Aneuploidy is a chromosomal state where abnormal numbers of specific chromosomes or chromosome sets exists within the nucleus.

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.

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, the most common types in humans are:

• Trisomy 21 (Down Syndrome)
• Trisomy 18 (Edward's Syndrome)
• Trisomy 13 (Patau's Syndrome)
• Trisomy 8 (Warkany syndrome 2)

Trisomy involving sex chromosomes include:

• Trisomy X (Triplo X Syndrome)
• Klinefelter's syndrome (XXY)
• XYY

Monosomy

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

Human genetic disorders arrising from monosomy are:

• Turner syndrome

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