Prevention — Genetic Disorders, Screening and Gene Therapy

Q1 — Sample Band 6 response (8 marks), annotated

From the table, each pregnancy of Priya and Arun has a 50% probability of producing an affected child with cystic fibrosis. This is because Arun is homozygous (CF/CF) and must transmit the CF allele to every child; whether the child is affected or a carrier depends entirely on whether Priya transmits her normal or CF allele (50:50). CF follows an autosomal recessive inheritance pattern — both alleles must be non-functional to produce disease. [1 — correct risk + inheritance pattern]

A screening test identifies individuals at elevated risk but does not provide a definitive diagnosis; NIPT (analysing cell-free fetal DNA in maternal blood from 10 weeks) is a screening test — a positive result must be confirmed. A diagnostic test definitively confirms or rules out a condition; amniocentesis (sampling fetal cells from amniotic fluid at 15–20 weeks) is diagnostic and can detect both chromosomal abnormalities and specific CFTR mutations. [1 — screening vs diagnostic with correct classification]

Option A (natural conception + NIPT → amniocentesis): The couple can conceive naturally and obtain early risk information from NIPT at 10 weeks without procedural risk. If NIPT indicates high CF risk (or ideally, targeted CFTR PCR is used on cfDNA), amniocentesis at 15–20 weeks provides a definitive answer at ~0.5% miscarriage risk. This sequence respects reproductive autonomy but means the couple must decide whether to continue or terminate an affected pregnancy — a profound and personal ethical choice. Timing disadvantage: result is available only partway through pregnancy. [1 — Option A criteria: timing, certainty, risk, ethics]

Option B (IVF + PGT): Embryos created via IVF are biopsied at the day-5 blastocyst stage and tested by PCR for CFTR mutations; only unaffected or carrier embryos are transferred. This provides the earliest possible certainty (before implantation) and avoids the need to consider termination. Significant trade-offs: IVF is invasive, expensive (~$10,000–$15,000 per cycle in Australia, not fully covered by Medicare), emotionally demanding, and not always successful per cycle. It also requires the production and testing of multiple embryos, raising ethical questions about the status of affected embryos that are discarded. [1 — Option B: timing, certainty, procedural demands, ethics]

Option C (natural conception + newborn Guthrie screening) is insufficient as a prevention strategy for this couple. The newborn screen detects CF at birth, not before; a positive result means the child already has CF and the disease cannot be undone. For this couple — who have a 50% per-pregnancy risk and know it in advance — newborn screening does not prevent disease, it simply confirms its presence after the fact. Management of CF with CFTR modulators (e.g. elexacaftor/tezacaftor) has significantly improved outcomes but is not a cure; the child still lives with a serious chronic condition. [1 — why Option C is not prevention for this family]

Comparing on three criteria: Timing — PGT (Option B) provides certainty before pregnancy is established; NIPT (A) provides early-pregnancy risk assessment; newborn screening (C) provides information after birth. Certainty — PGT and amniocentesis (A with follow-up) are diagnostic; NIPT (A first stage) is screening. Procedural risk — NIPT has no miscarriage risk; amniocentesis has ~0.5%; IVF has risks of hyperstimulation and low per-cycle success rates; Guthrie screen is minimal risk. [1 — explicit three-criteria comparison]

A justified recommendation must respect that different couples will weigh these trade-offs differently. Option B (PGT) best achieves prevention for a family at this risk level and with the means and willingness to pursue IVF — it is the only option that prevents an affected child being born or a pregnancy being terminated. However, Option A (NIPT + amniocentesis) is a valid alternative when IVF is not financially or physically feasible, or when the couple's values include a desire to conceive naturally. Option C alone is not an ethical recommendation as a prevention strategy when the risk is known in advance. [1 — justified recommendation acknowledging reproductive autonomy]

A high-quality answer also notes that carrier testing itself (confirming Priya's status before conception) was the crucial first step that made all other options meaningful — a point about the role of pre-conception genetic counselling in prevention. [1 — additional insight about pre-conception testing]

Marking criteria (8 marks):

• 1 mark — States 50% affected risk per pregnancy AND identifies CF as autosomal recessive.
• 1 mark — Correctly distinguishes screening (NIPT) from diagnostic (amniocentesis) tests with explanations.
• 1 mark — Evaluates Option A on timing, certainty, procedural risk, and ethical implications of termination decision.
• 1 mark — Evaluates Option B (PGT) on timing (pre-implantation), certainty (diagnostic), procedural demands and cost/ethics of embryo selection.
• 1 mark — Explicitly argues why Option C is insufficient for this family as a prevention strategy (detects, doesn't prevent).
• 1 mark — Compares on at least 3 criteria (timing, certainty, risk) across at least 2 of the options.
• 1 mark — Reaches a justified recommendation that does not oversimplify to one universally "correct" answer.
• 1 mark — Demonstrates additional insight (e.g. role of pre-conception carrier screening, CFTR modulator management, reproductive autonomy framing).

Q2 — Sample Band 6 response (7 marks), annotated

The claim contains a kernel of truth but is largely an oversimplification that misrepresents the current state of gene therapy. [1 — overall evaluative judgement]

What is scientifically valid: CRISPR-Cas9 has achieved genuine clinical breakthroughs. In November 2023, Casgevy (exa-cel) — developed by Vertex Pharmaceuticals and CRISPR Therapeutics — became the first CRISPR therapy approved globally (FDA, then UK MHRA) for sickle-cell disease and transfusion-dependent beta-thalassaemia. It edits patients' own blood stem cells to reactivate fetal haemoglobin production, addressing the root molecular cause of these conditions. This is a genuine scientific milestone that justifies cautious optimism about CRISPR's therapeutic potential. [1 — correct element with named approved example]

Inaccuracy 1 — "any genetic disease": CRISPR cannot currently correct any genetic disease. It works most readily in accessible cell types (blood stem cells). Delivering CRISPR machinery into organs such as the brain, muscle, or lung efficiently and safely remains a major unsolved challenge. Chromosomal disorders (trisomy 21, Turner syndrome, Klinefelter syndrome) arise from entire extra or missing chromosomes — CRISPR, which edits at the sequence level, cannot add or remove a whole chromosome. Multifactorial disorders (Type 2 diabetes, schizophrenia) involve hundreds of genetic variants interacting with environmental factors and are not approachable by single-gene editing. [1 — refutes "any disease" with specific limitation examples]

Inaccuracy 2 — off-target effects and safety: CRISPR-Cas9 can produce off-target edits — cuts at genomic sites similar to the intended target but distinct from it. In a clinical setting, an off-target edit in a tumour-suppressor gene could theoretically cause cancer. Current CRISPR therapies require extensive safety profiling and long-term follow-up; the technology is not yet universally safe across all applications or cell types. This is a non-trivial scientific limitation that the claim ignores. [1 — refutes "prevents any disease before symptoms" via off-target limitation]

Somatic vs germline editing: Casgevy edits somatic cells (blood stem cells) — changes that affect only the treated individual and are not heritable. Germline editing (modifying eggs, sperm or early embryos) would produce heritable changes and is banned in most countries, including Australia, because the long-term safety and ethical consequences of modifying the human germline for future generations are unknown and potentially catastrophic. A heritable off-target edit would be passed to all descendants of that individual. This is a fundamental reason why CRISPR cannot currently be used as a "prevention" strategy in the germline sense. [1 — somatic vs germline with justification for germline ban]

Screening remains essential: Even if CRISPR therapies expand dramatically, genetic screening programs are not made obsolete. NIPT, amniocentesis and the Guthrie card cover disorders not addressable by gene therapy (chromosomal abnormalities, multifactorial conditions, metabolic disorders such as PKU that are best managed by early dietary intervention rather than gene editing). Pre-conception carrier screening allows couples to understand their risk before pregnancy. These programs are population-level, low-cost interventions that deliver benefit at a scale and accessibility that AUD $3.5 million-per-patient CRISPR therapies cannot. [1 — screening programs remain necessary, with reasoning]

Overall judgement: The claim overstates the current reach of gene therapy. CRISPR represents a transformative but narrow breakthrough applicable, as of 2024, to two blood disorders. Screening programs remain the cornerstone of genetic disorder prevention at a population level. The two technologies are complementary, not competing. A scientifically defensible restatement would be: "CRISPR-Cas9 therapy has demonstrated proof-of-principle for treating specific single-gene blood disorders at the molecular level, but its clinical application is currently limited by delivery challenges, off-target risk, and the prohibition on germline editing; screening programs remain essential for detection and prevention across the full spectrum of genetic disease." [1 — accurate overall judgement with defensible reformulation]

Marking criteria (7 marks):

• 1 mark — States an overall evaluative judgement on the claim (partially valid, largely overstated, etc.).
• 1 mark — Correctly identifies the valid element with a named approved CRISPR therapy (Casgevy / exa-cel for sickle-cell disease / beta-thalassaemia, approved 2023).
• 1 mark — Refutes "any genetic disease" with at least one specific limitation: delivery to non-blood organs, inability to correct chromosomal disorders, or multifactorial disease complexity.
• 1 mark — Identifies a second scientific inaccuracy or limitation: off-target editing risks, cost/access barriers, or stage of clinical development.
• 1 mark — Correctly distinguishes somatic from germline editing and justifies why germline editing is not used as a prevention strategy.
• 1 mark — Argues that genetic screening programs (at least one named, e.g. NIPT, Guthrie card) remain necessary even alongside advancing gene therapies.
• 1 mark — Reaches an overall evidence-supported judgement that rejects the claim's oversimplification and offers a defensible restatement using precise lesson terminology.