Parental origin of meiotic error of the extra chromosome 21 as indicated by short tandem repeat (STR) polymorphisms in Down syndrome

Background – Down syndrome is the most common cause of mental retardation observed in approximately 1/230 pregnancies. This is the first epidemiological studies to categorize cases of Down syndrome by parent of origin of extra chromosome 21 meiotic error in Iranian Down syndromes. Methods – We studied 100 Down families using conventional cytogenetic and chromosome 21-specific markers. Five specific DNA markers, related to chromosome 21, were used to determine the origin of meiotic error to each case.Results – Chromosomal analysis revealed free trisomy 21 karyotype in 95%, mosaic trisomy karyotype in 5%, and none with chromosome translocation. The parent of origin of chromosome 21 trisomy was determined in 85% of the cases. Nondisjunctional error was determined as maternal in 86% and paternal in 14% of the cases.Conclusion – Although the exact mechanism behind chromosome nondisjunction has not been very well described, but in the case of free chromosome 21 trisomy the exact reasons behind the nondisjunction may vary between female and male who passed this extra chromosome to their child. The knowledge of Down syndrome of parental origin is one of the basic steps to evaluate accurately the risk factors and etiology of meiotic nondisjunction.