How does Down syndrome occur?

• A. An extra 18th chromosome.
• B. A missing chromosome 21.
• C. An extra 21st chromosome.
• D. A translocation involving chromosome 21.

Answer: (C)

Down syndrome occurs when there are three copies of chromosome 21 in the body's cells. This extra copy means there is additional genetic material from chromosome 21, which disrupts normal development and leads to the distinctive physical features and developmental differences seen in Down syndrome. The most common mechanism is nondisjunction during the formation of the egg (maternal meiosis), and this risk increases with maternal age. In some cases Down syndrome can also arise from a chromosome 21 translocation, but the defining idea is the presence of an extra chromosome 21 material. The other scenarios describe different problems (such as trisomy 18 or monosomy 21) that are not Down syndrome. So, an extra 21st chromosome best explains Down syndrome.