Biology • Year 12 • Module 6 • Lesson 1
Mutation, Alleles and Genetic Change
Apply the mutation-versus-reshuffling distinction to real antibiotic-resistance data, a classify-the-source table, a diagram critique and a Lederberg-style replica plating scenario.
1. Interpret the graph — resistance allele frequency before and after antibiotic use
A microbiology team sampled a hospital E. coli population at four time points: before any patients were treated, then at three points after a ward-wide ciprofloxacin regime began. At each time point they sequenced 200 random isolates and counted the percentage carrying a ciprofloxacin-resistance allele. The graph below shows the result. 7 marks
Stylised hospital surveillance data, after the kind of allele-frequency plots reported in clinical microbiology audits (e.g. Andersson & Hughes, Nat. Rev. Microbiol., 2010).
1.1 Describe the trend in the percentage of isolates carrying the resistance allele from Week 0 to Week 8. 2 marks
1.2 A student writes: "The antibiotic made the bacteria mutate so they became resistant." Using the lesson's language about mutation and selection, explain why this interpretation is biologically wrong, and what the graph is actually showing. 3 marks
1.3 Estimate the resistance frequency at Week 6 (between the Week 4 and Week 8 data points) and predict what would happen to the resistance frequency if ciprofloxacin use were stopped at Week 8 and another antibiotic substituted. Justify briefly. 2 marks
2. Classify each scenario — new allele or reshuffled allele?
For each scenario, write "New allele (mutation)", "Reshuffled (meiosis / fertilisation)", or "Frequency change (selection)" in the answer column, and add a one-sentence justification. 6 marks (1 classify + 1 justify per row)
| # | Scenario | Classification + justification |
|---|---|---|
| 2.1 | A single base substitution in a pigment gene changes a coding triplet, producing a protein that absorbs light at a slightly different wavelength. The change is present in one cell in a beetle population. | |
| 2.2 | Two heterozygous (Aa) parents have a child who inherits both a alleles. Neither allele is new in the population — they were already in the parents. | |
| 2.3 | Over thirty generations, the percentage of beetles in a population carrying the new pigment allele rises from 0.5% to 12% as predators become more common and the new pigment provides better camouflage. |
3. Diagram critique — what's wrong with this student's poster on antibiotic resistance?
A Year 12 student has drawn the poster shown in the placeholder below to explain how bacterial populations become resistant to an antibiotic. There are three biological errors in the poster. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
3.1 Error 1: What is wrong?
Correction:
3.2 Error 2: What is wrong?
Correction:
3.3 Error 3: What is wrong?
Correction:
4. Apply to a new scenario — replica plating of pre-existing mutants
In a famous 1952 experiment, Joshua and Esther Lederberg grew bacteria on a master plate with no antibiotic, then used a velvet pad to make replica plates that did contain the antibiotic. The same few colonies that grew on every replica plate could be traced back to the same colonies on the original (antibiotic-free) master plate. 5 marks
4.1 Using the lesson's framework, explain what the Lederberg replica-plating result demonstrates about when the resistance mutation occurred relative to antibiotic exposure. 2 marks
4.2 Why is this evidence specifically against the claim that "the antibiotic instructs bacteria to make the right mutation"? 2 marks
4.3 In one sentence, identify the role of natural selection in this scenario — even though selection did not create the resistance allele. 1 mark
Q1.1 — Trend description (2 marks)
The percentage of isolates carrying the resistance allele rises steeply from Week 0 to Week 8 — from 3% at baseline to 71% by Week 8 [1]. The rise is initially small (3% → 6% from Week 0 to Week 2) and then accelerates, with the largest absolute jumps between Week 4 (38%) and Week 8 (71%) [1].
Q1.2 — Why "antibiotics caused them to mutate" is wrong (3 marks)
The antibiotic does not induce the resistance mutation — mutation is random with respect to need [1]. The graph shows a small percentage of resistant bacteria already exist at Week 0 (3%), before antibiotic exposure; ciprofloxacin then kills the susceptible bacteria but spares the resistant ones, which reproduce and increase in frequency [1]. So the graph is recording a change in allele frequency driven by selection, not a change in DNA sequence caused by the antibiotic [1].
Q1.3 — Estimate + prediction (2 marks)
Estimate at Week 6: roughly 50–55% (linear interpolation between 38% and 71%) [1]. Prediction if antibiotic use stops and a different drug replaces it: the ciprofloxacin-resistance allele's frequency should plateau or fall, because the selective pressure favouring it has been removed; if carrying the allele has any metabolic cost in the absence of ciprofloxacin, frequency will decline over time [1]. Accept also: rise of a different resistance allele if cross-resistance exists.
Q2 — Classify each scenario (6 marks)
2.1 New allele (mutation). A change in the DNA sequence at a locus produced a new variant of the pigment gene — the textbook definition of mutation creating a new allele. [1 + 1]
2.2 Reshuffled (meiosis / fertilisation). Both a alleles already existed in the parents (Aa × Aa); fertilisation simply produced a new combination. No new allele was created. [1 + 1]
2.3 Frequency change (selection). The new pigment allele already exists in the population; its frequency rises across generations because individuals carrying it survive better when predators are common. Selection changes allele frequency, it does not create new alleles. [1 + 1]
Q3 — Diagram critique (6 marks)
3.1 Error 1 ("antibiotic causes bacteria to mutate"): Antibiotics do not induce the specific mutation needed for resistance. Correction: mutation occurs randomly with respect to need; the antibiotic only changes which existing variants survive and reproduce. [1 + 1]
3.2 Error 2 ("all bacteria gained the resistance allele after exposure"): Antibiotic exposure does not give every cell the resistance allele. Correction: a few cells already carried (or randomly acquired) the resistance allele before exposure; the antibiotic killed susceptible cells and the resistant ones reproduced, so the proportion of bacteria with the allele rose. [1 + 1]
3.3 Error 3 ("meiosis reshuffles alleles in bacteria"): Bacteria reproduce asexually by binary fission — they do not undergo meiosis. Correction: in bacteria, new alleles enter the gene pool by mutation (and by horizontal transfer such as conjugation or transformation), not by meiotic reshuffling. [1 + 1]
Q4.1 — When the mutation occurred (2 marks)
Because the same colonies appear on every replica plate in the same positions as the master (antibiotic-free) plate, the resistance mutations must have already been present in those colonies on the master plate, before they were ever exposed to the antibiotic [1]. Mutation occurred first, by chance, and antibiotic exposure only revealed which cells already had it [1].
Q4.2 — Why this refutes "the antibiotic instructs mutation" (2 marks)
If the antibiotic caused the right mutation, the resistant colonies would appear at random positions across the replica plates — not at the same positions as the master plate [1]. The match between master and replica colony locations is only explainable if the resistance allele existed before exposure, demonstrating that mutation is random with respect to need [1].
Q4.3 — Role of selection (1 mark)
Natural selection did not create the resistance allele; it simply allowed the rare, already-mutated cells to survive and reproduce while killing susceptible cells, increasing the resistance allele's frequency in the next generation [1].