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.

This article incorporates public domain text from The U.S. National Library of Medicine (http://ghr.nlm.nih.gov)