Gene therapy is defined as

• A. Testing an individual's genome for disease risk.
• B. Transplantation of normal genes into cells to replace missing or defective ones.
• C. Cloning an organism to produce identical offspring.
• D. Transferring genes between different species.

Answer: (B)

Introducing normal genes into cells to replace missing or defective ones is what gene therapy aims to do. This captures the idea of delivering a functional gene to correct a faulty one in the body's cells, with the goal of restoring normal function or halting disease progression. That’s why this description best fits gene therapy.

Genetic testing to assess disease risk is a screening or diagnostic activity, not a therapy. Cloning an organism is a reproductive technique to produce identical copies, not a treatment. Transferring genes between species describes genetic modification across species, which is about altering genomes rather than delivering a therapeutic gene to treat a disease in a patient.