trisomias

Trisomias are chromosomal abnormalities in which a cell contains three copies of a particular chromosome instead of the usual two. They arise from errors in cell division and can affect development, growth, and health. The condition can occur in autosomes or sex chromosomes and can be present in a mosaic form, where some cells carry the extra chromosome and others do not.

The most well-known trisomies affect autosomes: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). These conditions are associated with characteristic, but variable, patterns of physical features and medical issues, including congenital heart defects, developmental delays, and cognitive impact. Trisomies of sex chromosomes include trisomy X (XXX), Klinefelter syndrome (47,XXY), and XYY syndrome. Sex chromosome trisomies often present with more variable or milder physical and developmental effects compared with the major autosomal trisomies, and fertility can be affected.

Causes of trisomias primarily involve nondisjunction during meiosis, leading to an egg or sperm with an extra chromosome. Mosaic trisomies can result when nondisjunction occurs after fertilization. Translocations can also produce trisomic conceptuses. Maternal age is a known risk factor for many autosomal trisomies, particularly trisomy 21, while paternal age has a less consistent association.

Diagnosis commonly combines prenatal screening and diagnostic testing. Noninvasive prenatal testing using cell-free DNA can indicate a high risk for trisomies, followed by definitive testing such as amniocentesis or chorionic villus sampling. Postnatally, a karyotype confirms the diagnosis.

Management is multidisciplinary and focused on addressing medical needs, supporting development, and providing genetic counseling. There is no cure for trisomias, but early intervention and ongoing care can improve quality of life and outcomes.