A trisomy

A trisomy is a genetic condition where a person has an extra copy of a chromosome—so instead of two copies, there are three. Chromosomes carry our genes and are crucial for healthy development. This abnormality happens during the formation of reproductive cells (sperm or eggs). The extra chromosome can disrupt normal development and cause health problems or physical differences. Examples include Down syndrome, which is caused by an extra chromosome 21. Trisomies can vary in severity and impact, depending on which chromosome is affected.