Aneuploidy

Aneuploid
Chromosomes in Down syndrome, the most common human condition due to aneuploidy. Notice the three copies of chromosome 21 in the last row.
Classification and external resources
Specialty	Medical genetics
ICD-10	Q90-Q98
ICD-9-CM	758
MeSH	D000782
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Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes, which is called euploidy. An extra or missing chromosome is a common cause of genetic disorders, including some human birth defects. Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy originates during cell division when the chromosomes do not separate properly between the two cells.

Different species normally have different numbers of chromosomes from one another, and the terms "aneuploid" and "polyploid" refer to the chromosome number being different from the usual number for that species.

Chromosome abnormalities are detected in 1 of 160 live human births. Apart from , most cases of aneuploidy result in death of the developing fetus (miscarriage); the most common extra autosomal chromosomes among live births are 21, 18 and 13.

Most cells in the human body have 23 pairs of chromosomes, or a total of 46 chromosomes. (The sperm and egg, or gametes, each have 23 unpaired chromosomes, and red blood cells have no nucleus and no chromosomes.)

One copy of each pair is inherited from the mother and the other copy is inherited from the father. The first 22 pairs of chromosomes (called autosomes) are numbered from 1 to 22, from largest to smallest. The 23rd pair of chromosomes are the sex chromosomes. Normal females have two X chromosomes, while normal males have one X chromosome and one Y chromosome. The characteristics of the chromosomes in a cell as they are seen under a light microscope are called the karyotype.

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