Down's syndrome: trisomy 21

Down's syndrome is the most common single cause of learning disability in children of school age. Children with the syndrome typically have a round, flat face, and eyelids that appear to slant upwards. In addition to some learning disability, they also have an increased risk of infection (particularly respiratory and ear infections), and heart defects occur in about one-quarter of those with the syndrome.

The syndrome is named after John Langdon Down, a nineteenth century doctor who first described the condition in 1866. In 1959, the French physician Lejeune used chromosome-staining techniques to show that Down's syndrome is caused by an extra chromosome 21. Having one extra chromosome is known as trisomy,hence Down's syndrome is also known as trisomy 21.The extra chromosome usually comes from the egg cell due to non-disjunction of chromosome 21. About 70% of the non-disjunctions occur during meiosis I, when homologous chromosomes fail to separate; 30% occur during meiosis II, when sister chromatids fail to separate. Whether it occurs during meiosis I or meiosis II, non-disjunction leads to trisomy. In a few cases, the extra chromosome comes the father.

In about 3% of cases, Down's syndrome results from translocationof an extra chromosome 21. A region of the chromosome breaks off and rejoins with either the end of the other chromosome 21 or with another non-homologous chromosome (commonly chromosome 15). In these cases, a person may have the normal number of chromosomes, but one of the chromosomes will be abnormally long.