Down syndrome 101

Down syndrome is a chromosomal disorder named after the physician who recorded the first formal description of the syndrome, Dr. John Langdon Down.

The syndrome is marked by the presence of either one extra chromosome (47 instead of 46), or one chromosome with an extra part. This extra chromosomal material often causes cognitive impairment, developmental disability and the appearance of certain physical features such as a flatter facial structure and upward slanting eyes.

Down syndrome occurs in about one out of every 800 births, although this number is heavily influenced by older mothers, who are at greater risk of having children with the syndrome.

Around half of all babies born with Down syndrome are born with heart defects, but these can often be corrected with surgery. These babies are also more likely to have intestinal problems and to contract lung infections.

HISTORY

Dr. Down was working with patients with mental disabilities in a British hospital, and noticed that many of the patients shared the same physical features. He described the disorder in 1866, although he didn’t know what caused it.

Down initially described the patients as ‘mongoloids’ based on the notion that they looked like people from Mongolia, and for the next century, the disorder was closely tied with racist views.

It was not until 1958 that Dr. Jerome Lejeune discovered that Down syndrome was actually genetic. He found that the disorder occurred as a result of a trisomy of chromosome 21 (47 instead of 46 chromosomes), which is actually one of three forms of Down syndrome, and the most common.

The two other forms of the disease are translocation and mosaicism.

In the case of mosaicism, only some cells are affected by trisomy of the 21st chromosome. Translocation refers to the type of Down syndrome where part of chromosome 21 breaks off and attaches itself to another chromosome.