(h) the principles of, and potential for, gene therapy in medicine.

The idea

Gene therapy treats disease by delivering genetic material into a patient's cells so a faulty gene is corrected, replaced or supplemented, or so a harmful gene product is suppressed. Somatic therapy targets body cells such as lung epithelium or bone marrow; changes are not inherited. Germ-line therapy would alter gametes or embryos and is not used clinically in humans. Delivery may be in vivo (vector introduced directly, e.g. an adenovirus carrying a functional allele to lung cells) or ex vivo (cells removed, modified, checked and returned). Vectors include modified viruses and liposomes. Success depends on efficient, targeted delivery, stable expression and immune tolerance. Potential is greatest for single-gene recessive disorders such as cystic fibrosis or severe combined immunodeficiency; complex polygenic disease is far harder.

What earns the marks

• States that gene therapy introduces functional genetic material into a patient's cells to treat disease.
• Distinguishes somatic therapy (body cells, not inherited) from germ-line therapy (gametes or embryos, heritable, not used clinically in humans).
• Identifies delivery methods: viral vectors or liposomes, and in vivo versus ex vivo routes.
• Explains that potential is greatest for single-gene recessive disorders, while polygenic or multifactorial disease is much harder to treat.
• Recognises limitations: immune response to vector, insertional mutagenesis, and transient or unstable expression.

Common mistakes and how to put them right

• Confusing gene therapy with gene editing of embryos.

  Put it right: Clinical gene therapy is somatic and non-heritable; germ-line alteration is not standard practice.

• Assuming any disease can be cured.

  Put it right: Single-gene recessive conditions are the realistic targets; polygenic disease is far more complex.

• Thinking the vector itself is the medicine.

  Put it right: The vector is a delivery vehicle; the therapeutic agent is the genetic material.

• Believing one treatment gives permanent cure.

  Put it right: Expression may be transient, so repeat dosing may be needed.

Exam tips

• Always separate somatic from germ-line when asked about principles.
• Name a specific delivery route (viral vector, liposome) and say why targeting matters.
• Link potential to a named single-gene disorder and state one limitation to show balance.