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

Sources

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

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Trisomy 13 survival can exceed 1 year - Seen in 8% of Affected Infants
From OB/GYN News, 3/1/03 by Mitchel L. Zoler

BALTIMORE -- About 8% of infants born with either trisomy 13 or trisomy 18 survive longer than 1 year, Dr. Sonja A. Rasmussen said at the annual meeting of the American Society of Human Genetics.

"The perception among most physicians is that these children don't live very long," and, in fact, the median duration of survival was 7-10 days for infants with trisomy 13 and 10-15 days for infants with trisomy 18, she said.

"But the important thing for physicians to keep in mind when counseling parents is that some children live longer. Some can live longer than a year, and there are anecdotal reports of children living into their teens," said Dr. Rasmussen, a clinical geneticist at the National Center on Birth Defects and Developmental Disorders in Chamblee, Ga.

She and her associates analyzed survival statistics using population-based data sets. One set included data from 1969-1999 on 69 live-born infants with trisomy 13 and 106 live-born infants with trisomy 18. The data were collected by the Metropolitan Atlanta Congenital Defects Program.

The second data set included death certificate information in the Multiple-Cause Mortality Files for people who died in the United States during 1979-1999. These files included 5,515 people who died with trisomy 13 and 8,750 people who died with trisomy 18.

In the Metro Atlanta data set, among children with trisomy 13 the median duration of survival was 7 days, with 30.9% surviving for more than 30 days and 8.6% surviving for more than a year. Among children with trisomy 18 the median duration of survival was 14.5 days; 38.6% survived for more than a month, and 8.4% survived for more than a year.

In the death certificate data set, among children with trisomy 13 the median duration of survival was 10 days, 30.4% of children survived for more than a month, and 5.6% survived for more than a year. Among children with trisomy 18 the median duration of survival was 10 days, with 37.9% surviving for more than a month and 5.6% surviving for more than a year.

In general, the information in the Metro Atlanta data set is more reliable because Dr. Rasmussen confirmed the trisomy diagnosis by examining karyotypes in each patient's record. In the death certificate data set, a diagnosis of trisomy could not be independently confirmed.

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