nondisjunction

Nondisjunction occurs when chromosomes fail to segregate correctly during anaphase of cell division. In Meiosis I nondisjunction, homologous chromosome pairs fail to separate, producing gametes that are either disomic (containing both homologs) or nullisomic (containing neither). In Meiosis II nondisjunction, sister chromatids fail to separate after Meiosis I has already occurred normally, resulting in one gamete with two copies of one chromatid, one gamete missing that chromosome, and two normal gametes. Mitotic nondisjunction after fertilization leads to mosaicism, where an individual has two or more genetically distinct cell lines.

Clinical significance: Nondisjunction is the leading cause of aneuploidy in humans. Key examples include: - Trisomy 21 (Down syndrome) – most common cause is Meiosis I nondisjunction, strongly associated with advanced maternal age due to prolonged arrest of oocytes in prophase I. - Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome). - Sex chromosome aneuploidies: Klinefelter syndrome (47,XXY), Turner syndrome (45,X), and others. - Uniparental disomy can also result from nondisjunction events combined with 'trisomy rescue.'

Maternal age increases risk due to degradation of the meiotic spindle apparatus and cohesin proteins holding chromosomes together during the extended dictyate arrest of oocytes (which can last decades). Diagnosis is often made via karyotype analysis, and prenatal screening includes maternal serum markers, nuchal translucency ultrasound, and noninvasive prenatal testing (cell-free fetal DNA), with definitive diagnosis via amniocentesis or chorionic villus sampling.

Distinguishing nondisjunction from other mechanisms of aneuploidy (like Robertsonian translocation) is important, especially for Down syndrome, as translocation-based cases have different recurrence risk implications for genetic counseling.