Triszómia

Triszómia, or trisomy, is a chromosomal abnormality in which an individual has three copies of a chromosome instead of two. The extra genetic material can disrupt development and organ function. Most trisomies result in miscarriage, but a few are compatible with life, notably trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome).

Causes include nondisjunction during meiosis in one parent, producing a gamete with an extra chromosome; fertilization then yields trisomy. Mosaic trisomy arises when nondisjunction occurs after fertilization, so some cells are trisomic and others are not. Translocations can also cause trisomy if a chromosome segment is duplicated or exchanged without changing chromosome number.

Incidence varies by chromosome: trisomy 21 about 1 in 700 live births; trisomy 18 about 1 in 5,000; trisomy 13 about 1 in 12,000. Maternal age is a risk factor for nondisjunction, but trisomies occur in pregnancies of all ages.

Diagnosis uses prenatal screening and diagnostic tests, such as ultrasound markers and maternal serum screening, followed by confirmatory testing with amniocentesis or chorionic villus sampling. After birth, karyotyping confirms the diagnosis. Management is multidisciplinary, addressing medical problems, developmental support, and education. Prognosis varies by trisomy; Down syndrome has a broad range of outcomes, while Edwards and Patau syndromes have higher infant mortality.

Historically, trisomy 21 was described as Down syndrome by John Langdon Down; the concept of trisomy as a chromosomal abnormality was clarified with cytogenetics in the 20th century.