Polysomies

Polysomies are chromosomal abnormalities in which cells contain extra copies of one or more chromosomes or chromosome segments, resulting in more than the usual two copies per cell. They can involve entire chromosomes (whole-chromosome polysomies) or parts of chromosomes (segmental polysomies). Most cases arise from nondisjunction events during gamete formation or early embryonic development; mosaic polysomies occur when the extra copy is present only in a subset of cells.

Common examples include trisomies such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). Sex chromosome polysomies include XXX (Triple X syndrome) and XYY (Jacob syndrome), with other combinations such as XXYY also documented. Rare conditions involve higher-level or partial polysomies, such as tetrasomy 18p. The clinical presentation varies widely depending on the specific chromosome involved and the degree of mosaicism; features may include developmental delay, congenital anomalies, and characteristic physical findings for certain syndromes, while mosaic cases can have milder or more variable manifestations.

Diagnosis is achieved through genetic testing. Conventional karyotyping can detect whole-chromosome polysomies, whereas fluorescence in situ hybridization (FISH), chromosomal microarray, and other molecular methods can identify mosaicism and segmental polysomies. Prenatal testing (amniocentesis or chorionic villus sampling) can identify fetal polysomies.

Management is individualized and multidisciplinary, focusing on treating associated medical issues, maximizing development, and providing genetic counseling. Prognosis depends on the specific chromosome involved, the extent of mosaicism, and the presence of additional anomalies.