Our bodies are made from millions of cells, and inside every cell there are chromosomes. Chromosomes are made up of DNA that acts like a genetic code, with all the instructions that tell a cell what to do.
Most people have 46 chromosomes in each of their cells, but some people may have more or less than the usual 46 chromosomes.
Down syndrome is a genetic condition that arises during the very first stages of development, when abnormal cell division produces an extra copy of chromosome 21. A change in the number of chromosomes such as this is called aneuploidy.