Most cells in the human body have 23 pairs of DNA molecules called chromosomes, and the sex chromosomes are typically XX (female) or XY (male), although there are differences in sexual development. ‘Aneuploidy’ occurs when a person’s cells have an extra or missing chromosome. If this occurs in the sex chromosomes, a few differences like delayed development or changes in height can be seen around puberty.
Ancient DNA samples can erode over time and can be contaminated by DNA from other ancient samples or from people handling them. This makes it difficult to accurately capture differences in the number of sex chromosomes.
The team at the Crick developed a computational method which aims to pick up more variation in sex chromosomes. For the sex chromosomes, it involves counting the number of copies of X and Y chromosomes, and comparing the outcome to a predicted baseline (what you would expect to see).
The team used the new method to analyse ancient DNA from a large dataset of individuals collected as part of their Thousand Ancient British Genomes project across British history, identifying six individuals with aneuploidies across five sites in Somerset, Yorkshire, Oxford and Lincoln2. The individuals lived across a range of time periods, from the Iron Age (2500 years ago) up to the Post-Medieval Period (about 250 years ago).
They identified five people who had sex chromosomes which fell outside of the XX or XY categories. All were buried according to their society’s customs although no possessions were found with them to shed more light on their lives.