Which term best describes the condition where an individual has one fewer chromosome than normal?

The term that best describes the condition where an individual has one fewer chromosome than normal is monosomy. Monosomy specifically refers to the absence of one chromosome from the typical diploid set, resulting in a total chromosome count that is one less than what is normally expected in the species. For example, in humans, the normal chromosome number is 46 (23 pairs), so monosomy would refer to a condition where there are 45 chromosomes due to the loss of a single chromosome.

Aneuploid refers to any condition in which the chromosome number is not an exact multiple of the haploid number, encompassing both deletions and additions of chromosomes, which makes it a more general term than monosomy. Diploid denotes a normal cellular condition with two complete sets of chromosomes, and trisomy designates the presence of an additional chromosome, leading to a total of three copies of a particular chromosome. These definitions clarify why monosomy is the best description for having one fewer chromosome than normal.