Down Syndrome - Trisomy 21

Down syndrome (DS) or trisomy 21 is a chromosomal disorder caused by the presence of an extra copy of chromosome 21 in some or all cells. This condition belongs to aneuploidies and manifests with intellectual disability, typical facial appearance, and multiple comorbid pathologies. The syndrome was fully described in 1866 by a British physician, John Langdon Down, while the genetic cause, i.e., trisomy 21, was discovered in 1959 [1]. Trisomy 21 is the most common chromosomal anomaly in humans, caused by nondisjunction of chromosome 21 in a parent who is chromosomally normal. The incidence of DS is 1 in 690–750 births, and the risk of DS occurrence increases with the parental age (reaching 1 in 270 for women age 35–39) [1, 2, 3]. However, the probability of having a second child with DS is greater for a young female carrying a balanced translocation than for a healthy middle-aged woman [1].