1. The problem asks: Which genetic disorder is caused by a sex-linked gene? 2. Sex-linked genes are located on the sex chromosomes, typically the X chromosome. Disorders caused by mutations in these genes are called sex-linked disorders. 3. Let's analyze the options: - Sickle cell anemia: caused by a mutation in an autosomal gene (not sex-linked). - Tay-Sachs disease: caused by an autosomal recessive gene. - Hypercholesterolemia: usually autosomal dominant. - Hemophilia A: caused by a mutation in a gene on the X chromosome, making it a sex-linked disorder. - Turner syndrome: caused by a missing or incomplete X chromosome, a chromosomal disorder, not a gene mutation. 4. Therefore, the correct answer is Hemophilia A, which is a classic example of an X-linked recessive genetic disorder. Final answer: 4. hemophilia A