What term describes the presence of an extra chromosome, resulting in three copies instead of a pair?

The term that describes the presence of an extra chromosome, resulting in three copies instead of the usual pair, is trisomy. This condition occurs when there is an abnormal division of chromosomes during cell division, leading to a gamete with an extra chromosome. When this gamete participates in fertilization, the resulting zygote will then have three copies of that chromosome.

An example of trisomy is Down syndrome, which is caused by the presence of an extra copy of chromosome 21. Understanding this concept is crucial for recognizing how genetic variations can arise and how they lead to different genetic disorders.

The other terms listed refer to different chromosomal abnormalities or genetic concepts. Monosomy refers to the presence of only one copy of a chromosome instead of two, which results in significant genetic imbalances and is often lethal. The term genome refers to the complete set of genes or genetic material present in an organism. Antisomy is not a standard term used in genetics.