Trissomia

Trissomia, or trisomy, is a chromosomal condition in which a person has three copies of a chromosome instead of the usual two. It can be full trisomy, mosaic trisomy, or partial trisomy (segmental duplication).

Most trisomies arise from nondisjunction during gamete formation, producing a gamete with an extra chromosome that fuses with a normal one. Other mechanisms include translocations and post-zygotic errors that create mosaicism. Full trisomies other than certain common ones are generally not compatible with life; mosaic trisomies may vary in severity.

Down syndrome (trisomy 21) is the most common viable trisomy, with intellectual disability and characteristic facial features and potential congenital heart defects. Trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) are associated with severe congenital anomalies and higher infant mortality. Sex chromosome trisomies such as 47,XXX; 47,XXY (Klinefelter syndrome); and 47,XYY can be compatible with life, with a broad range of effects.

Diagnosis may occur prenatally through screening and diagnostic tests, including noninvasive cell-free fetal DNA testing and procedures like amniocentesis or chorionic villus sampling, or postnatally by karyotyping or microarray.

Prevalence varies; trisomy 21 occurs in about 1 in 700 live births and risk rises with maternal age. Other trisomies are rarer. Prognosis depends on the specific trisomy and mosaicism; individuals with Down syndrome can often live full lives with medical and educational support, while trisomies 18 and 13 have more limited survival.

Management is multidisciplinary, focusing on medical care, early intervention, education, and family support.