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As mentioned in Key Concept 11.4, mature organisms are often either diploid (for example, most animals) or haploid (for example, most fungi). Under some circumstances, triploid (3n), tetraploid (4n), or higher-order polyploid nuclei may form. Each of these ploidy levels represents an increase in the number of complete chromosome sets present. If there is nondisjunction of all of the chromosomes during meiosis I, diploid gametes will form. This can lead to autopolyploidy after fertilization. Autotriploids and autotetraploids have been important in some cases in species formation. A diploid nucleus can undergo normal meiosis because there are two sets of chromosomes to make up homologous pairs, which separate during anaphase I. Similarly, a tetraploid nucleus has an even number of each kind of chromosome, so each chromosome can pair with its homolog. However, a triploid nucleus cannot undergo normal meiosis because one-third of the chromosomes would lack partners. Triploid individuals are usually sterile.

Because polyploid nuclei have more chromosome sets, their cells tend to be larger. This has led to the use of polyploid plants in agriculture. Diploid bananas (2n = 22) are smaller and produce inedible seeds; triploid bananas (3n = 33) are larger and seedless. A similar phenomenon is seen in triploid seedless watermelon. Perhaps the best known, and certainly the most important, polyploid crop plant is wheat. In this case, hybridization occurred between species, forming new allopolyploid conditions:

Modern bread wheat is the result of two such events, which occurred around 8,000 to 10,000 years ago, resulting in a hexaploid. Wheat’s properties of grain formation and environmental adaptation thus come from three different ancestral species. Other allopolyploid crops include cotton, oats, and sugarcane.