Most human cells have 23 pairs of chromosomes and only one of them determines whether a fetus will be male or female. The union of an X chromosome with another Y gives rise to a boy, and if two X chromosomes join together, the final result is a girl. But biology is not an exact science, and variations are common.
The most common chromosomal abnormalities are known as aneuploidies (alteration in the number of chromosomes), the best-known example of which is Down syndrome (caused by an extra copy of chromosome 21). But genetic disorders can also specifically affect the sex chromosomes and then cause syndromes such as Turner, Klinefelter or Jacob.
Researchers at the University of Oxford have used ancient DNA from a large data set of individuals collected as part of the Thousand Ancient British Genomes project – spanning prehistory to the post-medieval period – to identify the first known people to have suffered this type of disease. alterations. A woman between 18 and 22 years old stands out who lived about 2,500 years ago, during the Iron Age, and who was missing one of the X chromosomes.
This person was affected by Turner syndrome (or monosomy X), a genetic condition in women characterized by shorter than average height, abnormal ovarian function, and sterility.
By investigating the details of the bones, specialists saw that it was unlikely that this young woman had gone through puberty and had menstruation, despite her age. It was shown that her syndrome was mosaic, meaning that some cells had one copy of the X chromosome and others had two.
As explained in a study published in the journal Communications Biology, experts used a new method to analyze ancient DNA, identifying five people who had sex chromosomes that were not within the XX or XY categories. All were buried according to the customs of their society in sites in Somerset, Yorkshire, Oxford and Lincoln.
The first known patient with Jacob syndrome (when a man has an extra Y chromosome, XYY) must be found in the early medieval period. Additionally, there are three other individuals with Klinefelter syndrome (when a male has an extra X chromosome, XXY) at various different periods.
The three people with Klinefelter syndrome shared some similarities: They were all slightly taller than average and showed signs of delayed development at puberty. One of them was buried in the medieval cemetery now situated beneath Longwall Quad, at Magdalen College, Oxford University itself.
“The results of this study open up exciting new possibilities for the analysis of sex in the past, going beyond binary categories in a way that would be impossible without the advances that have been made in the analysis of ancient DNA,” says Rick. Schulting, professor of Scientific and Prehistoric Archeology and co-author of the study.
Old genetic samples can erode over time and become contaminated with DNA from other samples or the people who handle them. This makes it difficult to accurately capture differences in the number of sex chromosomes, experts say.
To overcome these problems, the team of researchers developed a computational method that counts the copy number of the X and Y chromosomes and then compares the result with a predicted baseline (what one would expect to see).
