Mosaic Double Aneuploidy With Trisomy 21 and XYY Syndrome: A Case Report

Double aneuploidy involving trisomy 21 and XYY syndrome is an exceptionally rare chromosomal abnormality.Down syndrome is the most common autosomal aneuploidy, whereas XYY syndrome is a rare sex chromosome disorder.The coexistence of these two conditions may result in a complex clinical presentation, often dominated by features of trisomy 21.We report the case of a 24-month-old male presenting with typical dysmorphic features of Down syndrome and global developmental delay.Cytogenetic analysis revealed a mosaic karyotype, with 48,XYY,+21 detected in 55.1% of analyzed cells and 47,XY,+21 in 44.9%, confirming double aneuploidy.This case highlights the importance of cytogenetic analysis in patients with suspected trisomy 21 to identify additional chromosomal abnormalities that may influence prognosis and management.