Worked Solutions
Module 6: Genetic Change — Worked Solutions (HSC Biology)
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Worked examples for HSC Biology Module 6: Genetic Change. Each shows where the marks are awarded, the key idea, and a full model answer explained by your choice of tutor — Stella, Ella or Cassie.
How to use these
Attempt each question first, then check your answer against the model responses. Use the tutor tabs to read the solution in the style that suits you: Stella is direct and challenging, Ella is warm and explains the why, and Cassie is concise and analytical.
Genetic change questions reward clear cause-and-effect reasoning. Connect a mutation or selection pressure to its effect on the protein, the organism, and the population over time.
Example 1 — Point mutations and their effects
Question
A single base in a gene is changed from one nucleotide to another. Explain how this point mutation could result in either no change, a minor change, or a major change to the polypeptide produced. Use examples of mutation types in your answer.
Solution
A point mutation is a single base substitution, and its effect depends on how it changes the codon.
No change (silent mutation): because the genetic code is degenerate, several codons code for the same amino acid. If the new codon still specifies the same amino acid, the polypeptide is unchanged.
Minor change (missense mutation): the new codon specifies a different amino acid. One amino acid is swapped, which may slightly alter the protein's shape or function.
Major change (nonsense mutation): the new codon becomes a stop codon, ending translation early. The polypeptide is truncated and usually non-functional.
So the same type of point mutation can have no, minor or major effect depending on whether and how the amino acid sequence changes.
The key thing to understand is that a point mutation changes just one base, but the consequence depends on what that change does to the codon — and codons are read in triplets to specify amino acids.
Sometimes there's no change at all. The genetic code is degenerate, meaning more than one codon can code for the same amino acid. So if the altered codon still calls for the same amino acid, the protein comes out identical. We call this a silent mutation.
Other times there's a minor change. The new codon now codes for a different amino acid — this is a missense mutation. Swapping one amino acid may only slightly affect the protein, unless it sits at a critical site like an enzyme's active site.
And sometimes there's a major change. If the new codon becomes a stop codon — a nonsense mutation — translation halts early and the polypeptide is cut short, so it's usually non-functional.
That's why a tiny single-base change can range from completely harmless to severely disruptive: it all comes down to how the amino acid sequence is affected.
Point mutation = single base substitution. Effect depends on the new codon.
- Silent (no change): degenerate code; new codon → same amino acid → identical polypeptide
- Missense (minor change): new codon → different amino acid → one substitution; effect varies
- Nonsense (major change): new codon → stop codon → truncated, usually non-functional polypeptide
Same mutation class, different outcomes depending on the amino acid result.
Where the marks go
- 1 mark: Identifies a point mutation as a single base substitution
- 1 mark: Explains a silent mutation (no change) using code degeneracy
- 1 mark: Explains a missense mutation producing a minor (single amino acid) change
- 1 mark: Explains a nonsense mutation producing a major change via a premature stop codon
Key idea
A point mutation's effect depends on the codon it produces — silent (no change), missense (one amino acid changed) or nonsense (premature stop, truncated protein).
Example 2 — Natural selection and evolution
Question
A population of bacteria is exposed to an antibiotic over several generations. Explain, using the principles of natural selection, how the population becomes resistant to the antibiotic.
Solution
Start with variation. Within the bacterial population, random mutations produce genetic variation, and by chance some individuals carry an allele that confers antibiotic resistance.
The antibiotic is the selection pressure. When the population is exposed, non-resistant bacteria are killed, but the resistant individuals survive.
Those survivors reproduce — and because bacteria reproduce asexually, they pass the resistance allele to their offspring. Over successive generations the frequency of the resistance allele increases in the population.
The result is a population that is largely resistant. This is evolution by natural selection: differential survival and reproduction has changed the allele frequencies of the population over time.
This is a classic example of natural selection, so let's build it step by step.
It all begins with variation. Random mutations in the bacterial DNA mean the population isn't uniform — by chance, some individuals happen to carry an allele that makes them resistant to the antibiotic. Importantly, this variation already exists before the antibiotic is applied; the antibiotic doesn't create it.
The antibiotic then acts as a selection pressure. When the population is exposed, the non-resistant bacteria die, while the resistant ones survive — this is "survival of the fittest" in the technical sense of being best suited to that environment.
Because the survivors are the ones that reproduce, they pass on their resistance alleles to the next generation. Bacteria reproduce rapidly, so this happens quickly.
Over many generations the proportion of resistant individuals climbs and the resistance allele becomes common. That change in allele frequency over time is evolution — the population has adapted to its environment through natural selection.
Natural selection sequence:
- Variation: random mutations produce some resistant individuals (pre-existing)
- Selection pressure: antibiotic kills non-resistant bacteria
- Survival: resistant individuals survive exposure
- Reproduction: survivors reproduce, passing on resistance alleles
- Allele frequency change: resistance allele increases over generations
Outcome: a resistant population — evolution by natural selection.
Where the marks go
- 1 mark: States that genetic variation (from mutation) pre-exists in the population
- 1 mark: Identifies the antibiotic as the selection pressure
- 1 mark: Explains differential survival of resistant individuals
- 1 mark: Explains that survivors reproduce and pass on the resistance allele
- 1 mark: Concludes that allele frequency changes over generations, increasing resistance
Key idea
Natural selection acts on pre-existing variation: a selection pressure favours survival and reproduction of advantageous alleles, shifting allele frequencies over generations.