In the context of genetic disorders caused by an extra copy of a chromosome, the correct answer to the question is:
a. Down syndrome
Explanation:
1. Down syndrome is a genetic disorder that is caused by the presence of an extra copy of chromosome 21.
2. Individuals with Down syndrome typically have characteristic physical features, intellectual disabilities, and may experience certain medical issues.
3. This extra chromosome 21 disrupts the normal course of development and leads to the symptoms associated with Down syndrome.
4. Cystic fibrosis, Huntington's disease, and Trisomy 18 are genetic disorders caused by different genetic mutations or chromosomal abnormalities, not by an extra copy of chromosome 21.
