47XX21

47XX21 is a nonstandard shorthand used to describe a chromosomal pattern in which an individual has 47 chromosomes, two X sex chromosomes, and an extra chromosome 21. In standard cytogenetic notation, this configuration is written as 47,XX,+21 and corresponds to trisomy 21 in a person with female sex chromosomes.

Genetic basis: Trisomy 21 results from an extra copy of chromosome 21. Most cases arise from maternal meiosis nondisjunction, yielding full trisomy 21 in all cells. A minority of cases are mosaic, with some cells normal and others trisomic.

Clinical features: Individuals with trisomy 21 commonly present with hypotonia, characteristic facial features (epicanthal folds, flat nasal bridge), upward slanting palpebral fissures, Brushfield spots, a single transverse palmar crease, and mild to moderate intellectual disability. Congenital heart defects occur in about half, especially AV septal defects. Other issues include thyroid dysfunction, vision and hearing problems, and increased risk of gastrointestinal anomalies and early-onset Alzheimer’s disease in later life.

Diagnosis: Prenatal screening and diagnostic testing can detect trisomy 21; noninvasive prenatal testing indicates risk, while diagnostic tests include chorionic villus sampling or amniocentesis with karyotyping. Postnatal diagnosis is by karyotype or chromosomal microarray.

Management and prognosis: There is no cure. Care is multidisciplinary, focusing on developmental therapies, education, regular monitoring for heart, thyroid, hearing, and vision problems, and timely interventions. Life expectancy has increased substantially with improved medical care.

Epidemiology: Down syndrome occurs in about 1 in 700 live births. The sex chromosome complement of affected individuals varies; 47,XX,+21 denotes a female with trisomy 21.