Ancient DNA can give us useful information about our ancestors, but unfortunately samples can deteriorate or become contaminated over time, making analysis difficult. One area of ​​concern is detecting differences in chromosome number, such as extra or missing copies; this is called aneuploidy.

To combat this, the research team developed a new calculation method to detect more variation when counting the number of chromosomes in the human genome, specifically X and Y (also known as sex chromosomes). Most human cells have 23 pairs of chromosomes, including sex chromosomes. Females usually have two Xs (XX), while males have one X and one Y (XY).

The new method was applied to the Thousand Ancient British Genomes Project dataset and identified five individuals with sex chromosome aneuploidy; two of these were the first people known to have the genetic disorder.

“By precisely measuring sex chromosomes, we were able to show the first prehistoric evidence of Turner syndrome from 2,500 years ago and the earliest known incidence of Jacob syndrome from approximately 1,200 years ago,” said Kakia Anastasiadou, first author of the study, describing the findings. . in practice.

Turner syndrome is characterized by one X chromosome instead of the normal two, while Jacob syndrome is characterized by an extra Y chromosome.

Aneuploidy of sex chromosomes can sometimes affect development; The bones of an individual with Turner syndrome showed that she had not reached puberty or menstruated, although she was thought to be between 18 and 22 years old. But the analysis also showed that only some cells had only one copy of the X chromosome, while others had the normal two copies. This is called mosaicism.

The study also found three people with Klinefelter syndrome (an extra X chromosome, XXY) living at different times. It was also determined that an Iron Age baby had Down syndrome as a result of autosomal aneuploidy of the extra copy of chromosome 21, meaning that this affected the non-sex chromosome.

“It’s difficult to see the full picture of how these people lived and interacted with their society because they were not found with items or in unusual graves, but this may provide some insight into how concepts of gender identity developed over time,” Anastasiadou said. said. .

Co-author and evolutionary geneticist Pontus Skoglund added: “Our method can also classify DNA contamination in many cases and help analyze incomplete ancient DNA, so it can be applied to archaeological remains that are difficult to analyze.”

“Combining these data with burial contexts and possessions can allow for a historical perspective on how sex, gender, and diversity were perceived in past societies. I expect this type of approach will be used as the overall ancient DNA data resource continues to grow.”