What is a defining feature of an aneuploid organism?

An aneuploid organism is characterized by having an abnormal number of a particular chromosome, which means that instead of the standard diploid set, there is a gain or loss of one or more chromosomes. This can involve an individual having an extra copy of a chromosome (trisomy) or missing one (monosomy). For instance, Down syndrome is a well-known example of aneuploidy, where there is an extra copy of chromosome 21.

In contrast, having complete sets of chromosomes indicates a euploid organism, which is not indicative of aneuploidy. The presence of multiple missing chromosomes suggests a more severe condition that affects the organism’s viability, but it does not specifically define aneuploidy. Lastly, identical chromosome pairs pertain to a normal diploid state where each chromosome has a homologous pair, which again does not align with the definition of aneuploidy. Thus, the defining characteristic of an aneuploid organism is the abnormal number of a specific chromosome.