HSC Exam Practice

Sample response. Continuity of species is the ongoing existence of a species across successive generations, achieved when individuals reproduce successfully and pass their hereditary information (DNA) to viable offspring.

Marking notes. 1 mark for identifying continuity as persistence of a species across generations; 1 mark for linking continuity to inheritance / transfer of genetic material.

Sample response. Asexual reproduction involves a single parent and does not involve the fusion of gametes, producing offspring that are usually genetically identical to the parent apart from mutation. Sexual reproduction involves two parents (or two gametes), with fertilisation fusing haploid gametes to form a zygote, producing genetically varied offspring.

Marking notes. 1 mark for parent number (one vs two); 1 mark for gamete involvement (absent vs fertilisation of haploid gametes); 1 mark for naming the genetic consequence of each (identical / clones vs varied offspring).

Sample response. Acceptable named examples include: bacteria — binary fission; yeast or Hydra — budding; strawberry — vegetative propagation via runners (stolons); potato — vegetative propagation via tubers; some flatworms — fragmentation.

Marking notes. 1 mark per correctly named organism paired with the correct asexual mechanism (max 2). A correct organism without the mechanism, or vice versa, scores 0 for that entry.

Sample response. Sexual reproduction requires two haploid gametes, each carrying a different combination of alleles, to fuse during fertilisation. Because the two parents are genetically distinct and each gamete itself carries a unique allele combination, the resulting zygote inherits a new combination of alleles not present in either parent. Asexual reproduction relies on mitotic copying from a single parent, so allele combinations are not reshuffled and offspring are genetically near-identical apart from mutation.

Marking notes. 1 mark for identifying fusion of two genetically different gametes / two parents; 1 mark for explaining that this generates new allele combinations in offspring; 1 mark for contrast with asexual reproduction (no allele reshuffling — offspring genetically near-identical).

Sample response. First, hereditary information (DNA) must be transferred from parent to offspring during reproduction. Second, the offspring must themselves be viable — they must survive long enough to reproduce and pass that information on to the next generation.

Marking notes. 1 mark for transfer of DNA / hereditary information; 1 mark for viability of offspring (must survive to reproduce). Both required for full marks.

Sample response (a). In all three regions, the fully clonal Cavendish planting lost a substantially higher percentage of plants to TR4 than the mixed planting. Losses in the clonal blocks ranged from 71% (Region B) to 88% (Region C), while losses in the mixed blocks ranged from only 19% (Region B) to 31% (Region C). The pattern is consistent across regions, with the mixed planting losing roughly one-third or less of the clonal figure.

Sample response (b). The fully clonal Cavendish blocks consist of genetically near-identical individuals, all sharing the same susceptibility to TR4. Because there is no allele variation between plants, a pathogen capable of infecting one plant can infect virtually all plants in the block. The mixed planting contains sexually generated seedlings carrying allele combinations inherited from a resistant wild relative, so a proportion of plants possess alleles conferring resistance and survive infection. This is why higher genetic variation translates directly into lower whole-population losses.

Marking notes. Part (a) — 1 mark for identifying that clonal losses exceed mixed losses in all three regions; 1 mark for quoting at least one supporting figure from the data. Part (b) — 1 mark for identifying genetic uniformity in the clonal block as the source of uniform susceptibility; 1 mark for identifying that sexual reproduction introduces allele variation including resistance alleles; 1 mark for explicitly linking higher variation to reduced whole-population loss.

Sample response. The claim that sexual reproduction is always superior overstates the case: biology rewards different reproductive strategies in different environments, so superiority is context-dependent. In stable environments, asexual reproduction is highly effective. Bacteria reproducing by binary fission, and Cavendish bananas propagated clonally from a single ancestral plant, can rapidly fill a habitat or commercial system with copies of a successful genotype without needing mates or investing energy in gamete production. Under stable conditions, this efficiency is a continuity strength: more offspring per unit time means a higher chance that the species (or cultivar) persists. However, asexual reproduction routinely produces low genetic variation, so when conditions change — for instance when Tropical Race 4 of Fusarium oxysporum spread through Cavendish plantations — the genetic uniformity that powered the original success became a continuity weakness, leaving the entire crop susceptible. Sexual reproduction, by contrast, fuses two genetically distinct gametes during fertilisation, generating offspring with new allele combinations. Examples include flowering plants and most mammals. This variation makes it more likely that at least some offspring possess allele combinations suited to a changed environment, supporting continuity when conditions shift, but it comes at the cost of slower reproduction, mate-finding and higher energy investment per offspring. The two strategies therefore optimise different aspects of continuity: asexual reproduction optimises short-term population growth under stable conditions, while sexual reproduction optimises long-term resilience under changing conditions. The claim is therefore rejected: sexual reproduction is not universally superior but is favoured when environmental change is the dominant pressure on continuity.

Marking notes. 1 mark — defines or implicitly applies continuity of species (offspring + DNA transfer). 1 mark — names a valid asexual example with mechanism (e.g. binary fission, Cavendish clonal propagation). 1 mark — names a valid sexual example (e.g. flowering plants, mammals, coral spawning) and links to fusion of gametes. 1 mark — identifies the advantage of asexual reproduction in stable conditions (speed, efficiency, no mate). 1 mark — identifies the advantage of sexual reproduction in changing conditions (variation increases chance some offspring suit new conditions). 1 mark — uses a worked example of context dependence (Cavendish + TR4 or equivalent) showing that genetic uniformity becomes a vulnerability when the environment changes. 1 mark — reaches an explicit evaluative judgement that rejects "always superior" and frames superiority as environment-dependent.