DNA says you’re related to a Viking, a medieval German Jew or a 1700s enslaved African? What a genetic match really means
In 2022, we reported the DNA sequences of 33 medieval figures buried in a Jewish cemetery in Germany. Shortly after we made the data public, people began comparing their own DNA with that of 14th century German Jews, finding many “matches.” These medieval individuals had DNA fragments shared with thousands of people who uploaded their DNA sequence to an online database, the same way you share DNA fragments with your loved ones.
But what kind of relationship with a medieval character does a shared DNA fragment imply?
Turns out it’s not much that will help you in your family roots research.
We are population geneticists who work with ancient DNA. We understand how exciting it can be to find a genetic link to particular people who lived generations ago. But these DNA matches aren’t the close connections you might imagine. Here’s how it works.
Sequencing the DNA of those who lived long ago
Ancient DNA is a new and rapidly growing field, with a Nobel Prize awarded in 2022 to Svante Pääbo for his seminal work.
Using samples taken from skull bones or teeth, DNA researchers can sequence the DNA of people who lived 100,000 years ago. More than 10,000 ancient DNA sequences, or genomes, are currently available. These genomes, from the four corners of the world, have radically revolutionized scientists’ understanding of human origins.
A new trend in ancient DNA involves sequencing the genomes of “historical” individuals: those who lived during the last millennium.
Examples include genomes from Sweden, Norway, Denmark, Iceland, Poland, Southeast Europe, and London, Cambridge, and Norwich in the United Kingdom. Outside of Europe, scientists have sequenced historic genomes from East Asia, the Swahili Coast, South Africa, the Canary Islands, Lebanon, Machu Picchu, the Caribbean and the San Francisco Bay area. The genomes of African slaves from Delaware, Maryland, South Carolina, and St. Helena are also available.
Some historical genomes belong to named individuals, including Ludwig van Beethoven, the family of the last Russian tsar, the medieval Hungarian royal family, Lakota Sioux leader Sitting Bull, and King Richard III of England.
How might you compare your own DNA with that of these historical figures?
Several direct-to-consumer genetic testing companies, such as 23andMe, MyHeritage, or Ancestry, make reading your own genome sequence simple and affordable. They compare your DNA with that of their other customers. They identify relatives who share long, continuous sequences of identical DNA with you and bring you back those matches – from closest to most distant.
After an initial deliberation, 23andMe now allows customers to compare their genome with that of historical figures. Other genetic testing companies don’t yet do this, but passionate genealogists can take matters into their own hands. For example, the GEDmatch service allows users to upload their own DNA data, as well as the published DNA sequences of any historical figure. Once downloaded, GEDmatch will identify any user with whom you share genetic material.
A comparison of the DNA sequence of a chromosome between a 14th century German Jew and two living people who uploaded their DNA to GEDmatch. Each thin vertical bar represents a letter in the DNA sequence and is color-coded depending on whether it is a match or not. A shared DNA fragment appears between living person 1 and medieval person. GEDMatch
So what does a genetic match to a medieval person mean for your genealogy?
Surprisingly, very few.
Where genealogy and genetics diverge
The first thing to understand is how many ancestors you have in each past generation. A generation ago, you had two ancestors. Two generations ago, that doubled to four. Then eight, then 16. 30 generations ago, around the 12th century, you had more than a billion ancestors.
Obviously, at this point your ancestors include most of the people in your population who were alive at the time, excluding a small fraction who left no long-term descendants. This includes, if you have European origins, figures such as Charlemagne or Edward I, but also people from all medieval social classes. Your family tree reaches each of these ancestors through many lines.
Mathematical research demonstrates the following surprising fact. In a given population, the number of lines in your family tree that reach a specific medieval person is about the same between you and everyone else in the same population as you. In other words, everyone alive today is also related, genealogically, to all medieval peoples in that population.
The next step is to understand how many ancestors’ DNA you actually inherit from. Surprisingly again, very little.
Despite your millions of medieval ancestors, you inherit the DNA of only a tiny fraction of them. So we’re sorry, you probably didn’t inherit any DNA from Charlemagne or Edward I. For example, you only have about 2,000 genetic ancestors from the 12th century. In other words, your DNA sequence is a mosaic of about 2,000 “fragments,” each dating back to a single person from the 12th century.
Who are the medieval peoples from whom you inherited DNA? Each fragment of your DNA descends from a random line in your family tree – father from father’s mother, father from mother, etc. – with each past generation, by randomly selecting one of the two parents. The more lines in your family tree that reach back to a certain medieval person, the more likely you are to inherit that person’s DNA.
But remember that the number of family lines that reach a medieval person is roughly the same for all current individuals in a given population. Therefore, all individuals inherit DNA from any medieval person with very similar probabilities. So sharing genetic material with a particular medieval character is just a matter of chance, and everyone is playing the same game.
Here’s an analogy. Going to the casino and rolling a roulette ball to 24 does not mean that 24 is your special number. Anyone else could have gotten 24 as well. Likewise, sharing a DNA fragment with one of your millions of medieval genealogical ancestors does not signify any special relationship – beyond sharing a DNA fragment.
And if you don’t have a shared segment, you’re just out of luck. This does not mean that you are any less genealogically related to this medieval person than anyone else in your population who has a common segment.
As an aside, a “population” is not always well defined, but these arguments generally apply to people with similar backgrounds.
How to interpret a historical DNA match
Consider again the German Jews of the Middle Ages. Some Ashkenazi (European) Jews today will share their DNA with a particular medieval Jew. Some will share with another. Some won’t share with anyone. It’s a draw. And given that most Ashkenazi Jews today have very similar genealogical ties to medieval German Jews, the fact that the shared DNA fragment does not imply any unique genealogical relationship.
On the other hand, if you want to consider more recent ancestors, DNA matches can be informative. The same mathematical models show that the number of family lines reaching a particular historical person living about 200 or 300 years ago will be very different among people today. Therefore, a DNA match to someone from the 18th century implies a more specific genealogical relationship, which most other individuals today do not have.
This trend was demonstrated in a recent study by 23andMe. By comparing the genomes of 18th-century Maryland African slaves to those of more than 9 million of their customers, 23andMe discovered more than 41,000 living relatives, including a few almost direct descendants.
How long does a DNA match still have genealogical significance? For example, are DNA matches informative between the late Middle Ages and the 17th century? We don’t know yet. Future research will be needed to clarify this issue, as well as deviations from the simple model of a single, freely mixed population.
In the meantime, as scientists rapidly accumulate more and more historical genomic sequences, keep in mind the bizarre behavior of human genealogies when interpreting a DNA match.
This article is republished from The Conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you make sense of our complex world. It was written by: Shai Carmi, Hebrew University of Jerusalem and Harald Ringbauer, Max Planck Institute for Evolutionary Anthropology
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Shai Carmi is a paid consultant and holds stock options at MyHeritage.
Harald Ringbauer does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond his academic appointment.
