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.

Read more at Wikipedia.org

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How women weigh screening options; Trisomy 21 screening: 'no one size fits all'
From OB/GYN News, 5/15/04 by Miriam E. Tucker

NEW YORK -- It's important to offer patients a range of options for trisomy 21 screening, Dr. Fergal D. Malone said at an obstetrics symposium sponsored by Columbia University and New York Presbyterian Hospital.

The screening options that women choose will vary depending upon their perceived risk for having a child with Down syndrome. "Some want the highest detection rate, while others want the lowest false-positive rate. No one size fits all," said Dr. Malone, director of obstetric and gynecologic ultrasound and director of perinatal research at Columbia Presbyterian Medical Center.

At his institution, women are offered one of two screens for trisomy 21. They can choose first-trimester combined screening, in which nuchal translucency (NT), pregnancy-associated plasma protein A (PAPP-A), and free [beta]-human chorionic gonadotropin (hCG) are measured at 10-14 weeks and the results are provided. If the results are positive, chorionic villus sampling (CVS) is then offered.

Alternatively, a patient can opt for integrated screening, in which NT and PAPP-A are measured at 10-14 weeks, followed by the quad screen (serum alpha fetoprotein, hCG, unconjugated estriol, and inhibin A) at 15-16 weeks, and the results are given all at once as a single risk value; if the results are positive, amniocentesis is then offered to the patient.

In general, the integrated screen is optimal if speed is not a priority, while the first-trimester combined screen is better if speed is important, Dr. Malone said.

Overall, 85% of Columbia patients choose the integrated screen, while only 15% of women choose the first-trimester combination with the option of CVS. However, 90% of the women who choose the first-trimester combined screen are age 37 or older, while just 30% of those choosing integrated screening are in that age group.

It appears, then, that patients who perceive themselves to be at high risk for Down syndrome are more likely to choose an early test with early results. On the other hand, those who believe their risk is low tend to select the test with the lowest possible false-positive rate, which they perceive to be the most efficient, he said.

Interestingly, 88% of the women who had negative first-trimester screens decided to undergo amniocentesis anyway, while 60% of those with positive first-trimester screens refused a CVS and then waited until the second trimester to undergo amniocentesis.

It may be that many women of advanced maternal age already have decided that they will undergo an invasive test no matter what the results of their screening test are, and are using the first-trimester screen merely to determine whether their risk is high enough to go with CVS--which they perceive as more dangerous--or to wait for the "safer" test, amniocentesis.

Women with a positive first-trimester screen who refuse CVS may not feel the result was "bad enough" to justify the procedure, Dr. Malone speculated.

But regardless of their reasons. "There's a range of patients out there who want different things from their screening tests. We need to position ourselves over the next few months and years to be able to offer them a range of screening tests," he said.

BY MIRIAM E. TUCKER

Senior Writer

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