Most human cells contain exactly two copies of each chromosome. Trisomy is the presence of three copies of a particular chromosome. The extra genetic material disrupts normal development, and only a few trisomies are compatible with live birth.

Trisomy 21 causes Down syndrome and is the most common. Trisomy 18 and trisomy 13 produce more severe syndromes with high infant mortality. Sex-chromosome trisomies such as XXX, XXY, and XYY are generally milder. The usual cause is nondisjunction during meiosis, most often in maternal egg formation.

Prenatal screening and diagnostic tests detect trisomies with increasing accuracy. Maternal age is a well-established risk factor. Understanding the mechanisms of chromosome segregation continues to guide research into prevention and the biology of aneuploidy.

• Presence of three copies of a chromosome
• Most often caused by meiotic nondisjunction
• Trisomy 21 is the most common viable form
• Detected by prenatal and postnatal genetic tests