Which genetic condition is characterized by having an extra chromosome in a pair?

The genetic condition characterized by having an extra chromosome in a pair is trisomy. In human genetics, trisomy refers specifically to a situation where there are three copies of a particular chromosome instead of the usual two. This results from a nondisjunction event, where chromosomes fail to separate properly during cell division, leading to gametes that contain an abnormal number of chromosomes. When one of these gametes participates in fertilization, the resulting embryo can end up with three copies of that chromosome.

A well-known example of trisomy is Down syndrome, which is caused by the presence of an extra copy of chromosome 21. This additional genetic material affects development and can lead to various physical and intellectual characteristics associated with the condition.

Monosomy, on the other hand, involves the absence of one chromosome from a pair, while euploidy refers to a situation where the number of chromosomes is an exact multiple of the haploid number, typically considered normal in humans. Aneuploidy is a broader term that encompasses any abnormal number of chromosomes, including both trisomy and monosomy. Thus, while aneuploidy is related to the condition of having an extra chromosome, it does not specifically define the situation as clearly as trisomy does.