HSC Exam Practice

Sample response. Non-disjunction is the failure of homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) to separate correctly during cell division, producing daughter cells with an abnormal chromosome number. It most commonly occurs during meiosis — specifically during the formation of gametes (eggs or sperm) — rather than during mitosis, though mitotic non-disjunction can produce mosaicism.

Marking notes. 1 mark for defining non-disjunction as failure of chromosomes/chromatids to separate during cell division; 1 mark for identifying meiosis (gamete formation) as the relevant stage for producing heritable chromosomal abnormalities.

Sample response. A screening test identifies individuals at elevated risk of a condition but cannot confirm a diagnosis — it may generate false positives. NIPT is a screening test: it analyses cell-free fetal DNA in maternal blood from 10 weeks gestation to assess the risk of chromosomal abnormalities (e.g. trisomy 21) without procedural risk, but a positive result must be confirmed. A diagnostic test definitively confirms or rules out a specific condition. Amniocentesis is diagnostic: fetal cells are obtained from amniotic fluid (15–20 weeks) and karyotyped or genetically tested, providing a certain result at a ~0.5% miscarriage risk.

Marking notes. 1 mark for defining screening as risk identification with possible false positives; 1 mark for defining diagnostic as definitive confirmation; 1 mark for correctly applying both definitions to NIPT and amniocentesis respectively.

Sample response. Cystic fibrosis follows an autosomal recessive inheritance pattern. Carriers are phenotypically normal because they possess one functional CFTR allele and one mutant allele (e.g. delta-F508). The single functional allele produces enough CFTR protein to operate the chloride channel with sufficient efficiency — carriers can transport chloride ions and maintain normal mucus consistency. The disease phenotype requires both alleles to be non-functional (homozygous or compound heterozygous) so that CFTR is completely absent from cell membranes.

Marking notes. 1 mark for correctly identifying autosomal recessive. 1 mark for stating carriers have one functional allele producing sufficient CFTR protein. 1 mark for explaining that complete absence of functional CFTR (both alleles mutant) is required for the disease phenotype.

Sample response. CFTR normally functions as a chloride ion channel in the apical membrane of epithelial cells, allowing chloride ions to move out of the cell; water follows by osmosis, maintaining the hydration and low viscosity of secretions. The most common mutation (delta-F508) causes the CFTR protein to misfold and be degraded before reaching the membrane. Without membrane-bound CFTR, chloride cannot exit epithelial cells; water does not follow; mucus becomes abnormally thick and dehydrated. In the lungs, thick mucus impairs ciliary clearance and promotes chronic bacterial infections (e.g. Pseudomonas aeruginosa), causing progressive lung damage. In the pancreas, thick mucus blocks pancreatic ducts, preventing digestive enzymes from reaching the small intestine and causing malabsorption and failure to thrive.

Marking notes. 1 mark for identifying normal CFTR function as a chloride channel maintaining mucus hydration via osmosis. 1 mark for explaining that delta-F508 or equivalent mutation causes CFTR misfold/degradation before reaching membrane. 1 mark for consequence: no chloride transport → thick mucus. 1 mark for naming at least two organs/systems affected with a clinical consequence each (lungs → infection; pancreas → malabsorption; reproductive tract → infertility).

Sample response. Anticipation in Huntington's disease refers to the tendency for the condition to present with earlier onset and increased severity in successive generations. The molecular basis is triplet repeat expansion: the HTT gene contains a CAG trinucleotide repeat region; normal individuals have up to 35 repeats. During DNA replication in gamete formation, the repeat region is prone to slippage, causing the repeat number to expand. Offspring therefore inherit a longer repeat than their affected parent, producing a longer polyglutamine tract in the huntingtin protein that aggregates more readily in neurons, causing earlier and more severe neurodegeneration.

Marking notes. 1 mark for defining anticipation (earlier onset / greater severity in successive generations). 1 mark for identifying the CAG repeat expansion mechanism in HTT. 1 mark for explaining how a longer repeat causes more severe disease (longer polyglutamine tract → protein aggregation in neurons).

Sample response. Somatic gene editing modifies non-reproductive body cells (e.g. blood stem cells); changes affect only the treated individual and are not passed to offspring. Germline editing modifies eggs, sperm or early embryos; any changes are heritable and would be transmitted to all future descendants. Germline editing is banned in most countries because: the long-term effects of off-target edits introduced into the germline are unknown and could be passed through generations; it raises profound ethical concerns about modifying the human gene pool without consent of future persons; and the potential for eugenics (selection for non-medical traits) creates risks of discrimination and inequity.

Marking notes. 1 mark for distinguishing somatic (non-heritable, body cells only) from germline (heritable, modifies future generations). 1 mark for at least one scientific reason for the germline ban (off-target heritable effects, unknown long-term consequences). 1 mark for at least one ethical reason (consent of future generations, eugenics risk, inequity).

Sample response (a). Pathway B (NIPT + confirmatory amniocentesis) detected 43 trisomy 21 cases compared to 41 by Pathway A (NIPT only), a difference of 2 additional cases in the same 4200-pregnancy cohort. Pathway B also had a lower false positive rate (0.1% vs 0.3%). However, Pathway B resulted in 3 procedure-related fetal losses from amniocentesis, while Pathway A had zero such losses. The data therefore show a trade-off: Pathway B has slightly higher diagnostic accuracy and specificity, but at the cost of a small number of fetal losses attributable to the invasive confirmatory procedure.

Sample response (b). Despite its lower false positive rate and marginally higher detection, Pathway A may be preferred by patients who prioritise avoiding any procedure-related miscarriage risk. The 3 fetal losses in Pathway B represent an important harm — for a patient who would continue the pregnancy regardless of trisomy 21 status, or who values the life of a potentially affected fetus equally, the amniocentesis risk outweighs the incremental gain in diagnostic certainty. NIPT alone (Pathway A) is a screening test that reduces the pool of high-risk pregnancies and still detects the majority of trisomy 21 cases (41/43) without procedural risk. Patient values, reproductive intentions, and risk tolerance all appropriately influence which pathway is preferred — neither is universally superior.

Marking notes (a). 1 mark for identifying that Pathway B detects slightly more cases with a lower false positive rate (citing figures); 1 mark for identifying the procedural fetal loss risk in Pathway B vs zero in Pathway A (citing figures); 1 mark for framing this as a trade-off between accuracy and risk. Marking notes (b). 1 mark for identifying that Pathway A avoids procedural miscarriage risk and this may be preferable for patients who would continue the pregnancy regardless; 1 mark for explaining that NIPT as a screening tool still captures the large majority of affected cases; 1 mark for noting that patient values, not just clinical metrics, appropriately determine test selection.

Sample response (a). The graph shows that before Casgevy treatment, 0% of patients were free from severe vaso-occlusive crises in the preceding 12 months — all 29 trial participants had experienced at least one episode. After treatment, 93.5% of patients were free from VOC in the following 12 months, indicating a dramatic reduction in disease burden. A key limitation of this dataset is the small sample size (n = 29), which limits statistical power and generalisability; a larger randomised controlled trial with a control arm (e.g. standard care) would be required to establish efficacy with greater confidence. Other limitations include the short 12-month follow-up period (long-term safety and durability of the CRISPR edit are unknown) and the absence of a placebo/comparator group.

Sample response (b). Sickle-cell disease is caused by a point mutation in the HBB gene producing abnormal beta-globin (HbS), which polymerises when deoxygenated and causes red blood cells to adopt a sickle shape that blocks blood vessels (vaso-occlusive crises). Casgevy uses CRISPR-Cas9 guided by a specific RNA sequence to cut and disrupt the BCL11A gene in the patient's harvested blood stem cells; BCL11A is a repressor of fetal haemoglobin (HbF, using gamma-globin chains that do not polymerise with HbS). With BCL11A disrupted, HbF production is reactivated in the patient's red blood cells. HbF dilutes and functionally replaces HbS, preventing polymerisation and sickling, and so dramatically reducing the frequency of vaso-occlusive crises.

Marking notes (a). 1 mark for accurately describing the result (0% free before → 93.5% free after); 1 mark for identifying a valid limitation (small n, short follow-up, no comparator/control arm — accept any one well explained). Marking notes (b). 1 mark for identifying the molecular cause of sickling (HbS polymerisation due to HBB mutation). 1 mark for explaining the CRISPR mechanism (cuts BCL11A repressor gene in blood stem cells). 1 mark for explaining that HbF reactivation occurs as a result. 1 mark for linking HbF reactivation to reduced sickling and VOC reduction.

Sample response. The statement is scientifically and practically defensible — genetic screening and gene therapy address different points in the prevention timeline and target different categories of genetic disease, making them complementary tools rather than competitors.

Genetic screening programs operate at a population level before or early in pregnancy to identify affected or at-risk individuals. NIPT (from 10 weeks) screens for chromosomal abnormalities such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and monosomy X (Turner syndrome) — conditions caused by non-disjunction that gene therapy cannot currently address because they involve entire extra or missing chromosomes, not correctable point mutations. Australia's universal Guthrie card newborn screening detects metabolic disorders such as phenylketonuria (PKU) at 48–72 hours — enabling dietary treatment (phenylalanine-restricted diet) that prevents the neurological damage of PKU before it occurs. Pre-conception carrier testing identifies couples at risk of autosomal recessive conditions (e.g. CF, spinal muscular atrophy) before pregnancy, informing decisions about IVF with preimplantation genetic testing (PGT). These programs are population-level, cost-accessible, and cover a spectrum of disorder types including chromosomal and multifactorial disease that gene therapy cannot target. Their scientific strength is breadth; their limitation is that detection does not itself cure — decisions following diagnosis involve ethical complexity (termination, PGT, management).

Gene therapy, by contrast, acts after diagnosis to correct or compensate for the underlying molecular defect. Viral vector therapies (e.g. Zolgensma — AAV carrying SMN1 for spinal muscular atrophy; Luxturna for RPE65-associated blindness) deliver a functional gene copy to specific tissues. CRISPR-Cas9 therapy (Casgevy for sickle-cell disease and beta-thalassaemia, approved 2023) edits a patient's own blood stem cells to reactivate fetal haemoglobin production, dramatically reducing vaso-occlusive crises (93.5% of trial participants free from events post-treatment; CLIMB SCD-121). These therapies address the root cause at a molecular level for specific single-gene disorders in accessible cell types. Significant scientific limitations remain: delivery to organs such as brain, muscle, or lung is technically challenging; off-target CRISPR cuts at unintended genomic sites pose oncogenic risk; current approvals cover only two conditions; and cost (~AUD $3.5 million per Casgevy treatment) means equitable access is a profound practical and ethical challenge. Somatic editing affects only the treated patient — the disease allele remains in the germline and can be passed to offspring. Germline editing is banned due to unknown heritable off-target effects and ethical concerns about modifying future generations without their consent.

The ethical dimensions of each strategy differ: screening raises questions of reproductive autonomy, the potential for stigmatisation of conditions detectable prenatally, and the social implications of selecting against certain genotypes. Gene therapy raises questions of equitable access (who can afford $3.5M treatments), the ethics of somatic vs germline editing, and the risk of eugenics if selection for non-disease traits becomes possible. Neither strategy is ethically straightforward, but both are governed by existing regulatory frameworks.

In conclusion, the statement is justified: screening programs (NIPT, Guthrie, carrier testing, PGT) are currently indispensable for the full spectrum of genetic disease at a population level, covering chromosomal and multifactorial disorders for which gene therapy has no solution. Gene therapy is a narrower but transformative tool for specific single-gene disorders, best seen as a treatment modality that supplements rather than replaces screening. As CRISPR delivery technology matures and costs fall, the two strategies may increasingly interact — a genetically screened individual later treated by gene therapy — but they are not competing resources for the same clinical problem.

Marking criteria (8 marks):

1 mark — Defines or applies the distinction between genetic screening (population-level risk identification/detection) and gene therapy (molecular-level correction in diagnosed individuals) and explicitly frames them as complementary.

1 mark — Names and describes at least two different screening technologies with their specific disorder targets (e.g. NIPT → chromosomal; Guthrie → PKU/CF; carrier testing → AR conditions).

1 mark — Names and describes at least one approved gene therapy with its mechanism and condition (e.g. Casgevy/CRISPR for sickle-cell; Zolgensma for SMA; Luxturna for RPE65 blindness).

1 mark — Explains a scientific limitation of gene therapy that screening fills (e.g. CRISPR cannot correct chromosomal/multifactorial disorders; delivery limitations; somatic not germline; only approved for 2 conditions as of 2024).

1 mark — Explains a scientific or practical limitation of screening that gene therapy may partially address (e.g. screening detects but does not cure; requires termination/PGT decisions for prevention; newborn screening still requires lifelong dietary management for PKU).

1 mark — Evaluates ethical dimensions of at least one strategy (reproductive autonomy; access equity for expensive gene therapy; germline editing prohibition; stigmatisation; eugenics concerns).

1 mark — Uses precise scientific terminology consistently: gene therapy, somatic/germline editing, CRISPR-Cas9, vector delivery, NIPT, PGT, karyotyping, named disorders/genes.

1 mark — Reaches an explicit, evidence-supported overall judgement that the two strategies are complementary because they target different categories of disorder, different points in the prevention timeline, or different scales of application.