Nondisjunction

Nondisjunction is an error of cell division in which chromosomes that should move to opposite poles of the cell travel to the same pole instead. It can happen in meiosis I, when a pair of homologous chromosomes stays together, or in meiosis II or mitosis, when two sister chromatids fail to part. One daughter cell then ends up with an extra copy of the chromosome and the other with none.

If nondisjunction occurs as eggs or sperm form, a human gamete may carry 24 or 22 chromosomes instead of 23, and an embryo formed from it has a gain or loss of a whole chromosome in every cell, a state called aneuploidy. A gain gives trisomy, as in Down syndrome (three copies of chromosome 21), and a loss gives monosomy, as in Turner syndrome (a single X chromosome). Most other autosome gains or losses end in miscarriage, while extra or missing sex chromosomes are better tolerated. An error in meiosis I makes all four products abnormal, while an error in meiosis II leaves two of the four normal.

The chance of nondisjunction in eggs rises with the mother's age, which is why the likelihood of trisomy 21 increases in later pregnancies. Nondisjunction during mitosis after fertilization produces mosaicism, in which only some of a person's cells have an abnormal number.