QCE Biology 2021
Walkthrough of the 2021 QCE biology exam: what it assessed, strategy tips, and the common errors flagged in the official marker report.
- Marks
- 97
- Time
- 180 min
- Authority
- QCAA
- Updated
This paper was set on the 2019 QCAA syllabus. From the 2026 external assessments the 2025 syllabus applies, so some content and the exam format have changed. It is still useful practice, but check the current QCAA Biology syllabus and QCAA's 2026 key changes before relying on it. The mark and timing figures at the top of this page are a guide based on the 2024 papers (two papers, each with 90 minutes of working time), not QCAA's official structure for every year. Marks and question types vary, so use QCAA's own paper.
What this paper assessed
The QCAA Biology external assessment examines Unit 4: Heredity and continuity of life, while drawing on foundational understanding from Unit 3. The 2021 paper covered:
- DNA structure and gene expression: DNA replication, transcription and translation, the genetic code, and the regulation of gene expression.
- Inheritance: Mendelian genetics, autosomal and sex-linked inheritance, pedigree analysis, and variation arising from meiosis and mutation.
- Continuity of life on Earth: evidence for evolution, natural selection, speciation, and the use of molecular and fossil evidence to infer relatedness.
- Population genetics and biotechnology: allele frequencies, gene pools, and applications such as PCR, gel electrophoresis and DNA sequencing.
QCAA papers blend multiple choice with short response and an extended/complex response, and reward precise terminology, clear cause-and-effect reasoning, and the application of known principles to unfamiliar stimulus (graphs, pedigrees, data tables).
Structure and timing
QCAA sets the external assessment as more than one paper, and the number of marks, the sections and the question types differ from year to year. Open the official paper for the exact structure of this year; the marks and time shown at the top of this page are a guide only.
- Pace by marks. Share your working time in proportion to the marks on offer, so a 6-mark item gets about three times the time of a 2-mark item. Flag any multiple choice item that runs long and return to it.
- Use perusal time. Scan for the data-heavy items and the longest calculations or extended responses so you can plan them early.
- Subject habits. For a multi-mark explain or evaluate item, plan three or four linked points so each marking criterion is addressed, and keep a few minutes at the end to check that every explain or justify answer gives a reason, not just a description.
Worked practice questions (exam-style)
Question 1 (3 marks): A heterozygous brown-eyed man (Bb) has children with a blue-eyed woman (bb), where brown (B) is dominant. Determine the expected proportion of blue-eyed children and justify your answer with a genetic cross.
Set up the cross between Bb and bb. The Bb parent produces gametes B and b in equal proportion; the bb parent produces only b gametes.
| B | b | |
|---|---|---|
| b | Bb | bb |
| b | Bb | bb |
The offspring genotypes are Bb (brown) and bb (blue). Because blue requires two recessive alleles (bb), the expected proportion of blue-eyed children is 1/2 (50%). The justification is that each child inherits b from the mother and either B or b from the father with equal probability.
Question 2 (5 marks): Explain how a point mutation in a gene can lead to a non-functional protein, referring to transcription and translation.
A point mutation is a change in a single nucleotide base in the DNA sequence. During transcription, the altered DNA template is used to synthesise messenger RNA, so the mRNA carries the changed codon.
During translation, ribosomes read mRNA codons in groups of three. A substitution may produce a different codon: a missense mutation inserts a different amino acid, while a nonsense mutation creates a premature stop codon. Because the amino acid sequence (primary structure) determines folding, even one wrong amino acid can disrupt the protein's three-dimensional shape.
If the altered shape affects the active site or a structural region, the protein cannot perform its function - for example, an enzyme that can no longer bind its substrate. Thus a single base change can propagate through transcription and translation to yield a non-functional protein.
Question 3 (6 marks): A population of beetles on an island shows a shift from predominantly green to predominantly brown over 20 generations after a drought changed the vegetation. Using the theory of natural selection, explain this change and identify what data would strengthen the conclusion.
Natural selection acts on heritable variation within the gene pool: both green and brown alleles were present initially. After the drought, the vegetation became brown, so brown beetles were better camouflaged from predators - a selection pressure.
Brown beetles experienced higher survival (differential survival) and therefore higher reproductive success, passing the brown allele to more offspring. Over successive generations the frequency of the brown allele increased in the gene pool, shifting the population's phenotype - this is a change in allele frequency over time, the definition of evolution.
To strengthen the conclusion, useful data would include: predation rates on each colour morph before and after the drought, the allele frequencies measured each generation, and evidence that colour is heritable rather than environmentally induced. A control population on an unaffected island would help rule out other causes.
Common errors students made
The 2021 QCAA marker report noted that students frequently confused antigen and antibody, mixed up transcription and translation steps, gave conclusions that ignored the data shown, and failed to identify independent and dependent variables. Adding to those:
- Describing instead of explaining. "Explain" and "justify" items received a restatement of the trend with no causal mechanism.
- Vague selection language. Answers said organisms "wanted to" or "tried to" adapt, implying intent rather than differential survival of existing variation.
- Pedigree slips. Misreading shaded/unshaded symbols or assuming dominance from a single generation.
- Ignoring controls and variables. Experimental-reasoning items lost marks for not naming a control or confusing the variable being changed with the one being measured.
How to use this paper
Work the whole paper under timed exam conditions, keeping a short buffer at the end. Mark every response against the official QCAA marking guide and marker report, both available at the authority page linked at the top of this page. For any "explain" item that scored below full marks, rewrite it so each marking point is a separate, complete cause-and-effect sentence. Build a glossary of the terms most often confused (antigen vs antibody, transcription vs translation, allele vs gene) and re-test yourself a week later. Practise reading unfamiliar pedigrees and data graphs against the clock, since applying principles to novel stimulus is where marks are won.