HSC Exam Practice

Sample response. Genetic variation is the differences in alleles (and combinations of alleles) carried by individuals within a population. The three processes that generate it in a sexually reproducing population are mutation, meiosis, and fertilisation.

Marking notes. 1 mark for defining genetic variation in terms of alleles / DNA differences between individuals; 1 mark for naming all three processes (mutation, meiosis, fertilisation).

Sample response. Mutation is a change in DNA sequence (e.g. base substitution, insertion, deletion) that creates a new allele not previously present in the gene pool. Fertilisation, by contrast, does not change DNA sequence and does not create new alleles; it fuses two existing gametes at random, combining one set of maternal alleles with one set of paternal alleles to produce a new combination of existing alleles in the zygote.

Marking notes. 1 mark for mutation = sequence change producing new alleles; 1 mark for fertilisation = random fusion of gametes; 1 mark for the explicit "new allele vs new combination" distinction.

Sample response. (i) Independent assortment of homologous chromosomes during metaphase I, so different gametes receive different combinations of maternal and paternal chromosomes. (ii) Crossing over between homologous chromosomes during prophase I, which exchanges segments and produces recombinant chromatids.

Marking notes. 1 mark for independent assortment (correctly named); 1 mark for crossing over (correctly named). Description of timing/effect helpful but not required for full marks.

Sample response. Each parent produces genetically different gametes through meiosis, because independent assortment and crossing over reshuffle the alleles already present in that parent. Fertilisation then fuses one gamete from each parent at random, so each conception combines a different sperm with a different egg. This produces siblings with different combinations of existing parental alleles, even though no new allele has been created — variation arises entirely from reshuffling and random combination of existing alleles.

Marking notes. 1 mark for explaining that meiosis produces genetically different gametes (via independent assortment and/or crossing over); 1 mark for explaining that fertilisation is a random fusion of gametes; 1 mark for the conclusion that new combinations (not new alleles) account for sibling difference.

Sample response. Meiosis and fertilisation can only reshuffle and combine alleles that already exist in the gene pool. Mutation is essential because it is the only process that introduces genuinely new alleles into that gene pool — without mutation, the population's gene pool could never expand, and long-term genetic change driven by natural selection would not be possible.

Marking notes. 1 mark for identifying mutation as the only source of new alleles; 1 mark for linking this to long-term variation / maintenance of the gene pool over time.

Sample response (a). Allele Y is absent from the population from generation 0 to generation 15. It first appears at generation 15 at a very low frequency, then rises in a roughly logistic curve, reaching about 0.40 by generation 60. The rate of increase is fastest between approximately generations 25 and 45.

Sample response (b). The appearance of allele Y at generation 15 cannot be explained by meiosis or fertilisation alone, because they only reshuffle and combine existing alleles. A mutation in a germ-line cell must have produced allele Y by changing the DNA sequence. From generation 15 onward, meiosis distributes allele Y into a subset of gametes each generation, alongside the original allele, by independent assortment and crossing over. Fertilisation then combines these gametes randomly into the next generation's zygotes, so the proportion of zygotes carrying Y rises each generation — driving the increase in Y's frequency to about 0.40.

Marking notes. Part (a) — 1 mark for identifying allele Y is absent until generation 15; 1 mark for describing the rise after generation 15 with a supporting value (e.g. ≈0.40 at generation 60). Part (b) — 1 mark for attributing the appearance of Y at generation 15 to mutation (with reason); 1 mark for attributing the spread to meiosis (independent assortment / crossing over distributing Y into gametes); 1 mark for attributing the spread to random fertilisation combining those gametes into new zygotes each generation.

Sample response. The statement is partly correct but ultimately wrong. Meiosis and fertilisation are sufficient to explain a great deal of the genetic variation visible within a sexually reproducing population at any single time — including the routine genetic difference between siblings — but they cannot account for all genetic variation in a population, because they only reshuffle and combine alleles that already exist in the gene pool. Meiosis generates genetically different gametes from each parent through independent assortment of homologous chromosomes in metaphase I and crossing over between homologues in prophase I, both of which reshuffle existing maternal and paternal alleles into new combinations on the gametic chromosomes. Fertilisation then fuses one gamete from each parent at random, so a different combination of maternal and paternal alleles ends up in each zygote. These two processes adequately explain, for example, why two siblings can differ genetically with no new mutation in either parent: each parent produced a genetically distinct gamete through meiosis, and a different random pairing of gametes occurred at fertilisation. Mutation, however, is the only process that introduces genuinely new alleles into the gene pool — it changes DNA sequence (e.g. base substitution, insertion, deletion) and therefore produces variation that meiosis and fertilisation are incapable of generating. A worked example is provided by any newly observed allele, such as the human HBB sickle-cell allele: meiosis and fertilisation could never have produced this allele in a population whose ancestors did not carry it; it must have arisen by mutation, after which meiosis and fertilisation distributed it into new combinations across subsequent generations. Therefore the statement is wrong to dismiss mutation as unimportant: meiosis and fertilisation explain the combinations of variation visible in a single generation, but mutation alone explains where the new alleles behind that variation came from and is essential for long-term genetic change in populations.

Marking notes. 1 mark — states an overall evaluative judgement (partly correct but wrong overall, or equivalent). 1 mark — names independent assortment and crossing over as the meiotic mechanisms of variation. 1 mark — describes fertilisation as random fusion of gametes producing new allele combinations. 1 mark — applies meiosis + fertilisation correctly to a worked example (e.g. sibling difference) where no new mutation is needed. 1 mark — identifies mutation as the only source of new alleles and explains why meiosis/fertilisation cannot produce new alleles. 1 mark — uses a specific worked example showing mutation is essential (e.g. a named allele that must have originated from mutation, or any equivalent argument about long-term population change). 1 mark — reaches an explicit evaluative judgement that rejects the statement, distinguishing "new combinations" (meiosis + fertilisation) from "new alleles" (mutation), and links mutation to long-term variation / continuity of variation in a population.