nondisjunctioni

Nondisjunction is the failure of paired chromosomes to separate properly during cell division, leading to gametes or cells with an abnormal chromosome number. In meiosis, nondisjunction can occur during meiosis I when homologous chromosomes do not segregate, or during meiosis II when sister chromatids fail to separate. Nondisjunction can also occur during mitotic divisions after fertilization, which may produce mosaicism.

When nondisjunction occurs, the resulting gametes contain either an extra chromosome (trisomy) or are missing a chromosome (monosomy). Fertilization of such a gamete typically produces a zygote with aneuploidy. Most autosomal monosomies are embryonically lethal, whereas trisomies can be viable in some cases but often cause severe developmental disorders. Common viable autosomal trisomies include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). Sex chromosome nondisjunction yields conditions such as monosomy X (Turner syndrome) and extra sex chromosomes, including Klinefelter syndrome (XXY), Triple X syndrome (XXX), and XYY syndrome.

Maternal age is a well-documented risk factor for meiotic nondisjunction, particularly for autosomal trisomies. Other factors may influence risk, but the exact causes are not fully understood. Detection of aneuploidy can occur before birth through prenatal screening and diagnostic tests (such as cell-free DNA testing, karyotyping, or microarray analysis) or after birth through clinical evaluation and genetic testing. Management focuses on clinical care for the resulting conditions and varies by chromosome involved and the presence of mosaicism.