trisomy 21
Chromosomal condition causing Down syndrome, characterized by an extra copy of chromosome 21.
Full Definition
Trisomy 21 is the most common chromosomal abnormality, occurring in approximately 1 in 700 births and causing Down syndrome. The condition results from nondisjunction during meiosis, leading to three copies of chromosome 21 instead of the normal two. Clinical features include intellectual disability, characteristic facial features, hypotonia, congenital heart defects (present in about 40% of cases), and increased risk for certain medical conditions such as atlantoaxial instability, thyroid disorders, and early-onset Alzheimer disease. Maternal age is a significant risk factor, with incidence increasing dramatically after age 35.
Usage
Usage note: Use 'trisomy 21' in formal medical documentation; 'Down syndrome' in patient/family communication.
In Context
- "Prenatal screening suggested increased risk for trisomy 21, confirmed by amniocentesis." — Genetic counseling report
- "The newborn with trisomy 21 required echocardiogram to evaluate for congenital heart disease." — Discharge planning notes