What is the term for having an extra chromosome?

The correct term for having an extra chromosome is trisomy. This condition occurs when an individual has three copies of a particular chromosome instead of the usual two. For example, Down syndrome is caused by trisomy of chromosome 21, where there are three copies of that chromosome instead of the normal pair.

Trisomy results from an error during cell division, specifically nondisjunction, where chromosomes fail to separate properly. This leads to the formation of gametes with an abnormal number of chromosomes, which can result in an embryo with an extra chromosome when fertilization occurs.

The other terms refer to different chromosomal abnormalities or concepts: disomy refers to the typical two copies of each chromosome, polyploidy indicates having more than two complete sets of chromosomes (common in plants), and monosomy refers to the absence of one chromosome from a pair. Understanding these terms helps clarify the specific types of chromosomal variations that can occur in organisms.