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Classification & external resources
ICD-10 Q90.-Q98.
ICD-9 758
MeSH D000782

Aneuploidy is a change in the number of chromosomes that can lead to a chromosomal abnormality. In humans, in every cell of our bodies, we typically will have 46 chromosomes. The only exception to this is that our egg and sperm cells will only have half the number of chromosomes (23). The 46 chromosomes can be arranged into 23 pairs. From each pair, we typically will inherit one chromosome from our mother and one from our father. The first 22 pairs of chromosomes are referred to as autosomes, or body chromosomes. They are numbered from 1 through 22 from largest to smallest. The last pair of chromosomes are the sex chromosomes. Females will typically have two X chromosomes, while males will typically have one X chromosome and one Y chromosome.

The most common form of aneuploidy is Down syndrome, which is caused by an extra copy of chromosome number 21. [1]


Common forms

Name # Description
monosomy 1 Monosomy refers to the presence of only one chromosome (instead of the typical two in humans) from a pair. Partial monosomy occurs when only a portion of the chromosome has one copy, while the rest has two copies (see deletion (genetics)).
disomy 2 A disomy is the presence of two copies of a chromosome. For organisms, like humans, that have two copies of each chromosome (those that are diploid), it is the normal condition. For organisms that normally have three or more copies of each chromosome (those that are triploid or above), disomy is an aneuploidy. In uniparental disomy, both copies of the chromosome are from the same parent (with no contribution from the other parent).
trisomy 3 A trisomy is the presence of three copies, instead of the normal two, of a particular chromosome. The presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.
tetrasomy/pentasomy 4/5 A tetrasomy and a pentasomy are the presence of a respectively four or five copies of a chromosome. Although very rare, reported examples of tetrasomy and pentasomy in humans include the karyotypes XXXX (XXXX syndrome), XXXY, XXYY, XYYY, XXXXX, XXXXY, XXXYY, XXYYY and XYYYY.[2]

Changes in chromosome number may not necessarily be present in all cells in an individual. It may be detected in just a specific tissue or within different cells in a tissue. When the presence of chromosomal abnormalities occurs differentially within an individual, it is called chromosomal mosaicism. In general, as we would expect, individuals who are mosaic for a chromosome change tend to have a less severe form of the syndrome present than full trisomy individuals. Critical examples of mosaicism are found in leukemia cases, specifically; chronic lymphocytic leukemia (CLL) which is a trisomy of chromosome 12 and acute myeloid leukemia (AML) prognosis which is a trisomy of chromosome 8. Aneuploidy is common in cancerous cells.


  Aneuploidy is typically detected through karyotyping, a process where a picture of the chromosomes of an individual is analyzed. Other techniques include Fluorescence In Situ Hybridization (FISH), Quantitative Polymerase Chain Reaction (PCR) of Short Tandem Repeats, Quantitative Fluorescence PCR (QF-PCR), Quantitative Real-time PCR (RT-PCR) dosage analysis, Quantitative Mass Spectrometry of Single Nucleotide Polymorphisms, and Comparative Genomic Hybridization (CGH). These tests can also be done prenatally to detect aneuploidy in a pregnancy, either through amniocentesis or chorionic villus sampling.

Pregnant women over the age of 35 are typically offered various tests for aneuploidy in the fetus because the chance of these syndromes increases as the mother's age increases. For more information, see prenatal diagnosis.


  1. ^ Aneuploidy. Retrieved on 2007-12-20.
  2. ^ Linden MG, Bender BG, Robinson A (1995). "Sex chromosome tetrasomy and pentasomy". Pediatrics 96 (4 Pt 1): 672–82. PMID 7567329.
  • This article incorporates public domain text from The U.S. National Library of Medicine.

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Aneuploidy". A list of authors is available in Wikipedia.
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