Advanced Placement (AP) Biology Practice Exam

What process typically leads to Down Syndrome?

1. Gene inversion

2. Nondisjunction

3. Gene deletion

4. Chromosome translocation

The correct answer is: Nondisjunction

Nondisjunction is the process that typically leads to Down syndrome, which is also known as trisomy 21. This occurs when there is a failure of homologous chromosomes or sister chromatids to separate properly during cell division, specifically during meiosis. As a result, one gamete may end up with an extra copy of chromosome 21. When this gamete combines with a normal gamete during fertilization, the resulting zygote has three copies of chromosome 21, instead of the usual two. This genetic anomaly leads to the characteristics associated with Down syndrome, including physical traits and developmental challenges. Other processes like gene inversion, gene deletion, and chromosome translocation can affect genetic material and lead to various genetic disorders, but they do not cause Down syndrome. Gene inversion rearranges the order of genes on a chromosome, gene deletion removes segments of DNA, and chromosome translocation involves the transfer of a segment of DNA from one location to another, which can lead to different syndromes or cancers, but none of these processes are involved in the typical etiology of Down syndrome.