Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. It produces daughter cells with abnormal chromosome numbers.

Meiosis: A specialized cell division that produces haploid gametes through two successive divisions. Nondisjunction can occur in either meiotic division, producing gametes with extra or missing chromosomes.
Trisomy: A chromosome condition in which cells carry three copies of one chromosome instead of two. A gamete with an extra chromosome can produce a trisomic embryo after fertilization.
Meiosis I: The first meiotic division, in which homologous chromosomes normally separate. Failure of homologous chromosomes to separate here sends both copies into one daughter cell.
Chromosome lagging: The failure of a chromosome to keep pace with the spindle during cell division, often leading to its loss. A lagging chromosome can cause aneuploidy without both partners moving together to one daughter cell.
Anaphase: The stage of cell division when chromosomes or chromatids move toward opposite poles. Nondisjunction becomes apparent when chromosomes fail to move to opposite poles.
Monosomy: A chromosome condition in which cells carry one copy of a chromosome that is usually present in pairs. A gamete missing a chromosome can produce a monosomic embryo after fertilization.
Meiosis II: The second meiotic division, in which sister chromatids normally separate. Nondisjunction here leaves one gamete with both sister chromatids and another without either.
Polyploidy: The presence of more than two complete sets of chromosomes in a cell or organism. Nondisjunction usually changes individual chromosome counts, whereas polyploidy changes whole chromosome sets.
Cohesin: A protein complex that holds sister chromatids together until their separation during cell division. Faulty timing or loss of cohesion can disrupt the coordinated separation of chromosomes.
Down syndrome: A genetic condition usually caused by an extra copy of chromosome 21. Most cases result from chromosome 21 nondisjunction during gamete formation.

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