Who gets Down syndrome and why?
It is estimated that Down syndrome occurs in approximately 1 in 700-800 live births. It is caused by a chromosomal disorder in which an error in cell division results in the presence of an additional third chromosome 21 or “trisomy 21”. Instead of the normal number of 46 chromosomes in each cell, the individual with Down syndrome has 47 chromosomes. This chromosome abnormality is called Standard Trisomy 21. It is the most common type of Down syndrome and occurs in 90-95% of cases. The two other variants of Down syndrome are called Translocation Trisomy 21 and Mosaic Down syndrome. Apart from a few cases of Translocation Down syndrome, Down syndrome is not an inherited condition. A factor that does influence the incidence of Down syndrome is the age of the mother when she conceives. The incidence at various maternal ages is shown below.
