What type of relationship with a medieval person does a shared DNA fragment really imply? In 2022, we buried in a Jewish cemetery in
Germany. Not long after we made the , people started comparing their own DNA with that of the 14th-century German Jews, finding many “ .” These medieval individuals had DNA fragments shared with thousands of people who have uploaded their DNA sequences to an online database, the same way you share DNA fragments with your relatives. But what type of relationship with a medieval person does a shared DNA fragment imply? It turns out that there is not too much that will help with your family roots research. . We understand how exciting it can be to find a genetic link to particular people who lived many generations ago. But these DNA matches aren’t the tight ties you may be imagining. Here’s how it works. How could you compare your own DNA with that of these historical people? Ancient DNA is a new and rapidly growing field, with a to Svante Pääbo for his foundational work. Using samples taken from skull bones or teeth, of people who lived as far back as 100,000 years ago. , or genomes, are currently available. These genomes, which come from all corners of the world, have dramatically revolutionized scientists’ understanding of . A new trend in ancient DNA is sequencing the genomes of “historical” individuals: those who have lived during the past millennium. Examples include genomes from , , , , , , and , , and in the U.K. Outside Europe, scientists have sequenced historical genomes from , the , , the , , , the , and the . Genomes of enslaved Africans from , , , and are also available. Some historical genomes belong to named individuals, including , the family of the , , the Lakota Sioux leader , and of
England. How could you compare your own DNA with that of these historical people? 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 with you and report to you these matches – from the closest to the more distant. , 23andMe now lets customers . Other genetic testing companies don’t yet, but passionate genealogists can take matters into their own hands. For example, the service lets users upload their own DNA data, along with published DNA sequences of any historical people. Once uploaded, GEDmatch will identify any user with whom you share genetic material. So, what does a genetic match with a medieval person mean for your genealogy? Surprisingly, very little. The first thing to understand is how many ancestors you have in each past generation. One generation back, you have two ancestors. Two generations back, that doubles to four. Then eight, and 16. By 30 generations ago, around the 12th century, you have over one billion ancestors. Clearly, at this point, your ancestors include most people from your population who lived back then, excluding who left no long-term descendants. This includes, if you have European origins, notable people such as or , but equally also people of every medieval social class. Your family tree reaches each of these ancestors through . the following surprising fact. In any given population, the number of lines in your family tree that reach any specific medieval person is about the same between you and everyone else who belongs to the same population you do. In other words, everyone alive today is equally related, genealogically, to all medieval people from that population. The next step is to understand how many ancestors you actually inherit DNA from. Surprisingly again, very few. Despite your millions or more medieval ancestors, you inherit DNA from only a tiny . So, we’re sorry, you probably didn’t inherit any DNA from Charlemagne or Edward I. For example, you have only about 2,000 genetic ancestors from the 12th century. In other words, your DNA sequence is a mosaic of approximately 2,000 “fragments,” each tracing back to a single 12th-century person. Who are the medieval people whose DNA you inherited? Each fragment of your DNA descends from a random line up your family tree – father’s mother’s mother’s father, and so on – at each generation in the past, selecting at random one of two parents. The more lines in your family tree that reach a certain medieval person, the more likely you are to inherit DNA from that person. But remember, the number of family lines that reach a medieval person is about the same for all present-day individuals from a given population. Therefore, all individuals inherit DNA from any medieval person with very similar probabilities. So, sharing genetic material with one particular medieval person or another is just a matter of chance, and everyone is playing the same game. Here’s an analogy. Going to a casino and rolling a roulette ball onto 24 does not mean 24 is your special number. Anyone else might have rolled 24 as well. Similarly, sharing a DNA fragment with any one out of your millions of medieval genealogical ancestors does not mean any special relationship – beyond sharing a DNA fragment. And if you don’t have a shared segment, you just didn’t get lucky. It doesn’t mean you’re any less genealogically related to that medieval person than anyone else from your population who does have a shared segment. As a side note, a “population” is not always , but these arguments hold generally for people with similar origins. Consider again the medieval German Jews. Some present-day Ashkenazi (European) Jews will share DNA with one particular medieval Jew. Some will share with others. Some will share with none. It’s a lottery draw. And given that most Ashkenazi Jews today are genealogically related in a very similar way to the medieval German Jews, seeing that shared DNA fragment does not imply any unique genealogical relatedness. On the other hand, if you’re willing to consider more recent ancestors, DNA matches can be informative. The same show that the number of family lines reaching a particular historical person living around 200 or 300 years ago will be very different across present-day people. Therefore, a DNA match with an 18th-century person implies a more specific genealogical relationship, one that most other present-day individuals do not have. This pattern was demonstrated in a . Comparing the genomes of 18th-century enslaved Africans from
Maryland to more than 9 million of their customers, 23andMe discovered over 41,000 living relatives, . How far back in time does a DNA match still have genealogical meaning? For example, are DNA matches informative in the period between the late Middle Ages and the 17th century? We don’t know yet. Future research will be needed to clarify this question, as well as deviations from the simple . In the meantime, as scientists rapidly accumulate more and more historical genome sequences, keep the quirky behavior of human genealogies in mind when interpreting a DNA match.