þrílitningar

þrílitningar is a genetic term referring to a chromosomal condition in which a cell contains three copies of a chromosome instead of the usual two. The condition can involve autosomes or sex chromosomes. The most well-known viable autosomal trisomies are trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). Many other autosomal trisomies are not compatible with life and result in miscarriage. Trisomies of sex chromosomes also occur, including 47,XXY (Klinefelter syndrome), 47,XXX, and 47,XYY, each with its own clinical spectrum.

Causes and development: Trisomies typically arise from nondisjunction during meiosis in a parent, producing a gamete with an extra chromosome, or from mitotic nondisjunction after fertilization leading to mosaic trisomy. Maternal age is a risk factor for several autosomal trisomies, particularly trisomy 21, though the mechanism is not fully understood.

Diagnosis and detection: Prenatal testing can indicate trisomies and includes noninvasive screening (such as cell-free fetal DNA) and diagnostic procedures like chorionic villus sampling or amniocentesis with karyotyping or microarray. Postnatal diagnosis is made by cytogenetic analysis of blood or other tissues.

Phenotypic consequences: The clinical presentation varies by chromosome involved and by mosaic versus full trisomy. Down syndrome is associated with characteristic features, developmental delay, and possible congenital heart defects; other trisomies present with more severe anomalies and variability in viability. Sex-chromosome trisomies have distinct, often milder, but still variable phenotypes.

Etymology and usage: In Icelandic, þrílitningar literally means three chromosomes, formed from þrí- meaning three and litningar meaning chromosomes. The term is used in clinical genetics, cytogenetics, and medical literature.