HSC Biology rewards students who can do more than recall content. The 2025 exam asked students to compare processes, engage with stimulus material, design an epidemiological study and evaluate the impact of genetic technologies. NESA's marking feedback shows exactly which skills separated the better responses.
This guide breaks down the 2025 HSC Biology exam question by question. Every question is classified by module and topic using NESA's own mapping grid, so you can see exactly where the marks were. It also covers the answers, what better responses did and the areas NESA's markers said students need to improve. For the previous year's analysis, see our 2024 HSC Biology examiner report breakdown.
Key Takeaways
- Marks were spread almost evenly across the four Year 12 modules: Heredity 26, Genetic Change 26, Infectious Disease 25 and Non-infectious Disease and Disorders 23.
- Genetic Technologies was the single biggest topic, worth 15 marks, including the 7-mark evaluation in Question 30(b).
- Comparison questions (Q23, Q26, Q27, Q33(a)) were common. Markers wanted explicit similarities and differences, not two separate descriptions.
- Many responses didn't use the stimulus. Better responses referred to the data, diagrams and scenarios provided in Questions 21, 25, 27, 29 and 32.
- In extended responses, markers rewarded precise terminology (restriction enzymes, DNA ligase, plasmid vector, micro-injection) and a clear, consistent judgement.
HSC Biology Examination Structure
| Feature | Section I | Section II |
|---|---|---|
| Question type | 20 multiple-choice questions | Short-answer and extended-response questions (Q21–34) |
| Marks | 20 marks | 80 marks |
| Suggested time | About 35 minutes | About 2 hours 25 minutes |
| Skills tested | Recall, interpreting data and diagrams, applying concepts | Explaining, comparing, analysing stimulus, designing investigations and evaluating |
The exam has 5 minutes of reading time and 3 hours of working time, and is worth 100 marks. NESA-approved calculators are allowed. The HSC exam mark makes up 50% of the final HSC mark. The other 50% comes from school-based assessment.
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Marks by Module in the 2025 Exam
Every question below is classified using the mapping grid in NESA's 2025 marking guidelines, which links each question to a module and syllabus topic.
| Module | Questions | Marks | Share of Exam |
|---|---|---|---|
| Module 5: Heredity | MC 1, 8, 17, 18, 19; Q22(a), 27, 31, 33(a)–(b), 34(a) | 26 | 26% |
| Module 6: Genetic Change | MC 2, 4, 12, 16; Q23, 30, 33(c), 34(b) | 26 | 26% |
| Module 7: Infectious Disease | MC 3, 5, 6, 10, 11, 14; Q21, 22(b), 28, 29 | 25 | 25% |
| Module 8: Non-infectious Disease and Disorders | MC 7, 9, 13, 15, 20; Q24, 25, 26, 32 | 23 | 23% |
Marks by Syllabus Topic
| Topic | Module | Marks |
|---|---|---|
| Genetic Technologies | 6 | 15 |
| Immunity | 7 | 13 |
| Reproduction | 5 | 11 |
| Mutation | 6 | 11 |
| Epidemiology | 8 | 8 |
| Prevention, Treatment and Control | 7 | 7 |
| Technologies and Disorders | 8 | 6 |
| Causes of Infectious Disease | 7 | 5 |
| Homeostasis | 8 | 5 |
| Genetic Variation | 5 | 5 |
| Cell Replication | 5 | 5 |
| Prevention | 8 | 3 |
| DNA and Polypeptide Synthesis | 5 | 3 |
| Inheritance Patterns in a Population | 5 | 2 |
| Causes and Effects | 8 | 1 |
Source: NESA 2025 HSC Biology marking guidelines (mapping grid).
Section I: Multiple-Choice Breakdown
| Question | Module | Topic | What It Tested | Answer |
|---|---|---|---|---|
| 1 | 5 | Reproduction | Binary fission produces identical cells | B |
| 2 | 6 | Mutation | UV light as a mutagen | D (electromagnetic) |
| 3 | 7 | Immunity | Roles of phagocytes and lymphocytes | A |
| 4 | 6 | Mutation | Random change in allele frequencies | B (genetic drift) |
| 5 | 7 | Causes of Infectious Disease | Conditions for rapid disease spread | B |
| 6 | 7 | Prevention, Treatment and Control | Immunity from an antibody injection | D (acquired passive) |
| 7 | 8 | Homeostasis | Ectotherm or endotherm from data | C (endotherm) |
| 8 | 5 | Genetic Variation | Co-dominance | D |
| 9 | 8 | Technologies and Disorders | Blood composition before and after dialysis | A |
| 10 | 7 | Prevention, Treatment and Control | Trend in measles deaths after vaccination | C |
| 11 | 7 | Prevention, Treatment and Control | Herd immunity | C |
| 12 | 6 | Mutation | Reading a karyotype | D |
| 13 | 8 | Epidemiology | Incidence vs prevalence | A |
| 14 | 7 | Immunity | Antibody–antigen specificity | A |
| 15 | 8 | Technologies and Disorders | Order of steps in a cochlear implant | C |
| 16 | 6 | Genetic Technologies | Clones from embryo splitting | A (each other) |
| 17 | 5 | Cell Replication | Sequencing phases of cell division | C |
| 18 | 5 | Genetic Variation | X-linked dominant inheritance | B |
| 19 | 5 | Cell Replication | Base-pairing calculation | C (69.6 million) |
| 20 | 8 | Causes and Effects | Gene regulation and an oncogene | D |
Source: NESA 2025 HSC Biology marking guidelines (answer key and mapping grid). Descriptions of what each question tested are our own.
Multiple-Choice Questions to Watch
- Question 4: A storm that randomly kills 80% of a population changes allele frequencies by chance, not because of any advantage. That's genetic drift (a bottleneck), not natural selection.
- Question 6: Receiving ready-made antibodies gives acquired passive immunity. The person's own immune system didn't make them, so it isn't active.
- Question 13: A treatment that prolongs life without curing the disease doesn't change how many new cases occur (incidence), but people live with the disease for longer, so prevalence increases.
- Question 18: The bar-eye allele is dominant and on the X chromosome. A male passes his X to all daughters and his Y to all sons, so 100% of females and 0% of males have bar-eyes.
- Question 19: 42% of base pairs are C–G, so 58% are A–T. With 120 million base pairs (240 million nucleotides), thymine makes up 29% of nucleotides: 69.6 million.
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Section II: Question-by-Question Breakdown
| Question | Module | Topic | What It Tested | Marks | Key Answer |
|---|---|---|---|---|---|
| 21 | 7 | Immunity | Innate barriers to infection | 2 | e.g. skin is a physical barrier; stomach acid kills pathogens |
| 22(a) | 5 | Reproduction | Fungal reproduction | 2 | Spores or budding |
| 22(b) | 7 | Causes of Infectious Disease | Pathogen adaptation for transmission | 2 | e.g. spore release; cholera toxin causing diarrhoea |
| 23 | 6 | Genetic Technologies | Compare artificial insemination and artificial pollination | 3 | Both use gametes; animals vs flowering plants |
| 24(a) | 8 | Homeostasis | Thermoregulation mechanisms | 2 | Sweating (A), shivering (B) |
| 24(b) | 8 | Homeostasis | How shivering maintains homeostasis | 2 | Muscle contractions generate heat |
| 25 | 8 | Prevention | Evaluate a sunscreen claim using a UV graph | 3 | Claim is inadequate: UV ≥ 3 from about 8 am to 6 pm |
| 26 | 8 | Technologies and Disorders | Compare LASIK with another technology | 4 | e.g. myopia: LASIK vs concave lenses |
| 27 | 5 | Reproduction | Compare internal and external fertilisation using data | 6 | Group A internal, Group B external |
| 28(a) | 7 | Immunity | Roles of X, Y and Z in antibody production | 4 | Helper T cells, B cells, plasma cells |
| 28(b) | 7 | Immunity | Primary vs secondary antibody response | 2 | Secondary is faster and larger |
| 28(c) | 7 | Immunity | Role of memory cells | 3 | Rapid response on re-exposure |
| 29(a) | 7 | Causes of Infectious Disease | Why Varroa mite is an infectious disease | 2 | A parasitic pathogen spread between bees |
| 29(b) | 7 | Prevention, Treatment and Control | Two procedures to stop the spread | 4 | e.g. early detection and isolation, destroying hives |
| 30(a) | 6 | Genetic Technologies | Producing transgenic mosquitoes | 4 | Recombinant DNA and micro-injection into eggs |
| 30(b) | 6 | Genetic Technologies | Evaluate "Genetic technologies are beneficial for society" | 7 | Judgement with specific technologies |
| 31(a) | 5 | Genetic Variation | Inheritance pattern from a pedigree | 3 | Autosomal recessive |
| 31(b) | 5 | DNA and Polypeptide Synthesis | Identify a mutation using a codon chart | 3 | Point (substitution) mutation: valine → isoleucine |
| 32 | 8 | Epidemiology | Design an epidemiological study | 7 | Design with justified features |
| 33(a) | 5 | Cell Replication | Compare mitosis and meiosis | 3 | Mitosis: identical cells; meiosis: gametes |
| 33(b) | 5 | Reproduction | Relationship between two individuals | 2 | Individual 1 is the parent of Individual 2 |
| 33(c) | 6 | Mutation | Germline vs somatic mutations | 4 | Analysis of cells U, V, W and X |
| 34(a) | 5 | Inheritance Patterns in a Population | Why small populations fluctuate more | 2 | Smaller gene pool, less diversity |
| 34(b) | 6 | Mutation | Evaluate the effect of gene flow | 4 | Larger effect in the small population |
Key answers are based on the sample answers in NESA's 2025 marking guidelines. Extended-response questions accepted a range of answers. Topics follow NESA's mapping grid.
What the Markers Said: Module 5 – Heredity (26 marks)
Question 22(a) – Fungal reproduction
- Better responses gave detail on the process and its outcome. For example, a bud breaks off the parent fungus to form a genetically identical organism.
- Weaker responses named a process without explaining how it works.
Question 27 – Internal and external fertilisation (6 marks)
- Better responses used the egg data to support the comparison and gave both similarities and differences. Group A (monotremes, snakes, birds) uses internal fertilisation and produces few eggs. Group B (crabs, sea urchins, squid) uses external fertilisation and produces thousands to millions.
- Markers flagged students who confused external fertilisation with asexual reproduction, or fertilisation with parental care.
Question 31 – Pedigrees and mutations
- In 31(a), better responses justified autosomal recessive inheritance by referring to specific individuals in the pedigree, such as two unaffected parents with an affected child.
- Markers wanted the full term ("autosomal recessive", not just "recessive"), correct Punnett squares and allele letters that are easy to tell apart (Tt rather than Ss).
- In 31(b), better responses worked step by step from the template strand to mRNA to the amino acid chain using the codon chart, showing that valine is replaced by isoleucine: a point (substitution) mutation.
Question 33(a)–(b) – Mitosis, meiosis and relationships
- Better responses compared the processes of mitosis and meiosis (for example, both need DNA replication, but only meiosis halves the chromosome number), not just their outcomes.
- In 33(b), students needed to read the flow diagram and key carefully to see that Individual 1 provides the gamete that forms Individual 2.
Question 34(a) – Allele frequencies in small populations
- Better responses linked the smaller gene pool and lower genetic variation to bigger fluctuations in allele frequency.
What the Markers Said: Module 6 – Genetic Change (26 marks)
Question 23 – Artificial insemination vs artificial pollination
- Better responses gave a clear similarity and difference, using headings in a table or explicit words like "both" and "whereas".
- Common errors were describing each process separately instead of comparing them, and describing IVF instead of artificial insemination.
Question 30(a) – Producing transgenic mosquitoes
- Better responses recognised that germline cells must be modified so the trait passes to future generations.
- They described every step of recombinant DNA technology: isolating the SERPINE1 gene with restriction enzymes, inserting it into a plasmid vector with DNA ligase, amplifying it in bacteria and transferring it into mosquito eggs by micro-injection or viral vectors.
- Markers flagged students who wrote about breeding or cloning mosquitoes, or used vague phrases like "place it into the mosquito" instead of scientific terms.
Question 30(b) – Evaluating genetic technologies (7 marks)
- Better responses named specific technologies, such as recombinant insulin or Bt cotton, rather than "biotechnology" in general.
- They kept a clear, consistent judgement throughout and considered societal impacts: environmental, ethical, healthcare and economic, not just effects on one person.
Question 33(c) – Germline and somatic mutations
- Better responses explained that a germline mutation can be passed to offspring, while a somatic mutation stays in the individual. They then applied this to each of cells U, V, W and X.
- Markers recommended practising flow diagrams with a key.
Question 34(b) – Gene flow
- Better responses compared the impact of gene flow on both populations and made a judgement: introduced alleles have a much bigger effect in the small population.
- Markers noted that the same four alleles were introduced into both populations, that gene flow affects the gene pool (not the other way around), and that population size controls how much gene flow changes allele frequencies.
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What the Markers Said: Module 7 – Infectious Disease (25 marks)
Question 21 – Innate barriers
- Better responses explained how each component prevents infection and used components from the stimulus. For example, nasal hairs trap inhaled pathogens.
Question 22(b) – Pathogen adaptations
- Better responses linked an adaptation to transmission between hosts, not survival inside a host. For example, cholera bacteria produce a toxin that causes watery diarrhoea, which spreads the bacteria into water sources and on to new hosts.
Question 28 – The immune response to alpha-gal
- In 28(a), better responses clearly labelled X (helper T cells), Y (B cells) and Z (plasma cells) and explained how they interact to produce antibodies.
- In 28(b), better responses described both the speed and the size of the primary and secondary responses.
- In 28(c), better responses explained how memory cells form during the first exposure and then respond rapidly on re-exposure.
Question 29 – Varroa mite
- In 29(a), better responses identified the mite as a pathogen that is transmitted between organisms.
- In 29(b), better responses linked each procedure to how it stops the spread, and chose procedures that fit the scenario rather than generic ones.
What the Markers Said: Module 8 – Non-infectious Disease and Disorders (23 marks)
Question 24 – Thermoregulation
- Better responses identified a cooling mechanism (such as sweating) and a warming mechanism (such as shivering).
- Students needed to interpret the flow chart carefully and link mechanism B to heat generation.
Question 25 – Evaluating a sunscreen claim
- Better responses made a definite judgement using the graph. UV levels were 3 or above from about 8 am to 6 pm, so "use sunscreen between 10 am and 4 pm" leaves people unprotected for hours.
- Markers wanted an evidence-based judgement that linked the Cancer Council advice, the product claim and the graph.
Question 26 – LASIK vs another technology
- Better responses named a specific disorder (such as myopia), explained how each technology refracts light onto the retina, and gave both similarities and differences.
- Weaker responses compared only cost, accessibility and healing time from the stimulus instead of how the technologies treat the disorder.
Question 32 – Designing an epidemiological study (7 marks)
- Better responses showed a clear understanding of the features of an epidemiological study, used the stimulus (the three regions, the age groups and the time scale) and justified each design feature in terms of reliability and validity.
- Markers recommended using the stimulus to support design choices, such as using the isolated region as a control group.
💡 Explore the complete guide to HSC command terms to master words like compare, evaluate and justify.
Common Mistakes Across the 2025 Exam
- Describing instead of comparing: In Q23, Q26, Q27 and Q33(a), separate descriptions of two things didn't earn full marks.
- Ignoring the stimulus: Many responses didn't refer to the diagrams, graphs or data provided, even when the question required it.
- Generic answers: Generic procedures in Q29(b), "biotechnology" in Q30(b) and non-scientific wording in Q30(a) all limited marks.
- Vague or inconsistent judgements: In "evaluate" questions (Q25, Q30(b), Q34(b)), markers wanted a clear judgement supported by evidence.
- Confusing related concepts: IVF and artificial insemination, external fertilisation and asexual reproduction, fertilisation and parental care, and germline and somatic mutations.
- Imprecise terminology: "Recessive" instead of "autosomal recessive", and unclear allele letters in Punnett squares.
How to Prepare for HSC Biology
- Practise comparison questions using a "similarity / difference" structure or a table with clear headings.
- Learn the steps of recombinant DNA technology with precise terms, and prepare named examples of genetic technologies and their impacts.
- Practise designing investigations, including epidemiological studies, and justify each feature in terms of validity and reliability.
- Work with stimulus material every time: quote data, refer to labels and use the scenario in your answer.
- Revise the immune response as a sequence, from antigen presentation to plasma and memory cells.
- Complete full past papers under timed conditions to practise managing 3 hours of working time.
For more preparation:
- see our 2024 HSC Biology examiner report breakdown
- studying Chemistry too? See our 2025 HSC Chemistry examiner report breakdown
- understand HSC performance bands
- read how the HSC works
- see the HSC exam timetable 2026 for this year's exam date.
Need Help With HSC Biology?
Our HSC Biology tutors have scored highly in the subject themselves and work one-on-one with students to master the content, write sharp comparisons and evaluations, and use stimulus material the way markers expect.
Sources: NSW Education Standards Authority, 2025 HSC Biology examination, marking guidelines and HSC marking feedback 2025.
