from different companies. There are two more players which you should be aware of. The first is GEDMATCH which hosts various different tools for “painting” your chromosomes under the heading “Admixture” and then “Ad-Mix Utilities.” The other are offerings from a company called DNA Tribes. Hopefully these will be covered in a future lesson.
The basic premise is the same no matter where you test. These tools attempt to give you a breakdown of your ancestral origins. There are some important limitations and caveats. First, it is necessary to understand that your paper family tree and your DNA are not identical. Huh? Let’s see why not. Let’s go back to our first lesson and the Visual DNA Chart. Look at our 8 great grandparents across the top. Let us for the sake of discussion have each of these grandparents have a different ancestral origin.
“Great grandpa Blue” (paternal) = 100% Polish
“Great grandma Yellow” (paternal) = 100% English Colonial
“Great grandpa Green” (paternal) = 100% Irish
“Great grandma Pink” (paternal) = 50% Irish 50% English Colonial
“Great grandpa Gray” (maternal) = 50% African 50% English Colonial
“Great grandma Orange” (maternal) = 50% Norwegian 50% Swedish
“Great grandpa Light blue” (maternal) = 50% Native American 50% Portuguese
“Great grandma Salmon” (maternal) = 100% Chinese
The estimated amount of DNA of the great grandchildren would be about 12.5% from each great grandparent, so our estimate would look like this:
25% English Colonial
6.25% each of African, Native American, Norwegian, Portuguese, and Swedish
However the DNA we inherit from each great grandparent is not equal and those great grandparents with more than one ancestry may give unequal shares of that ancestry to their children and then their children give more or less to their children and so forth. So in the DNA lottery you may have gotten 13% from your Chinese (salmon) great grandma but your half Native American and half Portuguese (lt blue) great grandpa may have only given you 12% of his DNA and not in equal portions so you may have only retained 3% of the Native American and the other 9% is Portuguese. This does not change who are great grandparents are. Our DNA only reflects what we win from each great grandparent in the DNA lottery. So in reality great grandchildren with the same parents might look like this:
Each of these great grandchildren has the same great grandparents but they did not inherit the same DNA. So given this example you can see that by the time great grandchild B passes their DNA to their children some may not reflect any discernible Native American. Another particular problem with Native American AIMs is that they have some of the same ancient origins as Far East DNA so sometimes a program is unable to know which is which and may report Native American as Asian or visa versa. Similar problems creep in with heavily admixed (mixed origin) countries or ethnicity.
In the beginning there were often only a few major categories represented by AIMs: European, Asian and African. As time goes on these categories got more specific: Northern European versus Southern European or North African versus Central Africa versus East Africa versus West Africa. In some cases algorithms have trouble discerning the difference between Scandinavian and British or French from German. Today we get some very specific origins at some companies some of the time. My husband has very specific ” East Cork & Waterford” at Ancestry. I have specificity on my Swedish at 23andme with my top locale Dalarna County which is accurate. All ancestral origins in their current state are estimates of your background based on your AIMs. Since these represent an even smaller subset of your total genome you need to use these as indicators rather than hard and fast percentages.
My Sweet Valentine
My grand-daughter “Valentine” along with her maternal grandparents, and parents have their atDNA tested. The expected shared DNA would be about 50% with each parent and about 25% with each grandparent. In addition it is expected that several intact chromosomes would be shared between Valentine and each grandparent. (Likely range of 1-4 with Grandmother and 3-5 with Grandfather)
The reality is quite different than the expectation (the comparisons involve V3 to V4 comparison so the missing pieces or slight overlap is likely the cause— in reality those gaps or overlaps do not exist).
Father shares: 3719 cM in 24 segments for about 49.59% shared DNA
Mother shares: 3720 cM in 23 segments for about 49.60% shared
Maternal grandfather shares: 1523 cM in 25 Segments for about 20.30% shared; 1 chr: X
Maternal grandmother shares: 2204 cM in 32 segments for about 29.38% shared; 5 chr: 10,15,20,21,22
So Valentine got only one full chromosome from her maternal grandpa and this we know most come from his mother since he gets his X from his Mom so this X went through 3 transmission events intact. A bit of a surprise but it happens. Valentine got nearly 30% of her grandmother’s DNA via her Mother and only about 20% from her grandfather, rather than the average 25% from each. In addition she got 5 “intact” segments from grandmother and only one from Grandfather.
So Kitty is right: “Autosomal DNA is like box of chocolates … you never know what you will get.“
3 Generations 6 Women 1 Family by Anne Swayne
DNA Simulation by Paul Rakow
Kelly Wheaton Copyright 2020. All rights Reserved.