What does aneuploidy refer to in genetic terms?

Aneuploidy specifically refers to a condition where there is an abnormal number of chromosomes in a cell, which can manifest as either the presence of extra copies or a deficiency of certain chromosomes. This can lead to various genetic disorders, such as Down syndrome, which results from the presence of an extra copy of chromosome 21. Thus, the defining characteristic of aneuploidy is the alteration in the number of individual chromosomes rather than entire sets.

In contrast to the other choices, the addition of whole sets of chromosomes describes polyploidy, which is a different phenomenon. The complete absence of chromosomes refers to a more extreme state of chromosomal loss, while the exact duplication of all chromosomes pertains to the process of cell division and replication rather than the specifics of aneuploidy.