A change in the number of chromosomes leads 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, or the presence of an extra chromosome in each cell. Monosomy, or the loss of one chromosome from each cell, is another kind of aneuploidy.

Aneuploidy is common in cancerous cells. Molecular biologist Peter Duesberg has proposed that it may even be the cause of, and not a symptom of, most cancers (PMID 15085930). This view is still hypothetical, but is increasingly respected by mainstream cancer researchers.

Disomy

A disomy is the presence of a pair of chromosomes, or the normal amount for some organisms including humans. It is not a disorder, or aneuploid, but is the absence of aneuploidism.

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. 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 (Edward's syndrome)
Trisomy 13 (Patau syndrome)
Trisomy 9
Trisomy 8 (Warkany syndrome 2)

Trisomy involving sex chromosomes includes:

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

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

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

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Trisomy
From Gale Encyclopedia of Childhood and Adolescence, 4/6/01

Chromosomes in the human body generally come in pairs. Most people have 23 pairs of chromosomes, for a total of 46. When an extra chromosome is present in one of the pairs (resulting in three instead of two), the abnormality is labeled trisomy. Scientists have numbered the 23 pairs of human chromosome, and the trisomy abnormality may occur in several different chromosomes. For example, when the trisomy abnormality occurs in chromosome 21, the result is Down syndrome. Babies born with Down syndrome usually survive into adulthood.

Other chromosomal abnormalities are trisomy 13, Patau's syndrome, and trisomy 18, Edward's syndrome. Both are relatively rare but serious conditions that cause severe mental retardation and physical deformities. Babies born with either of these trisomies usually do not survive beyond their first year.

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Gale Encyclopedia of Childhood & Adolescence. Gale Research, 1998.

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