Aneuploidy

Aneuploidy is a condition in which a cell has an abnormal number of chromosomes that is not an exact multiple of the normal haploid set, with one or a few chromosomes extra or missing. It is a leading cause of miscarriage and of some genetic syndromes.

A cell missing one chromosome of a pair is monosomic and one with an extra copy is trisomic. In humans trisomy 21 causes Down syndrome, and changes in the number of sex chromosomes cause conditions such as Turner syndrome (45,X) and Klinefelter syndrome (47,XXY). Most other changes are fatal early in development, and aneuploidy is found in a large share of early miscarriages.

The usual cause is nondisjunction, the failure of chromosomes or sister chromatids to separate in meiosis or mitosis. The risk of trisomy increases with the age of the mother, probably because of the long pause that egg cells spend in meiosis. Failures of the spindle checkpoint or defects of the centromere can contribute, and the result is a gamete with too many or too few chromosomes.

Most tumors are aneuploid, with large gains and losses of chromosomes that change gene dosage and may drive cancer growth. Some people have mosaic aneuploidy, in which only some cells are affected. Aneuploidy differs from polyploidy, the whole-genome multiplication that is normal in many plants. See chromosomes and genetics.

• Chromosomes — the structures whose number changes
• Meiosis — the division where nondisjunction often occurs
• Centromere — the region that guides segregation