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The condition in which the chromosome number of the cells of an individual is not an exact multiple of the typical haploid set for that species. The nomenclature employs the suffix somic, as the following examples illustrate. Down syndrome (q.v.) and Turner syndrome (q.v.) are examples of a human trisomic and monosomic, respectively. Nullisomics result from the loss (2N − 2) and tetrasomics from the gain (2N + 2) of a chromosome pair. If more than one different chromosome is lost or gained, the condition is described as doubly monosomic (2N − 1 − 1) or doubly trisomic (2N + 1 + 1). Early studies of aneuploids led to the conclusion that genes carried by specific chromosomes controlled morphological traits. For example, in Datura stramonium (q.v.) extra doses of chromosome G broaden and reduce the seed capsule and increase the size of the spines . See Chronology, 1934, Blakeslee; hyperploid, hypoploid, symbols used in human cytogenetics, polyploid.

Aneuploidy in the Jimson weed, Datura stramonium. Extra doses of G chromosomes make the seed capsules smaller and broader and their spines larger. After Blakeslee, 1934. Drawings of Datura seed capsules by E. John Pfiffner.

Subjects: Genetics and Genomics.

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