Down syndrome or Down’s syndrome is a congenital condition caused by the presence of an additional copy of chromosome 21 in a person’s cells. This is also referred to as trisomy 21.

Humans usually have 46 chromosomes in every cell, with 23 inherited from each parent. Due to the extra copy of chromosome 21, people with Down’s syndrome have 47 chromosomes in their cells. This additional DNA causes the physical characteristics and developmental problems associated with the syndrome.

Down’s syndrome was first described by an English physician John Langdon Down in 1862, who helped to differentiate the condition from mental disability. He used the term “mongoloid” to describe the condition, due to his opinion that children with Down’s syndrome shared similar physical features to people from the Blumenbach’s Mongolian race. This term for the condition became less common after the 1970’s due to its inaccuracy and the fact that it was considered pejorative.

Historically, many individuals with Down’s syndrome were killed, abandoned or ostracised from society. In the 20th century, it was common for these individuals to be institutionalized and they did not receive appropriate treatment for the associated medical complications such as heart disorders, vision defects and intestinal problems. Many children with Down’s syndrome therefore used to die during infancy or early adulthood.

As the eugenics movement came into being, the forced sterilization of individuals with Down’s syndrome was introduced to 33 of the 48 states that then existed in the United States. The sterilization programs reached large proportions until protests from the general public led to their discontinuation. At this point in history, the cause of Down’s syndrome was not understood. It was assumed that several genetic factors, older maternal age, birth injuries, and injury during pregnancy caused the illness.

It was only in the mid-twentieth century, (the 1950s) that karyotype techniques were discovered, which could be used to help identify the shape and number of chromosomes. This led to the understanding that trisomy 21 was the cause of Down’s syndrome, a finding that was reported by Jérôme Lejeune in 1959. Lejeune’s claim that he discovered the extra copy of chromosome 21, however, has been disputed and in 2014, the Scientific Council of the French Federation of Human Genetics awarded Marthe Gautier with the Grand Prize for this discovery.