HSC Exam Practice

Sample response. A mutation is a change in the DNA sequence of a gene. An allele is a variant form of a gene located at the same locus on a chromosome. Mutation is the routine source of new alleles, because a change in DNA sequence inside a gene produces a new version of that gene.

Marking notes. 1 mark for correct definition of mutation (change in DNA sequence); 1 mark for correct definition of allele (variant form of a gene); 1 mark for explicit relationship (mutation produces new alleles).

Sample response. A gene is a DNA region with a biological function, usually coding for a protein or functional RNA. An allele is a specific variant of that gene at the same locus on homologous chromosomes — different alleles may differ by one or more DNA bases. A gene pool is the population-level concept: it is the total collection of every allele of every gene present in a population.

Marking notes. 1 mark for correct definition of gene; 1 mark for correct definition of allele as a variant of a gene; 1 mark for gene pool as a population-level total of alleles (must reference population, not individual).

Sample response. Meiosis produces genetically different gametes from a single parent by reshuffling chromosomes and alleles through independent assortment and crossing over. Fertilisation randomly combines two gametes from two parents to form a zygote, producing a new combination of alleles. Neither process normally creates a new allele — they recombine alleles that already exist in the parental gene pool.

Marking notes. 1 mark for meiosis (independent assortment and/or crossing over reshuffling alleles); 1 mark for fertilisation (random combination of two gametes producing a new allele combination); 1 mark for the explicit statement that neither creates a new allele.

Sample response. "Random with respect to need" means the environment does not direct or instruct the specific DNA change that an organism requires. Mutations happen by chance, regardless of whether the change is useful, neutral or harmful. Selection may later act on the random variation produced — increasing the frequency of an allele that happens to help survival or reproduction — but the mutation itself is not produced because it was needed.

Marking notes. 1 mark for stating that the environment does not direct or instruct mutation; 1 mark for stating that mutations occur by chance regardless of usefulness; 1 mark for distinguishing the random origin of the mutation from the later, non-random action of selection.

Sample response. Natural genetic change arises through biological processes that occur without deliberate human direction — for example, mutation, meiosis, fertilisation, gene flow and natural selection. Induced genetic change is caused or directed by human technologies — for example, cloning, recombinant DNA, or CRISPR gene editing.

Marking notes. 1 mark for correct distinction (without human direction vs caused/directed by human technology); 1 mark for at least one valid example of each category.

Sample response (a). The percentage of E. coli isolates carrying the ciprofloxacin-resistance allele rises steeply with time after the antibiotic regime began — from 3% at Week 0 to 38% at Week 4 and 71% at Week 8. The rise is approximately monotonic and the increase between Week 4 and Week 8 is the largest absolute jump.

Sample response (b). Mutation is the original source of the resistance allele: at some point a chance change in DNA sequence produced the variant that confers resistance, and the allele was therefore already present at low frequency at Week 0 (3%) before any patients were treated. Selection then accounts for the rising frequency: once ciprofloxacin is used ward-wide, susceptible isolates die and resistant isolates survive and reproduce, so the proportion of resistant isolates in the population rises across Weeks 4 and 8. The antibiotic does not create the resistance allele in any isolate — it changes which isolates leave descendants.

Sample response (c). Disagree. At Week 0, before ward-wide antibiotic use began, 3% of isolates already carried the resistance allele. This shows the allele existed in the population before exposure, so the antibiotic cannot have created it. The graph records a change in the allele's frequency, not its origin.

Marking notes. (a) — 1 mark for describing a rise from Week 0 to Week 8; 1 mark for quoting at least two specific data values. (b) — 1 mark for identifying mutation as the source of the resistance allele; 1 mark for noting that the allele was already present at Week 0 (or before exposure); 1 mark for identifying selection as the process driving the frequency rise; 1 mark for explicit statement that selection changes frequency without creating new alleles. (c) — 1 mark for stating disagreement; 1 mark for citing the Week 0 value (3% / non-zero baseline) as evidence that the allele predates antibiotic use.

Sample response. The claim that "bacteria mutate so they can survive antibiotic treatment" is biologically misleading. It implies that bacteria produce, on demand, the specific DNA change that helps them survive a threat — which inverts the causal order of mutation and selection. Mutation is a change in the DNA sequence of a gene. It occurs randomly with respect to need: the environment does not instruct or direct what mutation will occur, so resistance alleles are not produced because they would be useful, but by chance copying errors and other random events in DNA. Such alleles exist in bacterial populations at low frequency regardless of whether antibiotics have ever been used.

Natural selection is a separate process and operates only on heritable variation that already exists. When an antibiotic is added to a bacterial population, cells carrying a resistance allele survive and reproduce while susceptible cells die. Over generations, the proportion of bacteria carrying the resistance allele rises sharply, even though no new allele has been created. The antibiotic changes which bacteria leave descendants; it does not author the DNA change that confers resistance.

The classic Lederberg replica-plating experiment provides direct evidence for this ordering. Lederberg and Lederberg (1952) grew bacteria on a master plate with no antibiotic, replica-plated onto plates containing the antibiotic, and showed that the surviving resistant colonies could be traced back to the same positions on the master plate — meaning the resistance mutation existed before the bacterial cells had ever been exposed to the antibiotic. If antibiotics had induced the right mutation on demand, the surviving colonies would have appeared at random positions, not in correspondence with the master plate.

A defensible reformulation of the popular claim is therefore: "Mutation occurs randomly and continually in bacterial populations, occasionally producing resistance alleles. When antibiotics are used, selection acts on any such pre-existing variation, raising the frequency of the resistance allele in the population." The original wording is wrong not because resistance is unrelated to antibiotic use, but because it confuses the source of new alleles (random mutation) with the process that changes their frequency (selection). The popular claim is therefore rejected.

Marking notes. 1 mark — identifies the order-of-events flaw (mutation occurs first, selection later acts on it). 1 mark — defines mutation correctly. 1 mark — correctly explains "random with respect to need". 1 mark — identifies natural selection as the process that changes the allele's frequency without creating new alleles. 1 mark — uses a named, valid example (Lederberg replica plating, fluctuation test, or empirical resistance allele frequency data) to support the position. 1 mark — provides a defensible reformulation that uses lesson terminology (mutation, allele, frequency, selection). 1 mark — reaches an explicit evaluative judgement that the popular claim is rejected on biological grounds.