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Nondisjunction: Definition

Nondisjunction is the failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate properly during cell division, producing gametes or cells with an abnormal chromosome number (aneuploidy). Its consequences include trisomies such as trisomy 21 (Down syndrome), sex-chromosome aneuploidies like XXXXY, and — in birds — rare sex-ratio exceptions explained by nondisjunction of sex chromosomes Etiology of nondisjunction in flies and people Maternal age effect review Canary nondisjunction progeny analysis.

Mechanisms: How Chromosomes Fail to Separate

Live-cell imaging reveals the cell-biological roots of nondisjunction: maloriented chromosomes (syntelic/merosyntelic attachments) that lag at anaphase fail to segregate. In cold-treated crane fly spermatocytes, anaphase lag and nondisjunction increased with recovery time (43%→72% lag; 1%→8% nondisjunction) Malorientations cause nondisjunction (crane fly). In mouse oocytes, unstable kinetochore attachments cause whole-bivalent nondisjunction: hybrid oocytes had only 81.5% attached kinetochores (vs 100% in CD-1) and 30% nondisjoined chromosomes in young hybrids Whole-bivalent nondisjunction in mouse oocytes.

Outcomes and Evidence Limits

Molecular analyses of 18 leukemic trisomy 21 cases found the extra chromosome was predominantly maternal in origin, often with reduced recombination, but found no systematic difference in nondisjunction mechanism between leukemic and nonleukemic Down syndrome Nondisjunction in trisomy 21 leukemia cases. Familial clustering of sex-chromosome aneuploidy and trisomy 21 (an XXXXY male with trisomy 21) suggests inherited predisposition to nondisjunction in rare families Familial nondisjunction cluster (Lancet, 1961). Uncertainty: mechanistic data come largely from model organisms (crane fly, mouse, Drosophila) whose spindle biology may not fully generalize to human oocytes, and hypotheses such as the microcirculation model of the maternal age effect rest on circumstantial review evidence. Evidence that age-dependent spindle dysfunction contributes to human female nondisjunction would be challenged if large cohort studies showed nondisjunction rates independent of oocyte age or attachment stability.