HSC Exam Practice

Sample response. A genetic disease is a disease caused by inherited mutations or chromosomal abnormalities; it is present from conception, non-infectious, and cannot be transmitted between individuals (e.g. cystic fibrosis — caused by a mutation in the CFTR gene). An infectious disease is caused by a pathogen (bacteria, virus, fungi, parasite) that can be transmitted from one individual to another (e.g. influenza — caused by influenza A/B viruses). The key distinction is that genetic diseases arise from the individual’s own DNA, while infectious diseases are caused by foreign organisms that enter and replicate in the body.

Marking notes. 1 mark for a correct definition of genetic disease (inherited mutation / chromosomal abnormality, non-infectious, present from conception) with a named example. 1 mark for a correct definition of infectious disease (caused by pathogen, transmissible) with a named example. 1 mark for explicitly identifying the key distinguishing feature (own DNA vs external pathogen / non-transmissible vs transmissible).

Sample response. Gene: CFTR (Cystic Fibrosis Transmembrane conductance Regulator) on chromosome 7; the most common mutation is F508del. Normal protein: CFTR is a Cl− ion channel in the apical membrane of epithelial cells; its normal function is to secrete chloride ions into the airway (and other) lumens, and water follows by osmosis to hydrate the mucus layer. Consequence of loss: without functional CFTR, Cl− is not secreted, water does not follow, and mucus becomes thick and dehydrated. In the lungs, this thick mucus accumulates in the airways, providing a medium for chronic bacterial colonisation (e.g. Pseudomonas aeruginosa, Staphylococcus aureus) leading to progressive inflammation and lung damage.

Marking notes. 1 mark for naming the gene (CFTR / Cystic Fibrosis Transmembrane conductance Regulator) and chromosome (chr 7). 1 mark for describing normal CFTR function accurately (Cl− channel in epithelial apical membrane; Cl− secretion → water follows by osmosis → mucus hydrated). 1 mark for the immediate cellular consequence (Cl− not secreted → water not secreted → mucus dehydrates and thickens). 1 mark for the physiological organ consequence (thick mucus in lungs → chronic bacterial infections → progressive lung damage; or pancreatic duct blockage → malabsorption).

Sample response. CF is autosomal recessive because the CFTR mutation is a loss-of-function: the mutant protein simply fails to form a functional Cl− channel. One functional CFTR allele produces enough working CFTR protein to maintain adequate Cl− secretion and keep mucus hydrated; carriers (Cc) are therefore unaffected. Both alleles must be mutated (cc) before the disease manifests — hence recessive. HD is autosomal dominant because the HTT mutation is a gain-of-function: the expanded CAG repeat produces a mutant huntingtin protein with an abnormally long polyglutamine (polyQ) tract that is actively toxic to striatal neurons. Having one normal HTT allele does not prevent the toxic mutant protein (produced by the other allele) from accumulating and destroying neurons. One mutant allele is therefore sufficient to cause disease — hence dominant.

Marking notes. 1 mark for identifying CF as loss-of-function (mutant CFTR is absent/non-functional as a channel). 1 mark for explaining why loss-of-function = recessive (one functional allele is sufficient; carriers are unaffected because one copy produces enough protein). 1 mark for identifying HD as gain-of-function (mutant huntingtin acquires a new toxic action; it is not simply missing). 1 mark for explaining why gain-of-function = dominant (one normal allele does not protect against the toxic mutant protein produced by the other allele).

Sample response. Gene: PAH (Phenylalanine Hydroxylase) on chromosome 12. PKU mutations produce a non-functional phenylalanine hydroxylase enzyme, which normally converts phenylalanine (an essential dietary amino acid) to tyrosine in the liver. Without this enzyme, phenylalanine accumulates in the blood to toxic concentrations. Elevated phenylalanine crosses the blood–brain barrier and competitively inhibits the transport of other large neutral amino acids into the brain, disrupting neurotransmitter synthesis and impairing myelin formation. This progressive neuronal damage causes intellectual disability, seizures, and behavioural disturbance in an untreated child.

Marking notes. 1 mark for naming the gene (PAH, phenylalanine hydroxylase) and its normal function (converts phenylalanine to tyrosine in liver). 1 mark for explaining phenylalanine accumulation (enzyme non-functional → phenylalanine not converted → accumulates in blood). 1 mark for mechanism of neurological damage (elevated phenylalanine crosses blood–brain barrier → disrupts amino acid transport / myelin synthesis → neurons damaged → intellectual disability/seizures).

Sample response. Huntington’s disease (48 repeats): caused by a single gene (HTT), autosomal dominant, 100% penetrance at 40+ repeats — every individual who inherits the allele will develop the disease if they live long enough. Environmental factors are not required to trigger the disease; the mutant protein accumulates regardless of lifestyle or environment. Type 1 diabetes: polygenic (multiple genes contribute, particularly HLA genes on chromosome 6), not autosomal dominant or recessive in the classic Mendelian sense. Penetrance is incomplete — identical twin concordance is only ~50%, meaning genetic predisposition is necessary but not sufficient. Environmental factors (viral infection such as enteroviruses, dietary antigens, gut microbiome changes) are required to trigger the autoimmune destruction of beta cells in genetically susceptible individuals.

Marking notes. 1 mark for HD: single gene, autosomal dominant, 100% penetrance (40+ repeats). 1 mark for HD: environmental factors not required — the disease manifests solely from the genetic mutation. 1 mark for T1D: polygenic (HLA genes, multiple loci), incomplete penetrance (~50% MZ concordance). 1 mark for T1D: environmental factors (viral, dietary, microbiome) required to trigger autoimmune beta cell destruction in genetically susceptible individuals.

Sample response. The PKU mutation causes loss of phenylalanine hydroxylase function in both children; the genetic cause is identical. The difference in outcome arises entirely from substrate availability. In an untreated child, dietary phenylalanine from protein foods is consumed but cannot be converted to tyrosine; it accumulates progressively in blood and brain from the first days of life, and the neurological damage is irreversible once it occurs. In a child diagnosed at birth (via the Guthrie heel-prick test) and immediately placed on a low-phenylalanine diet, phenylalanine intake is restricted — the substrate that would otherwise accumulate to toxic levels is not supplied. The enzyme is still absent, but without its substrate, there is nothing to accumulate; the brain develops normally because phenylalanine never reaches toxic concentrations. The genetic mutation is present in both children but the metabolic consequence is prevented by dietary management.

Marking notes. 1 mark for identifying that both children have the same mutation (loss of PAH function) and that the difference in outcome is not genetic. 1 mark for explaining that dietary restriction limits phenylalanine intake → substrate does not accumulate → brain is not damaged. 1 mark for explicitly stating that the genetic mutation is still present but its phenotypic consequence is prevented by removing the substrate — the gene is unchanged but the metabolic cascade is blocked.

Sample response (a). MZ concordance for Type 1 diabetes is approximately 50%, while for Huntington’s disease (40+ repeats) it is approximately 100%. This difference reveals that in HD, the genetic factor (expanded HTT allele) is both necessary and sufficient to cause the disease — 100% MZ concordance means that every individual who inherits the allele will develop HD if they live long enough, with no additional environmental requirement. For Type 1 diabetes, the 50% MZ concordance shows that genetic factors contribute substantially (compare to DZ concordance of ~5–10%, much lower despite sharing 50% of DNA) but are not alone sufficient — additional non-genetic (environmental) factors must also be present to produce the disease.

Marking notes (a). 1 mark for correctly reading and describing the data (T1D ~50% MZ, HD ~100% MZ) with reference to the difference. 1 mark for interpreting HD 100% concordance correctly: genetic factor necessary and sufficient; 1 mark for interpreting T1D 50% concordance correctly: genetic factor necessary but not sufficient; environmental factors also required.

Sample response (b). Identical twins share 100% of their DNA, so if Type 1 diabetes were caused purely by the inherited genotype, concordance would approach 100% (as it does in HD). The ~50% concordance indicates that having the high-risk HLA gene variants (e.g. HLA-DR3 or DR4) is necessary but not sufficient — an environmental trigger must also occur. The most likely triggers are viral infections (particularly enteroviruses such as Coxsackievirus B, which infect the pancreas), dietary antigens introduced at specific developmental windows, or alterations in gut microbiome composition. Because identical twins, although genetically identical, are not exposed to exactly the same viruses, the same diet at the same developmental stage, or the same microbial communities, approximately 50% of co-twins who carry the genetic risk never encounter the specific environmental trigger — and therefore never develop the autoimmune response that destroys their beta cells. The other twin who does develop T1D presumably encountered the necessary trigger at a critical developmental window, initiating autoimmune beta cell destruction.

Marking notes (b). 1 mark for identifying that 100% shared DNA should produce ~100% concordance if purely genetic, and that ~50% concordance proves environmental factors are also required. 1 mark for correctly identifying the environmental trigger concept (viral infection / dietary antigen / gut microbiome) and linking to autoimmune beta cell destruction. 1 mark for explaining why MZ twins may not share the same environmental exposure despite identical genomes (different viral exposures, diet timing, microbiome) — accounting for why concordance is 50% and not 100%.

Sample response. The gene–protein–phenotype framework is highly useful in explaining genetic disease mechanisms because it identifies the precise molecular step at which a mutation disrupts normal physiology, enabling targeted explanation and, increasingly, targeted treatment.

In cystic fibrosis, the framework traces the pathway from CFTR gene mutation (most commonly F508del on chromosome 7) → misfolded CFTR protein degraded before reaching the cell membrane → no Cl− channel in the apical epithelial membrane → Cl− not secreted → water not secreted by osmosis → dehydrated, thick mucus → chronic lung infections and progressive lung destruction. This framework has directly informed treatment: CFTR modulator therapies (e.g. Trikafta) were designed specifically because researchers identified that the F508del protein is present but misfolded, allowing corrector drugs to rescue its folding and a potentiator to restore its gating. Without the gene–protein–phenotype framework, this targeted intervention would not have been possible.

In phenylketonuria, the same framework traces PAH gene mutation → non-functional phenylalanine hydroxylase → phenylalanine not converted to tyrosine → phenylalanine accumulates to toxic levels → crosses blood–brain barrier → disrupts myelin synthesis and amino acid transport → intellectual disability. Understanding that the mechanism is substrate accumulation rather than a missing end-product (tyrosine) directly informed the management strategy: remove the substrate (restrict dietary phenylalanine) rather than supplement the product. Australia’s universal newborn PKU screening program (Guthrie heel-prick test, 1960s) enabled dietary intervention from birth, transforming the outcome from severe intellectual disability to normal development. Again, the framework enabled the solution.

A contrasting case is Huntington’s disease: HTT gene CAG expansion → mutant huntingtin protein with polyQ tract → toxic gain-of-function aggregates in striatal neurons → progressive neuronal death → chorea, cognitive decline, death. Unlike CF and PKU, the mechanism here is gain-of-function (not loss), so treatments cannot simply restore a missing function or remove a substrate. The framework reveals why HD is currently much harder to treat: researchers must prevent the toxic protein from aggregating, clear existing aggregates, or silence the HTT gene — all active areas of clinical research but none yet clinically available. The framework is still useful; it explains why management of HD remains palliative, not curative.

Overall, the gene–protein–phenotype framework is highly useful as an explanatory and therapeutic guide: it reveals the mechanism at each step, clarifies why loss-of-function mutations can sometimes be managed by removing a substrate or replacing a channel, and explains why gain-of-function diseases such as HD require fundamentally different approaches. Its limitation is that it simplifies polygenic conditions (e.g. Type 1 diabetes, where many genes contribute and environment interacts) into a framework that was designed for single-gene mutations, requiring supplementary concepts (polygenic risk, gene–environment interaction, penetrance) to maintain accuracy.

Marking criteria.

• 1 mark — Correctly applies the gene–protein–phenotype framework to at least one disease with all three steps named accurately (gene → protein change → physiological consequence).
• 1 mark — Applies the framework to a second disease and explicitly compares the mechanism (e.g. loss-of-function CF vs gain-of-function HD, or loss-of-function PKU vs CF).
• 1 mark — Links the framework to a specific management or treatment strategy for at least one disease (e.g. PKU dietary management removes substrate; CFTR modulators target the misfolded protein; HD has no targeted treatment because gain-of-function mechanism is harder to block).
• 1 mark — Evaluates the usefulness of the framework — identifies at least one strength (explains mechanism, informs targeted treatment) and at least one limitation (over-simplifies polygenic/multifactorial conditions; does not account for gain-of-function diseases as well as loss-of-function).
• 1 mark — Reaches an explicit overall evaluative judgement that is evidence-based (e.g. “the framework is highly useful for single-gene loss-of-function diseases but requires supplementary concepts for polygenic or gain-of-function conditions”) using precise lesson terminology throughout.