HSC Exam Practice

Sample response. Genetic variation is the set of differences in genetic makeup (allele combinations and alleles) between individuals within or between populations of the same species.

Marking notes. 1 mark for identifying differences in genetic makeup / alleles between individuals; 1 mark for situating it within a population (or between members of the same species).

Sample response. A new allele combination is a rearrangement of alleles already present in the parents — the alleles themselves are unchanged but their pairing in offspring is novel; this is produced by crossing over, independent assortment and random fertilisation. A new allele is a DNA sequence that did not previously exist in the parents and so is genuinely novel; this is produced by mutation.

Marking notes. 1 mark for defining a new allele combination (rearrangement of existing alleles); 1 mark for defining a new allele (changed DNA sequence / not previously present); 1 mark for correctly assigning sources (three reshufflers vs mutation).

Sample response. During prophase I of meiosis, paired homologous chromosomes come close together (synapsis) and exchange corresponding segments at chiasmata. This produces recombinant chromosomes carrying a new mix of maternal and paternal alleles along their length. Gametes therefore carry chromosomes whose allele combinations are different from those in either parent, increasing offspring variety.

Marking notes. 1 mark for identifying the stage (meiosis prophase I) and that exchange occurs between homologous chromosomes; 1 mark for stating that segments are exchanged; 1 mark for the genetic consequence (new combinations of existing alleles in gametes).

Sample response. Full siblings inherit alleles from the same two parents, so they share a large proportion of their genetic material — this explains the resemblance. However, meiosis in each parent produces genetically varied gametes through crossing over (recombinant chromosomes) and independent assortment (random distribution of homologous pairs). Random fertilisation then pairs different gametes to form each sibling, so each sibling inherits a different reshuffle of the parental alleles, which is why they are not identical.

Marking notes. 1 mark for shared parents explaining resemblance; 1 mark for meiosis producing varied gametes (crossing over and/or independent assortment named); 1 mark for random fertilisation producing different gamete-pairings between siblings.

Sample response. Mutation. Mutation changes the DNA base sequence itself, so it can produce an allele whose sequence did not previously exist in the population. The other three sources (crossing over, independent assortment, random fertilisation) can only rearrange alleles that are already present — they cannot generate a sequence that is not already there.

Marking notes. 1 mark for identifying mutation as the source acting on DNA sequence; 1 mark for explaining the contrast with the three reshufflers (they cannot generate sequence not already present).

Sample response (a). Distinct-gamete count rises exponentially with haploid number — each extra chromosome pair doubles the count (2ⁿ), producing a straight line on a log axis. For humans (n = 23) the value sits between 10⁶ and 10⁷ on the log scale, giving approximately 8 × 10⁶ (≈ 8 million) distinct gametes by independent assortment alone.

Sample response (b). Independent assortment lets each homologous pair orient independently at metaphase I, contributing one binary "maternal-or-paternal" choice per pair. With n = 4 there are only 2⁴ = 16 possible maternal/paternal mixes per gamete, but with n = 23 there are 2²³ ≈ 8.4 × 10⁶. The huge gap reflects the doubling per extra chromosome pair, not a difference in the mechanism itself — the same metaphase I behaviour is doing more work because there are more pairs to assort.

Sample response (c). Crossing over multiplies the count further by producing many possible recombinant versions of each chromosome (so the same chromosome is no longer simply "maternal" or "paternal"). Random fertilisation multiplies one parent's gamete count by the other parent's (≈ 8.4×10⁶ × 8.4×10⁶ ≈ 7×10¹³ zygote types per human couple). Mutation increases variation in a qualitatively different way — by adding new alleles to the population's pool, not in the existing count at all.

Marking notes. Part (a) — 1 mark for identifying the exponential / doubling trend; 1 mark for a reasonable estimate (≈ 8 million / 10⁶–10⁷). Part (b) — 1 mark for connecting the gap to 2ⁿ doubling per extra pair; 1 mark for citing metaphase I orientation as the mechanism. Part (c) — 1 mark for correctly stating the effect of crossing over and random fertilisation on the count; 1 mark for distinguishing mutation as adding new alleles rather than scaling existing combinations.

Sample response. The claim is largely incorrect, although it contains a small grain of truth. It is correct that mutation is unique: it is the only one of the four sources of variation that changes the DNA sequence itself, and therefore the only source that can introduce a genuinely new allele into a population. Without mutation, the allele pool of a population would be fixed and could never expand. In this very specific sense, mutation has irreplaceable evolutionary importance — only mutation generates novel sequence on which selection can act over long timescales. However, the claim's broader assertion that the other three sources cannot "generate anything new" misunderstands what counts as new variation. Crossing over, during prophase I of meiosis, exchanges segments between homologous chromosomes to produce recombinant chromosomes carrying new combinations of existing maternal and paternal alleles. Independent assortment, at metaphase I, distributes homologous pairs into gametes in many different combinations — 2ⁿ per individual. Random fertilisation then combines any one of a parent's many genetically varied gametes with any one of the other parent's, multiplying these counts together to produce an enormous number of possible offspring genotypes — for humans, on the order of 10¹³ or more per couple, even before crossing over is counted. These rearrangements are not nothing: they generate the genetic diversity natural selection acts on every generation. In a population that already contains many alleles, virtually all of the offspring-level variation a parent generates is reshuffled rather than freshly mutated. Without reshuffling, every gamete a parent produced would carry the same chromosomes in the same arrangement, no offspring would meaningfully differ, and selection would have no within-generation variation to act on — even if mutation continued to add new alleles. Both kinds of variation are therefore biologically important but for different reasons. The three reshuffling sources generate the short-term, this-generation genetic differences that drive selection now; mutation generates the long-term raw material that expands the allele pool over evolutionary time. The claim is therefore rejected: mutation is unique but not the only biologically important source of variation — reshuffling existing alleles is essential to the moment-to-moment functioning of evolution, and a population that had mutation alone (with no meiotic reshuffling or random fertilisation) would generate variation far more slowly than one with the full system.

Marking notes. 1 mark — defines or implicitly applies genetic variation as differences in allele combinations and alleles. 1 mark — correctly identifies the unique role of mutation (only source acting on DNA sequence / only source of new alleles). 1 mark — describes crossing over with mechanism (homologous chromosome segment exchange in meiosis prophase I). 1 mark — describes independent assortment with mechanism (random orientation of homologous pairs at metaphase I; 2ⁿ outcomes). 1 mark — describes random fertilisation and its multiplicative contribution to offspring genotype diversity. 1 mark — explicitly applies the new allele combination vs new allele distinction and identifies which sources do which. 1 mark — uses a quantitative or comparative argument (e.g. 2²³ ≈ 8 × 10⁶ gametes; ≈ 10¹³ zygotes per couple) to show that reshuffling is not negligible. 1 mark — reaches an explicit evaluative judgement that mutation is necessary but not sufficient and that all four sources are biologically important for different reasons.