Aneuploidy

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 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.

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)