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Aneuploidy
Specialty	Medical genetics

Aneuploidy is a chromosomal state in which abnormal numbers of specific chromosomes or chromosome sets exist within the nucleus.

A change in the number of chromosomes can lead to a chromosomal disorder. These changes can occur during the formation of reproductive cells (eggs and sperm) or in early fetal development. In humans the most common form of aneuploidy is trisomy (2n+1, where 2n means a normal diploid chromosome number), or the presence of an extra chromosome in each cell. Monosomy (2n-1), or the loss of one chromosome from each cell, is another kind of aneuploidy.

Aneuploidy is common in cancerous cells.

The most commonly observed forms of aneuploidy

Monosomy

Monosomy is the presence of only one chromosome from a pair in a cell's nucleus. Monosomy is a type of aneuploidy. Partial monosomy occurs when the long or short arm of a chromosome is missing.

Human genetic disorders arising from monosomy are:

• X0 (Turner syndrome)
• cri du chat syndrome -- a partial monosomy caused by a deletion of the end of the short (p) arm of chromosome 5

Disomy

A disomy is the presence of a pair of chromosome. For diploid organisms, such as humans, it is the normal condition. For organism that are normally triploid or above, disomy is an aneuploidy. It can also refer to cells that are normally haploid, such as gametes.

In uniparental disomy, the disomy refers to two copies of the chromosome from one of the parents (with no contribution from the other parent).

Trisomy

A trisomy is the presence of three, instead of the normal two, chromosomes of a particular numbered type in an organism. Thus the presence of an extra chromosome 21 is called trisomy 21. Most trisomies, like most other abnormalities in chromosome number, result in distinctive birth defects. Many trisomies result in miscarriage or death at an early age.

A partial trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes, or if one of the chromosomes has two copies of part of its chromosome. A mosaic trisomy is a condition where extra chromosomal material exists in only some of the organism's cells.

While a trisomy can occur with any chromosome, few babies survive to birth with most trisomies. The most common types that survive without spontaneous abortion in humans are:

• Trisomy 21 (Down syndrome)
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome)
• Trisomy 9
• Trisomy 8 (Warkany syndrome 2)

• Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester.

Trisomy involving sex chromosomes includes:

• XXX (Triple X syndrome)
• XXY (Klinefelter's syndrome)
• XYY (XYY syndrome)

Sources

This article incorporates public domain text from The U.S. National Library of Medicine.

See also

• Ploidy
• Polyploidy

External links

• Aneuploidy Testing
• Aneuploidy FAQ
• Genetics of Aneuploids

• Genetics
• Genetic disorders