aneuploidy (an-you-ploy-dee) n. -- a condition in which a set of chromosomes in a nucleus are not complete. An extra chromosome may be added, or one may be missing. (Contrast with polyploidy, which is the occurence of multiple complete sets of chromosomes.)

Examples of aneuploidy include Down syndrome (trisomy 21), Klinefelter's syndrome (XXY sex chromosomes), and Turner's syndrome (XO sex chromosomes). Examples do not include Fragile X Syndrome or any instance where part of a chromosome is removed or duplicated into the genome.

Aneuploidy is often used in genetic engineering, usually in plants, in order to generate larger flowers or brighter flowers, or more disease resistance, or something of the like. Aneuploidy can also be forced on certain fungi (Neurospora and the like) in order to analyze segregation during meiosis.

It's worth noting, in humans at least**, that aneuploidy is most commonly trisomy of a given chromosome (13, 18, 21, 22, etc.) rather than monosomy. This is because the autosomes are incapable of maintaining the cell with only one copy, thus killing the new embryo before it ever implants into the uterus; as such, monosomies in autosomes are never seen. Also, only certain trisomies are ever maintained; for example, trisomy 4 would be lethal. The exception to this rule is the sex chromosomes. A cell can have any number of sex chromosomes, so long as at least one is an X chromosome.

** -- I'm not sure about anything else. If you are, feel free to write it up.

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