down's syndrome: cytogenetic studies in 150 cases in tehran

cytogenetic studies were performed on 150 cases of down's syndrome (ds) in iran. the standard trisomy 21 was found in 132 (88 % ) and translocation-trisomy 21 (+21) in 18 (12%) patients, i.e., t(21,21) in 1(0.63%) and mosaicism in 17(11.33%) cases. the comparison of the frequencies for mosaicism between different populations such as denmark, hungary, egypt, iraq, india, australia and iran demonstrated a difference in geographic distribution. there was a high incidence in the north of europe towards egypt and iraq which decreased towards iran and further towards the eastern region in the indian ocean and india and further decreased towards australia. statistical analyses demonstrated significant differences between the data in iran and copenhagen, hungary and australia for mosaicism and translocation +21, and india, for translocation +21. the occurrence of translocation +21 decreased significantly from denmark towards egypt in africa and iraq in southwest asia, then it increased from iran towards australia in the pacific ocean. the comparison of cells having satellite associations (sa), significantly indicated the involvement of two and three sas in ds cases. the study on the position of chromosomes in the metaphase plate, the occurrence of chromatid breaks and endoreduplication did not present any significance in ds cases.