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Number of sets of chromosomes in the nucleus of a cell. In normal human body cells, chromosomes exist in pairs, a condition called diploidy. During meiosis the cell produces sex cells (gametes), each containing half the normal number of chromosomes, a condition called haploidy. When an egg and a sperm unite in fertilization, the diploid condition is restored. Polyploid cells have three or more times the number of chromosomes found in haploid cells; polyploid organisms usually cannot reproduce. Aneuploid cells have an abnormal number of chromosomes that is not a multiple of the haploid number. Aneuploidy is most often caused by an error leading to an unequal distribution of chromosomes during division.
This entry comes from Encyclopædia Britannica Concise. For the full entry on ploidy, visit Britannica.com.