Which is the most common chromosomal anomaly seen?

Down syndrome is by far the most common and best known chromosomal disorder in humans and the most common cause of intellectual disability. It is characterized by mental retardation, dysmorphic facial features, and other distinctive phenotypic traits. Down syndrome is primarily caused by trisomy of chromosome 21; this is the most common trisomy among live bihs. 21 trisomy is a result of maternal non-disjunction.