Advanced Placement (AP) Biology Practice Exam 2026 - Free AP Biology Practice Questions and Study Guide

An organism with an incomplete or abnormal chromosomal number due to the failure of chromosomes to separate during meiosis is best described by the term aneuploidy. This condition occurs when there is an atypical number of chromosomes, leading to an imbalance that can result in various genetic disorders. Aneuploidy typically arises from nondisjunction, which is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This can lead to gametes with extra or missing chromosomes, and when these gametes contribute to fertilization, the resulting organism will have an abnormal chromosomal count.

In contrast, overexpression pertains to genes being expressed at higher-than-normal levels, while translocation involves the rearrangement of parts between nonhomologous chromosomes, both concepts unrelated to the specific issue of chromosomal number. Polyploidy, on the other hand, refers to organisms having multiple complete sets of chromosomes, which is quite different from the singular extra or missing chromosomes characteristic of aneuploidy.