Footnote
a Trisomy 21 is a congenital defect causing mental retardation. Chromosomes normally come in pairs, but babies born with trisomy have an extra chromosome on one of the pairs. Trisomy 21 affects chromosome 21.
a Trisomy 21 is a congenital defect causing mental retardation. Chromosomes normally come in pairs, but babies born with trisomy have an extra chromosome on one of the pairs. Trisomy 21 affects chromosome 21.