"Polyploidy is the condition in which a normally diploid cell or organism acquires one or more additional sets of chromosomes. In other words, the polyploid cell or organism has three or more times the haploid chromosome number. Polyploidy arises as the result of total nondisjunction of chromosomes during mitosis or meiosis. Polyploidy is common among plants and has been, in fact, a major source of speciation in the angiosperms. Particularly important is allopolyploidy, which involves the doubling of chromosomes in a hybrid plant. Normally a hybrid is sterile because it does not have the required homologous pairs of chromosomes for successful gamete formation during meiosis. If through polyploidy, however, the plant duplicates the chromosome set inherited from each parent, meiosis can occur, because each chromosome will have a homologue derived from its duplicate set. Thus, polyploidy confers fertility on the formerly sterile hybrid, which thereby attains the status of a full species distinct from either of its parents. It has been estimated that up to half of the known angiosperm species arose through polyploidy, including some of the species most prized by man. Plant breeders utilize this process, treating desirable hybrids with chemicals, such as colchicine, that are known to induce polyploidy. Polyploid animals are far less common, and the process appears to have had little effect on animal speciation." Encyclopedia Britannica Online, "Polyploidy", April 22, 2005. "The multiplication of entire sets of chromosomes is known as polyploidy. A diploid organism carries in the nucleus of each cell two sets of chromosomes, one inherited from each parent; a polyploid organism has three or more sets of chromosomes. Many cultivated plants are polyploid: bananas are triploid, potatoes are tetraploid, bread wheat is hexaploid, some strawberries are octaploid. All major groups of plants have polyploid species, but they are most common among flowering plants (angiosperms), of which about 47 percent are polyploids. Polyploidy is rare among gymnosperms, such as pines, firs, and cedars, although the redwood, Sequoia sempervirens, is a polyploid. Most polyploid plants are tetraploids. Polyploids with three, five, or some other odd-number multiple of the basic chromosome number are sterile, because the separation of homologous chromosomes cannot be achieved properly during formation of the sex cells. Some plants with an odd number of chromosome sets persist by means of asexual reproduction, particularly through human cultivation; the banana is one example. Polyploidy is a mode of quantum speciation that yields the beginnings of a new species in just one or two generations. There are two kinds of polyploids: autopolyploids, which derive from a single species, and allopolyploids, which stem from a combination of chromosome sets from different species. Allopolyploid plant species are much more numerous than autopolyploids. An allopolyploid species can originate from two plant species that have the same diploid number of chromosomes. The chromosome complement of one species may be symbolized as AA, and the other BB. An interspecific hybrid, AB, will usually be sterile owing to abnormal chromosome pairing and segregation during formation of the gametes at meiosis. But chromosome doubling may occur as a consequence of abnormal mitosis, in which the chromosomes divide but the cell does not. In a hybrid, this results in a cell with four sets of chromosomes, AABB. Tetraploid plant cells may proliferate and produce branches and flowers. Because there are two chromosomes of each kind, functional diploid gametes with the constitution AB can be produced by the tetraploid flowers. The union of two such gametes at fertilization produces a tetraploid individual (AABB). In this way, self-fertilization in plants makes possible the formation of a tetraploid individual as the result of a single abnormal cell division. Autopolyploids originate in a similar fashion, except that the individual in which the abnormal mitosis occurs is not a hybrid. Self-fertilization thus enables a single individual to multiply and give rise to a population. This population is a new species, since polyploid individuals are reproductively isolated from their diploid ancestors. A cross between a tetraploid and a diploid yields triploid progeny, which are sterile." Encyclopedia Britannica Online, "Evolution > The process of evolution > Species and speciation > The origin of species > Polyploidy", April 22, 2005.