Which condition describes having three copies of a chromosome?

The condition described by having three copies of a chromosome is trisomy. This occurs when an individual has three homologous chromosomes instead of the typical two in diploid organisms. Trisomy is often associated with specific genetic disorders, such as Down syndrome, which is caused by an extra copy of chromosome 21.

Monosomy refers to the presence of only one copy of a particular chromosome instead of the normal two, leading to a lack of genetic material for that chromosome. Tetrasomy involves having four copies of a chromosome, and hexaploidy refers to having six sets of chromosomes in total. These terms describe different chromosomal conditions, but only trisomy specifically pertains to the scenario of having three copies of a single chromosome.