Which term best describes the genetic alteration that leads to Down syndrome?

The correct answer is aneuploidy, which describes a genetic alteration involving an abnormal number of chromosomes. Down syndrome specifically results from an individual having an extra copy of chromosome 21, leading to a total of three copies rather than the usual two. This condition is known as trisomy 21 and exemplifies aneuploidy, where the number of chromosomes deviates from the typical diploid state.

To further clarify the context, other terms such as point mutation refer to a change in a single nucleotide in the DNA sequence, which is not applicable to Down syndrome since the condition involves an entire chromosome rather than just a small sequence change. Gene expression involves the process where information from a gene is used to synthesize a functional gene product, typically a protein, and does not relate directly to the genetic basis of chromosomal abnormalities. Changes in chromosome structure refer to alterations in the structure or arrangement of chromosomes (such as deletions or duplications) but do not specifically denote an abnormal number of chromosomes, which is the defining feature of aneuploidy. Thus, aneuploidy is the most appropriate descriptor for the genetic alteration leading to Down syndrome.