Trisomy 21 (Down Syndrome)

Trisomy 21 results from an additional copy of chromosome 21. Meiotic nondisjunction accounts for approximately 95% of cases, Robertsonian translocation for about 3%, and mosaicism for about 2%. The extra chromosome changes gene dosage during development; it does not produce one uniform clinical picture. Mosaicism may produce fewer or milder features, but phenotype cannot be predicted reliably from the mechanism alone. The clinical pattern commonly includes hypotonia, developmental delay, characteristic facial features, and variable intellectual disability. The findings that most affect early clinical risk are not the facial features but the associated conditions: congenital heart disease occurs in approximately half of children, thyroid dysfunction is more common, conductive and sensorineural hearing loss may interfere with language development, and airway anatomy and hypotonia increase the risk of obstructive sleep apnea. A child’s developmental progress should therefore be interpreted alongside hearing, vision, sleep, thyroid function, nutrition, and cardiac status. A new plateau in language or behaviour may reflect an untreated sensory or medical problem rather than a primary change in cognition.