Trisomia

Trisomia, in genetics, refers to a chromosomal abnormality in which an individual has three copies of a chromosome instead of the usual two. The condition can involve autosomes or sex chromosomes and may occur in all body cells (constitutional trisomy) or in a subset of cells (mosaic trisomy).

Most trisomies arise from nondisjunction during meiosis, when an egg or sperm contributes an extra chromosome to the embryo. Translocations can also produce trisomy if a fragment or whole chromosome is joined to another chromosome. Mosaic trisomy occurs when nondisjunction happens after fertilization, leading to two or more cell lines. Some trisomies result from inherited rearrangements in a parent with a balanced translocation or inversion.

Common autosomal trisomies include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). Down syndrome is characterized by intellectual disability, distinctive facial features, hypotonia, and a higher risk of congenital heart defects; life expectancy has improved with comprehensive medical care. Edwards and Patau syndromes involve more severe congenital anomalies and higher infant mortality, though the range of outcomes varies among individuals. Sex chromosome trisomies such as XXX, XXY (Klinefelter syndrome), and XYY often present with milder or more variable features, including reproductive or learning differences.

Diagnosis can be prenatal, via noninvasive tests (NIPT) and diagnostic procedures (amniocentesis or chorionic villus sampling with karyotyping or microarray), or postnatal by chromosome analysis of blood. Management is multidisciplinary and supportive, addressing medical complications and development, with genetic counseling for families.

Epidemiology varies by trisomy type; fetal losses are common, and the likelihood of a live birth or survival depends on the specific trisomy and its severity.