HSC Exam Practice

Sample response. Meiosis is the cell division that produces four genetically varied haploid (n) daughter cells (gametes) from a single diploid (2n) parent cell. It halves the chromosome number from the diploid state.

Marking notes. 1 mark for identifying meiosis as producing four daughter cells (or as a reduction division producing gametes); 1 mark for identifying the daughter cells as haploid (n) / having one set of chromosomes.

Sample response. Meiosis I separates homologous chromosome pairs into different daughter cells and is therefore the reduction division — chromosome number is halved from diploid (2n) to haploid (n). Meiosis II separates sister chromatids in each haploid cell in a division similar in outline to mitosis, and does not change the chromosome number.

Marking notes. 1 mark for identifying meiosis I as the division that separates homologous chromosomes (reduction division); 1 mark for identifying meiosis II as the division that separates sister chromatids; 1 mark for stating the effect on chromosome number for each (meiosis I halves, meiosis II does not change).

Sample response. Fertilisation fuses two haploid (n) gametes to form a diploid (2n) zygote, restoring the species' normal chromosome number rather than doubling it. Together with meiosis, this keeps the chromosome number stable from one generation to the next.

Marking notes. 1 mark for stating that fertilisation fuses two haploid gametes to form a diploid zygote (n + n = 2n); 1 mark for linking this to maintenance of the species' chromosome number across generations.

Sample response. During meiosis I, homologous chromosomes pair up and exchange corresponding segments between non-sister chromatids. Because the segments swapped already exist in the genome, crossing over relocates existing alleles to new combinations on each chromosome — it does not change the DNA base sequence of any allele. New alleles arise only through mutation, a separate process not part of meiosis.

Marking notes. 1 mark for identifying that crossing over exchanges corresponding segments between homologous chromosomes; 1 mark for identifying that the alleles exchanged already exist (new combinations, not new alleles); 1 mark for identifying mutation as the process that creates new alleles.

Sample response. (i) Each gamete contains n = 6 chromosomes. (ii) The zygote contains 2n = 12 chromosomes, restored by fertilisation.

Marking notes. 1 mark for gamete count (6); 1 mark for zygote count (12).

Sample response. Crossing over occurs during prophase of meiosis I, when homologous chromosomes pair and exchange corresponding segments between non-sister chromatids, producing new combinations of existing alleles within a chromosome. Independent assortment occurs at metaphase of meiosis I, when each homologous pair lines up at random so that either the maternal or paternal homologue can move to either pole, producing different combinations of whole chromosomes in each gamete.

Marking notes. 1 mark for naming both events (crossing over and independent assortment); 1 mark for correct timing of each (prophase I and metaphase I, or "during meiosis I" for both); 1 mark for describing what each event changes (allele combinations within a chromosome vs. which homologue enters which gamete).

Sample response (a). In every species, the chromosome number is at its full diploid value in the parent germline cell (46, 8, 14), halves to the haploid value after meiosis I and remains at that haploid value through meiosis II (23, 4, 7), and then returns to the original diploid value in the zygote after fertilisation. The same 2n → n → n → 2n pattern is conserved across the three species despite their different absolute chromosome numbers.

Sample response (b). The drop between the parent germline bar and the post-meiosis-I bar reflects meiosis I as a reduction division. During meiosis I, homologous chromosome pairs are separated into different daughter cells, so each daughter cell receives only one homologue of each pair instead of both. Because the parent cell had two sets of chromosomes (one from each parent) and the daughter cells now have only one set, the chromosome number is halved exactly.

Sample response (c). The zygote bar equals the parent germline bar because fertilisation fuses two haploid (n) gametes to form a diploid (2n) zygote, restoring the species' normal chromosome number. This restoration is what maintains continuity of species across generations — without meiosis halving first, fertilisation would double chromosome number each generation; without fertilisation restoring afterwards, gametes could not develop into viable diploid offspring.

Marking notes. Part (a) — 1 mark for describing the 2n → n → n → 2n pattern; 1 mark for noting it is conserved across all three species. Part (b) — 1 mark for identifying meiosis I as the reduction division; 1 mark for explaining homologous chromosome separation as the mechanism. Part (c) — 1 mark for explaining fusion of two haploid gametes restores 2n; 1 mark for linking restoration to continuity of species across generations; 1 mark for explicitly invoking the dual role (meiosis halves + fertilisation restores).

Sample response. Meiosis is essential to sexual reproduction for two complementary reasons that together support both continuity of species and variation among offspring. First, it is a reduction division. Meiosis I separates each homologous chromosome pair into different daughter cells, halving the chromosome number from diploid (2n) to haploid (n); meiosis II then separates sister chromatids but does not change chromosome number. The reason this matters is that fertilisation, which fuses two haploid gametes, restores the diploid chromosome number in the zygote rather than doubling it. Without meiosis, gametes would still be diploid, and after one generation the offspring would be tetraploid (4n), then octoploid (8n), and so on — chromosome number could not remain stable and continuity of species would fail. Meiosis and fertilisation therefore function as a paired mechanism: meiosis halves, fertilisation restores, and chromosome number is conserved from one generation to the next. Second, meiosis is the major routine source of genetic variation among offspring. Two processes contribute. Crossing over during prophase of meiosis I exchanges corresponding segments between non-sister chromatids of homologous chromosomes, producing new combinations of existing alleles on each chromatid. Independent assortment at metaphase of meiosis I lines up homologous pairs at random, so each gamete inherits a different combination of maternal and paternal homologues — in humans, 2²³ ≈ 8.4 million combinations from independent assortment alone. Importantly, crossing over does not create new alleles; it reshuffles existing ones. New alleles arise only through mutation. The claim is therefore supported: meiosis is essential not just for producing gametes but specifically for producing haploid gametes (continuity) and genetically varied gametes (variation). These two outcomes are not separable; both arise from the way meiosis I works. Continuity of species across generations depends on the first; the capacity of populations to respond to changing environments depends on the second. The two together make meiosis a more important process than mitosis for the evolutionary success of sexually reproducing species.

Marking notes. 1 mark — defines meiosis as producing haploid gametes from a diploid parent / identifies it as reduction division. 1 mark — identifies meiosis I specifically as the reduction step (separation of homologous chromosomes). 1 mark — links meiosis to fertilisation (n + n = 2n) and explains why this maintains chromosome-number stability across generations. 1 mark — uses a counter-factual or logical consequence (e.g. without meiosis, chromosome number would double each generation). 1 mark — identifies crossing over as a source of variation with correct timing (prophase I) and correct effect (new combinations of existing alleles within a chromosome). 1 mark — identifies independent assortment as a source of variation with correct timing (metaphase I) and correct effect (which homologues enter which gamete). 1 mark — distinguishes new combinations of existing alleles from new alleles (acknowledges mutation as the source of new alleles). 1 mark — reaches an evaluative judgement that meiosis is essential for both continuity and variation, and that the two outcomes are inseparable in meiosis I.