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Polyploidy is the state of a cell or organism having more than two paired (homologous) sets of chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes—one set inherited from each parent. However, some organisms are polyploid, and polyploidy is especially common in plants. In addition, polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues. This is known as endopolyploidy. Species whose cells do not have nuclei, that is, prokaryotes, may be polyploid, as seen in the large bacterium Epulopiscium fishelsoni. Hence ploidy is defined with respect to a cell. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Males of bees and other Hymenoptera, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations. The gametophyte generation is haploid, and produces gametes by mitosis, the sporophyte generation is diploid and produces spores by meiosis.