Polyploidy is the condition of some biological cells which containing more than two homologous sets of chromosomes. Polyploid types are termed according to the number of chromosome sets in the nucleus: triploid (three sets; 3n), tetraploid (four sets; 4n), pentaploid (five sets; 5n), hexaploid (six sets; 6n), etc. A normal cell is a haploid and only has one set of chromosomes. In some instances not all the chromosomes are duplicated and the condition is called aneuploidy.

Polyploidy has occurred often in the evolution of plants. The process can begin if diploid (2n) gametes are formed. These can arise in the gametes when they are formed by mitosis instead of Meiosis.

Polyploidy is much rarer in animals. It is found in some insects, fishes, amphibians, and reptiles. Until recently, no polyploid mammal was known. However, the 23 September 1999 issue of Nature reports that a polyploid (tetraploid; 4n 3D 102) rat has been found in Argentina.

The liver cells of the Argentinean rat are larger than those of its diploid relatives, and its sperm are huge in comparison. Normal mammalian sperm heads contain some 3.3 picograms (10-12 g) of DNA; the sperm of the rat contains 9.2 pg.

Animal polyploidy is much rarer, with it being found in some insects, fishes, amphibians, and reptiles. When a newly-arisen tetraploid (4n) plant tries to breed with its ancestral species (a backcross), triploid offspring are formed. Generally these are sterile because they cannot form gametes with a balanced assortment of chromosomes.