Biology • Year 12 • Module 5 • Lesson 8
Meiosis — Reduction Division and Continuity Across Generations
Build HSC Band 5–6 extended-response technique on meiosis: chromosome-number stability across generations, the two sources of variation, and what meiosis means for continuity of species.
1. Data + scenario — meiotic non-disjunction and chromosome-number stability
8 marks Band 5–6
Scenario. The graph below shows the percentage of human conceptions affected by a meiotic non-disjunction event (an error in which a homologous chromosome pair fails to separate during meiosis I) plotted against the age of the mother at conception. The trend has been replicated in many populations and is the basis for the increased risk of conditions such as Trisomy 21 with maternal age. (Modelled curve, simplified after Hassold & Hunt, 2001, Nature Reviews Genetics 2: 280–291.)
Figure 1.1. Percentage of human conceptions affected by meiotic non-disjunction at maternal age 20–45. Modelled curve simplified after Hassold & Hunt (2001), Nature Reviews Genetics 2: 280–291.
Q1. Using the figure and your understanding of meiosis, analyse why correct functioning of meiosis is essential for chromosome-number stability across generations, and evaluate how the data on age-related non-disjunction supports the claim that meiosis is a "reduction division" rather than just another form of cell division. In your response you must:
- Define meiosis and explain what is meant by reduction division, including the role of meiosis I.
- Use specific data from Figure 1.1 (at least two age points) to describe how non-disjunction risk changes with maternal age.
- Explain what happens at the cellular level when meiosis I fails (non-disjunction) and what the chromosome count of the resulting zygote will be after fertilisation.
- Reach an evidence-based judgement on what the figure tells us about the importance of meiosis for continuity of species across generations.
- Use precise lesson terminology — homologous chromosomes, diploid (2n), haploid (n), reduction division, fertilisation — throughout.
2. Data + scenario — sources of variation in meiosis
8 marks Band 5–6
Scenario. A genetics class models the variation generated in human meiosis using only the two homologous pairs that carry genes for hair colour (Pair X) and eye colour (Pair Y). Two real published findings frame the discussion:
- Humans have 23 homologous pairs, so independent assortment alone produces 223 ≈ 8.4 million different chromosome combinations per gamete (Pierce, Genetics: A Conceptual Approach, 2017).
- In humans, an average meiosis produces 2–3 crossover events per chromosome arm (Sun et al., 2004, Hum. Mol. Genet. 13: 2363–2376), each generating new combinations of existing alleles on the resulting chromosomes.
| Source of variation | When during meiosis? | What changes? | Estimated contribution to gamete variation |
|---|---|---|---|
| Independent assortment | Metaphase of meiosis I | Which homologue (maternal or paternal) of each pair enters which gamete | 223 ≈ 8.4 × 106 combinations |
| Crossing over | Prophase of meiosis I | Combinations of alleles within a chromosome (segments swapped between homologues) | multiplies independent-assortment variation by many further orders of magnitude |
| Mutation | Any time DNA is copied | The actual DNA base sequence of an allele (creates new alleles) | Not a meiotic mechanism — separate process |
Q2. Discuss the contribution of meiosis to genetic variation between offspring. Use the data in the table above to justify the lesson's claim that "crossing over creates new combinations of existing alleles, not new alleles." In your response you must:
- Distinguish independent assortment and crossing over, including when in meiosis each occurs and what each changes.
- Use at least one quantitative figure from the table above to support your discussion.
- Explain why crossing over does not create new alleles, and identify the process that does.
- Reach an evaluative judgement on whether meiosis alone could produce all the genetic variation observed in a species, or whether something else is also required.
- Refer explicitly to the lesson's framing of meiosis as supporting both continuity (chromosome-number stability) and variation (allele reshuffling).
Q1 — Sample Band 6 response (8 marks), annotated
Meiosis is the cell division that produces four haploid (n) daughter cells from a single diploid (2n) parent cell — in humans, four gametes with 23 chromosomes from a parent cell with 46. [1 — defines meiosis with ploidy] Meiosis I is called a reduction division because it separates each homologous chromosome pair into different daughter cells, halving the chromosome count from 2n to n; meiosis II then separates sister chromatids but does not change chromosome number. [1 — defines reduction division and identifies meiosis I]
Figure 1.1 shows that the percentage of human conceptions affected by meiotic non-disjunction is low and approximately flat below the mid-30s (around 3–5% at age 25 and roughly 8% at age 35) but rises steeply afterwards, reaching approximately 25–35% by age 45 — a sevenfold increase over the same age range. [1 — uses at least two specific data points from Figure 1.1]
When meiosis I fails through non-disjunction, both members of a homologous pair end up in the same daughter cell. After meiosis II, the four resulting gametes carry n+1 = 24 (×2) or n−1 = 22 (×2) chromosomes instead of the correct 23. At fertilisation, the n+1 gamete fusing with a normal n=23 gamete produces a zygote with 47 chromosomes — three copies of the affected chromosome. [1 — explains cellular consequence of non-disjunction; 1 — derives the 47-chromosome zygote]
This is direct evidence that meiosis must function as a reduction division: when meiosis I fails to halve the chromosome number correctly, fertilisation cannot restore the species' normal 2n count, and the zygote carries the wrong chromosome number from generation one. [1 — links the data to the necessity of reduction division]
The age dependence shown in Figure 1.1 underlines how tight the constraint on meiosis is across generations: even a sevenfold increase in error rate above age 35 still leaves most meioses correct, because chromosome-number stability is essential for offspring viability. Meiosis is not "just another form of cell division" — it is the specialised reduction division without which fertilisation would double the chromosome number every generation and continuity of species would fail. [1 — overall evaluative judgement linking data to continuity of species]
Marking notes (per criterion).
- 1 mark — Correctly defines meiosis with ploidy change (2n → n).
- 1 mark — Defines reduction division and identifies meiosis I as the reduction step (separation of homologues).
- 1 mark — Uses at least two specific data points from Figure 1.1 (e.g. ≈5% at age 25, ≈25–35% by age 45) to describe the trend.
- 1 mark — Explains the cellular consequence of non-disjunction (both homologues end up in the same daughter cell; gametes carry 24/22 chromosomes instead of 23).
- 1 mark — Derives the correct zygote chromosome number after fertilisation (47).
- 1 mark — Links Figure 1.1 explicitly to the necessity of meiosis I as a reduction division (when it fails, the chromosome count is wrong from generation one).
- 1 mark — Reaches an evaluative judgement that the data supports meiosis as essential for continuity of species, not just another form of cell division.
- 1 mark — Uses precise lesson terminology throughout (homologous chromosomes, diploid/haploid, reduction division, fertilisation).
Q2 — Sample Band 6 response (8 marks), annotated
Meiosis contributes to genetic variation between offspring through two complementary mechanisms that operate during meiosis I. Independent assortment occurs at metaphase of meiosis I, when each homologous chromosome pair lines up at random and either the maternal or the paternal homologue can move to either pole — so which combination of 23 homologues a gamete inherits is randomised. Crossing over occurs earlier, during prophase of meiosis I, when non-sister chromatids of paired homologues exchange corresponding segments, generating new combinations of alleles within each chromosome. [1 — distinguishes when each occurs; 1 — distinguishes what each changes]
The table makes the quantitative impact clear. Independent assortment alone produces 223 ≈ 8.4 million different chromosome combinations per gamete in humans. Crossing over then multiplies this further: with 2–3 crossovers per chromosome arm, almost every chromosome in every gamete carries a unique reshuffled allele combination. Together these mechanisms make the probability of two genetically identical gametes from the same meiosis vanishingly small. [1 — uses at least one quantitative figure from the table; 1 — explains how the two combine]
Crucially, crossing over does not create new alleles. It only relocates segments that already exist, producing new combinations of existing alleles on the daughter chromatids. New alleles arise through mutation — a change to the DNA base sequence — which is a separate process listed in the third row of the table and not a meiotic mechanism. [1 — clearly distinguishes "new combinations" from "new alleles"; 1 — names mutation as the source of new alleles]
Meiosis therefore cannot, on its own, produce all the genetic variation observed in a species — it can only reshuffle the alleles already present. Mutation is also required to generate the original variation that meiosis then redistributes. However, meiosis is enormously efficient at this redistribution, and combined with fertilisation it is the dominant source of routine differences between offspring within one generation. [1 — evaluative judgement that meiosis alone cannot produce all variation; mutation is also needed]
This is the lesson's two-purpose framing of meiosis: it supports continuity by halving chromosome number so fertilisation restores rather than doubles it, and it supports variation by reshuffling existing alleles through both crossing over and independent assortment. [1 — links explicitly to lesson's continuity + variation framing]
Marking notes (per criterion).
- 1 mark — Identifies the timing of each variation source (independent assortment at metaphase I; crossing over at prophase I).
- 1 mark — Identifies what each changes (which homologue goes where vs. allele combinations within a chromosome).
- 1 mark — Uses at least one quantitative figure from the table (e.g. 223 ≈ 8.4 million; 2–3 crossovers/arm).
- 1 mark — Explains how the two mechanisms combine to multiply variation.
- 1 mark — Clearly distinguishes "new combinations of existing alleles" from "new alleles".
- 1 mark — Names mutation as the source of new alleles and identifies it as not a meiotic mechanism.
- 1 mark — Reaches an evaluative judgement that meiosis alone is insufficient — mutation is also required to generate the variation that meiosis redistributes.
- 1 mark — Links explicitly to the lesson's framing of meiosis as supporting both continuity (chromosome-number stability) and variation (allele reshuffling).