Some organisms have more than two complete chromosome sets. The general term for this chromosomal alteration is polyploidy, with the specific terms triploidy (3n) and tetraploidy (4n) indicating three or four chromosomal sets, respectively. One way a triploid cell may be produced is by the fertilization of an abnormal diploid egg produced by non disjunction of all its chromosomes. An example of an accident that would result in tetraploidy is the failure of a 2n zygote to divide after replicating its chromosomes. Subsequent normal mitotic divisions would then produce a 4n embryo.Polyploidy is fairly common in the plant kingdom. The spontaneous origin of polyploidy individuals plays an important role in the evolution of plants. In the animal kingdom, polyploid species are much less common, although they are known to occur among the fishes and amphibians. Researchers in Chile were the first to identify a polyploid mammal, a rodent whose cells are tetraploid.

Figure: A tetraploid mammal. The somatic cells of this burrowing rodent, Tympanoctomys barrerae, have about twice as many chromosomes as those of closely related species. Interestingly, its sperm's head is unusually large, presumably a necessity for holding all that genetic material. Scientists think that this tetraploid species may have arisen when an ancestor doubled its chromosome number, presumably by errors in mitosis or meiosis within the animal's reproductive organs.

Additional research has found that a closely related species also appears to be tetraploid. In general, polyploids are more nearly normal in appearance than aneuploids. One extra (or missing) chromosome apparently disrupts genetic balance more than does an entire extra set of chromosomes.

Figure: Aneuploids can be produced through non disjunction in (a) meiosis I, (b) meiosis II, and (c) mitosis.
The gametes that result from meiosis with non disjunction combine with a gamete (with blue chromosome) that results from normal meiosis to produce the zygotes.