Down syndrome is the most common autosomal abnormality and occurs in approximately 1 per 700 live births. Down syndrome accounts for about one third of all moderate and sever mental handicaps in school-aged children. To reveal genetic epidemiology of Down syndrome, 545 karyotypes of referred cases to the author were evaluated. The frequencies of three cytogenetic variants of Down syndrome were trisomy 21 (77.5%), mosaicism (18%) and chromosomal translocation (4.5%). Male to female ratio was 1.34. The mean age of their mothers and fathers were 29.03 years (median=27 years) and 34.6 years (median=33 years), respectively. Therefore, mean age of Iranian mothers with Down syndrome is six years less than those in the western countries.

Keywords: Down syndrome, trisomy 21, karyotype, genetic epidemiology

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