Cytogenetics of Down Syndrome

Down syndrome is the most common autosomal chromosomal aneuploidy in human and the best known of all malformation syndromes associated with developmental delay, mental retardation and several characteristic physical features. The birth prevalence of Down syndrome is approximately 1 in 650-1000 live-born children world-wide which make the syndrome the most common cause of mental retardation. According to the American College of Obstetricians and Gynecologists reports a woman’s risk for having a child with Down syndrome is 1 in 1,000 at age of 30, 1 in 400 at age of 35, and 1 in 100 at age of 40. Generally, more than 95% of Down syndrome individuals have trisomy 21 which results from non-disjunction error during gametogenesis in chromosome 21. About 2-4% results from a translocation of chromosome 21, while only 1-2% is mosaicism that showing a normal cell line additionally to trisomy. Mosaic Down syndrome individuals may be phenotypically less severely affected than Individuals in trisomy or translocation, but their conditions are indistinguishable in all other aspects.