Most human cells carry exactly 46 chromosomes. Aneuploidy describes any departure from that normal number—extra chromosomes or missing ones. The imbalance disrupts gene dosage and usually causes developmental problems.

Trisomy 21, the cause of Down syndrome, is the most familiar example. Monosomy X produces Turner syndrome. Many aneuploidies end in miscarriage because the embryo cannot tolerate the severe disruption. Errors in chromosome segregation during meiosis are the usual source.

Prenatal screening and diagnostic tests detect aneuploidy through karyotyping, chromosomal microarray, or cell-free DNA analysis. Maternal age increases the risk for certain trisomies. Understanding aneuploidy helps clinicians counsel families and researchers explore the mechanisms of chromosome segregation.

• Abnormal number of chromosomes
• Includes trisomy and monosomy
• Often arises during meiosis
• Frequently affects development