Understanding our own ancient DNA is a little different than contemporary DNA that we use for genealogy, but it’s a continuum between the two with a very long umbilical cord between them, then, and now. And just when you think you’re about to understand autosomal DNA transmission and how it works, the subject of ancient DNA comes up. This is particularly perplexing when all you wanted in the first place was a simple answer to the question, “who am I and who were my ancestors?” Well, as you’re probably figured out by now, there is no simple answer.
In a nutshell – we know that every generation gets divided by 50% when we’re talking about autosomal DNA transmission.
So you inherit 50% of the DNA of each of your parents. They inherited 50% of the DNA of each of their parents, so you inherit ABOUT 25% of the DNA of each of your grandparents.
Did you see that word, about? It’s important, because while you do inherit exactly 50% of the DNA of each parent, you don’t inherit exactly 25% of the DNA of each grandparent. You can inherit a little less or a little more from either grandparent as your parents 50% that you’re going to receive is in the mixer.
This is also true for the 12.5% of each of your great-grandparents, and the 6.25% of each of your great-great-grandparents, and so forth, on up the line.
The chart below shows the percentages that you share from each generation.
|Relationship to You
||Approximate % Of Their DNA You Share
So, here’s the question posed by people trying to understand their ethnicity.
If I have 3% Melanesian (or Middle Eastern, Indo-Tibetan or fill-in-the-blank ethnicity), doesn’t that mean that one of my great-great-great-grandparents was Melanesian?
There are really two answers to this question. (I can hear you groaning!!!)
If the amount is 25% (for example) and not very small amounts, then the answer would be yes, that is very likely what this is telling you. Or maybe it’s telling you that you have two different great-grandparents who have 12.5 each – but those relatives are fairly close in time due to the amount of DNA that came from that region. See, that was easy.
However, the answer changes when we’re down in the very small percentages, below 5%, often in the 1 and 2% range. This answer isn’t nearly as straightforward.
The Dandelion – Your Ancestor
The answer is the dandelion.
The dandelion is one of your ancestors who lived in the Middle East, let’s say, 20,000 years ago, maybe 30,000 years ago. In case you’re counting generations, that is 800 to 1200 generations ago. The percentage of DNA you would carry from a single ancestor who lived 20,000 years ago, assuming you only descended from that ancestor 1 time, is infinitesimally small. There are more zeroes following that decimal point than I have patience to type. Let’s call that ancestor Xenia and let’s say she is a female.
However, you did inherit DNA from many of your ancestors who lived 20,000 years ago, thousands of them, because all of them, through their descendants, make up the DNA you carry today. So infinitesimally small or not, you do carry some of the DNA of some of those ancestors. It’s just broken into extremely small pieces today and their individual contributions to you may be extremely small. You don’t carry any DNA from some of them, actually, probably most of them, due to the recombination event, dividing their DNA in half, happening 800 times, give or take.
Now, given that your ancestors’ DNA is divided in every generation by approximately half, and we know there are about 3 billion base pairs on all of your chromosomes combined, this means that by generation 32 or 33, on average, you carry 1 segment from this ancestor. By generation 45, you carry, on average, .00017 segments of this ancestor’s DNA. And for those math aficionados among us, this is the mathematical notation for how much of our ancestor’s DNA we carry after 800 generations: 4.4991E-232.
But, we also know that this dividing in half, on the average, doesn’t always work exactly that way in reality, because some of those ancestors from 20,000 years ago did in fact pass their DNA to you, despite the infinitesimal odds against that happening. Some of their DNA was passed intact generation after generation, to you, and you carry it today. The DNA contributed by any one ancestor from 800 generations ago is probably limited to one or two locations, or bases, but still, it’s there, and it’s the combined DNA of those ancient ancestors that make us who we are today.
The autosomal DNA of any specific ancestor from long ago is probably too small and fragmented to recognize as “theirs” and attribute to them. Of course, the beauty of Y DNA and mitochondrial is that it is passed in tact for all of those generations. But for autosomal DNA and genealogy, we need hundreds of thousands of DNA pieces in a row from a particular ancestor to be recognizable as “theirs.” When we measure DNA for genealogy, what we are measuring is both centiMorgans, a measure of distance between chromosome positions (length) and the number of contiguous SNP (Single Nucleotide Polymorphism) base locations that match (quantity). The values from these calculations tells us how closely we are related to people, because remember, DNA is divided in each generation so there is a mathematically predictable amount we will share with specific relatives.
Here is an example from a Family Finder comparison table showing both centiMorgans and matching SNPs with a second cousin.
The matching threshold for genealogical significance is either 5 or 7 cM depending on which of the major companies you are using. At Family Tree DNA, if you match above the threshold, then you can view down to 1cM, which is the case above. Another match criteria is the number of SNPs, or locations, matching contiguously. Anything below about 500-800 is considered to be a population match, not a genealogical match, unless you also have a significant number of genealogical matches at higher cMs and segments with this person.
OK, where is all of this going?
Think of your ancestor 20,000 years ago as the dandelion. Now, blow.
Xenia lived in the Middle East. Where might her descendants land, over time, with every new generation? In Europe? In Asia? In India? In America via the Native Americans through Asia? In North Africa? Where?
So let’s say that groups of descendants settle across the globe. Let’s say that her mitochondrial haplogroup is X. Yes, haplogroup X is found both in Europe and in Asia and in the Native Americans, so this is actually a good example. So Xenia carried mitochondrial haplogroup X and we know for sure via mitochondrial DNA testing that indeed, Xenia’s seeds were scattered to all of the winds. The only place we haven’t found Xenia’s children is in Subsaharan Africa and the Australian archipelago, at least not yet.
Ok, so now that we know where her children and their children went, let’s go back to ancient DNA.
The way ethnicity is determined is by studying the frequency with which a specific allele or group of alleles is found in any particular population. Two “pure” examples come to mind.
The first example is the Duffy Null allele that is only found in the Subsaharan African populations. Currently this marker is found in about 68% of American blacks and in 88-100% of African blacks. If you have the Duffy Null allele, you have African heritage. Of course, you don’t know which line or which ancestor it came from, or how far back in time, but it assures you that you do in fact have African heritage. It could have been from an ancestor long ago. It could have been very recent. This is one of the factors considered when determining percentage of ethnicity.
A second example is the STR marker known as D9S919 which is present in about 30% of the Native American people. The value of 9 at this marker is not known to be present in any other ethnic group, so this mutation occurred after the Native people migrated across Beringia into the Americas, but long enough ago to be present in many descendants. There is also no other known marker that is only found only among Native Americans, although I expect as we move into full genome sequencing we will discover more. You can test this marker individually at Family Tree DNA, which is the only lab that offers this test. If you have the value of 9 at this marker, it confirms Native heritage, but if you don’t carry 9, it does NOT disprove Native heritage. After all, many Native people don’t carry it. Again, you don’t know how long ago this marker was introduced into your ancestry.
These two examples are very unique because the markers are found only in certain groups. Generally, with the rest of the DNA values, they are found in different amounts, or frequencies, in different parts of the world and ethnic groups.
So, if you’re trying to determine the ethnicity of an individual, you’re going to compile a huge data base of percentages of DNA values found of Ancestrally Informative Markers (AIMs) in different parts of the world.
So, you would compare the participant’s values against your data base and you will come up with those regions or ethnicities that are present most often in your comparison. This is exactly what the products and services that provide you with your ethnicity percentages do – and how accurate the results are depend highly on the data base itself, the amount of data, and the quality of data. Dare I mention Ancestry’s issue that they’ve had since they first began offering their autosomal product over a year ago where everyone seems to have Scandinavian ancestry? Ancestry doesn’t share with us their sources, so as a community we have no idea how they have come up with these numbers.
You can easily compare your autosomal results in nauseating detail at both 23andMe and Family Tree DNA by testing with both companies, or by testing with either 23andMe or Ancestry and transferring your autosomal results to Family Tree DNA. All 3 of these companies will give you a somewhat different result, but they should be in the same ballpark. You can also then download your raw data file from any of those vendors and upload it to www.gedmatch.com where you can then do ethnicity comparisons using a variety of tools. These tools, an example shown below, will have much more variance and detail than the vendor’s tools or results. And because of that, they tend to be more confusing as well.
Many people with small amounts of minority admixture are disappointed with the results through the vendors, especially if their Native American admixture doesn’t show. I wrote extensively about this in my series, The Autosomal Me, so I won’t rehash it here, but using the GedMatch tools is very enlightening, as you can see above with my results. And do I really have Indo-Tibetan and Indo-Iranian ancestors?
Back to Xenia and her descendants. Let’s say that Xenia’s descendants settled in four primary locations. One is in the Middle East – they never left home. One is in Asia and from there, to the Americans to become the Native Americans and lastly, to Europe. Now let’s say there is a pocket of them in the Altai region of Asia and a pocket in France. The Altai is the ancestral home of the Native Americans and could explain the Indo-Tibet result, above. We’ll call that Central Asia. And France is where my Acadian ancestors were from. Hmmm….this is getting confusing. To make matters even more confusing, I might well descend from both groups, who originally descended from Xenia.
Let’s say that I do in fact carry small segments of Xenia’s DNA. Now let’s say that this same DNA is found in a group of people in Central Asia, maybe in Tibet, it’s published in an obscure journal someplace, and it finds its way into a data base. Voila – there you go – I now have a match in Central Asia in a place called Indo-Tibet. But do I really?
Does this mean that my ancestor was from Central Asia? Not necessarily. And if so, maybe not recently, but the people from that location for some reason share some of the DNA that I carry. The question of course is why, how and when?
What this really means to you is a matter of degrees. If you have a few matches from obscure regions, along with very small percentages, it is likely a result of the dandelion’s dispersion. If you have a lot of matches, meaning a high percentage hit rate, from a particular region, pay attention, it probably has some genealogical significance.
It’s no wonder people are confused by this! Now, just think how many dandelions you have. In 15 generations, you have 32,768 ancestors. In fact, this is how we know for sure that we all descend from the same ancestor multiple times. Our number of ancestors quickly exceeds the world population. In 30 (25 years) generations, in about the year 1263, we reach about 1 billion ancestors. In 1750, there were 791 million people on Earth, in 1600, 580 million, in 1500, 458 million and in 1000, 310 million.
We know that we very likely descend several times from a much smaller group of ancestors from isolated local populations. However, just looking at the 32,000+ ancestors in 15 generations, it’s still an entire dandelion field!!!
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