The condition in which an organism has three or more complete sets of chromosomes (see genome
) in its nuclei. Polyploids originate when gametes containing more than one chromosome set fuse. Such gametes are formed when chromosomes fail to separate during anaphase 1 of meiosis. Consequently gametes are diploid instead of haploid, and fertilization results in triploid or tetraploid individuals. Polyploid individuals may be produced as a result of multiplication of chromosome sets from one species (*autopolyploidy
) or by combining sets of chromosomes from different species (*allopolyploidy
). In either case the polyploid offspring may be incapable of reproducing with their parents and so constitute new species. Polyploidy is common in flowering plants (40% of dicotyledons and 60% of monocotyledons are polyploid) and has probably contributed significantly to their evolution. In polyploid organisms harmful recessive alleles are more likely to be masked by normal dominant alleles. Polyploids also have a greater store of genetic variability and thus evolutionary potential is higher in polyploid populations.
In contrast to plants, polyploidy is extremely rare in animals because the sex-determining mechanism often depends on chromosome numbers and ratios. These are upset by polyploidy, and consequently polyploid animals are inviable or sterile. See also hybrid