Chromosome 21 Down syndrome
Down syndrome (DS) is the most common chromosomal abnormality. The syndrome named after L. Down, who described the associated clinical features in 1866. End of the 1950’s when human chromosome studies became available Down’s syndrome proved to be caused by 3 instead of the normal 2 copies of chromosome 21. Throughout the world, the overall prevalence of DS is 10 per 10,000 live births, by definition of rare DS is not. In countries where abortion is illegal such as Ireland and the United Arab Emirates, its prevalence is high. In Denmark, France and the Netherlands DS prevalence is low, due to the available screening test during pregnancy and a high percentage of DS pregnancy terminations.
When suspected the clinical diagnosis of Down syndrome, the diagnosis should be confirmed with chromosome analysis.