What is the term for the condition where an individual has three copies of a chromosome?

The term for the condition where an individual has three copies of a chromosome is trisomy. In human genetics, trisomy is specifically associated with conditions such as Down syndrome, where there is an extra copy of chromosome 21. This occurrence leads to an abnormal number of chromosomes and is a type of aneuploidy, which refers to deviations from the typical number of chromosomes in a cell.

The presence of three copies of a chromosome can disrupt the normal balance of gene expression and result in various phenotypic traits and medical challenges. Each chromosome typically exists in pairs (one from each parent), so having a triplicate can significantly affect development and health.

Understanding trisomy is critical in genetics as it highlights how variations in chromosome number can lead to specific disorders and the importance of chromosome integrity during cell division.