t21

T21, in medical and genetic contexts, most commonly refers to Trisomy 21, the presence of an extra copy of chromosome 21. This chromosomal condition is the most common cause of intellectual disability and affects about 1 in 700 live births. The extra chromosome typically arises from nondisjunction during meiosis, but it can also result from a Robertsonian translocation or, less commonly, mosaicism where some cells have the extra chromosome and others do not.

There are three main forms of T21. Free trisomy 21 is the most common, in which all cells carry three copies of chromosome 21. Mosaic trisomy 21 involves a mix of cells with two or three copies. Translocation Down syndrome occurs when part of chromosome 21 is attached to another chromosome, often due to a Robertsonian translocation such as t(14;21) or a similar rearrangement; in this form, the total chromosome count may be normal, but there is extra 21 material.

Clinical features vary but commonly include mild to moderate intellectual disability, hypotonia (reduced muscle tone), and distinctive facial features. Congenital heart defects, often present at birth, occur in a substantial fraction of individuals. Other associated health issues can include thyroid conditions, hearing and vision problems, accelerated risk of leukemia in childhood, and later development of early-onset Alzheimer disease in some adults.

Diagnosis may occur prenatally through screening and diagnostic tests (such as nuchal translucency ultrasound and invasive testing with karyotyping or microarray) or postnatally by confirming the karyotype. Management is multidisciplinary, emphasizing early intervention, educational planning, regular medical care, and monitoring for associated conditions. Life expectancy has improved markedly with modern care, and many individuals with T21 lead healthy, fulfilling lives.