Biology • Year 12 • Module 6 • Lesson 6
Fertilisation, Meiosis and Mutation as Causes of Genetic Variation
Apply the three sources of variation to family data, allele-frequency models, and a student diagram. Connects directly to Module 5 Lesson 13 (meiosis and gamete variation).
1. Apply to family data — three siblings, two parents
A geneticist sequences four unlinked loci (A, B, C, D) in two parents and their three children. Each parent's diploid genotype is shown. None of the parents has experienced any new mutation at these loci in their lifetimes. 7 marks
| Individual | Locus A | Locus B | Locus C | Locus D |
|---|---|---|---|---|
| Parent 1 | A1A2 | B1B1 | C1C2 | D1D2 |
| Parent 2 | A3A3 | B2B3 | C3C3 | D3D4 |
| Child 1 | A1A3 | B1B2 | C1C3 | D1D3 |
| Child 2 | A2A3 | B1B3 | C2C3 | D2D4 |
| Child 3 | A1A3 | B1B3 | C2C3 | D1D4 |
1.1 The three children carry different combinations of the parents' alleles. Identify the two processes from this lesson that together explain this difference, given that no new mutation has occurred. 2 marks
1.2 Inspect locus A. Explain why Child 1 and Child 3 can both be A1A3 while Child 2 is A2A3. Refer to meiosis and fertilisation in your answer. 3 marks
1.3 A fourth child is later born with genotype A4A3 at locus A, even though no parent ever carried an A4 allele. Which source of variation is the only plausible explanation? Justify your answer in one sentence. 2 marks
2. Interpret a model — allele frequency over generations
The graph below models the frequency of two alleles, X and Y, at the same locus in a small population over 50 generations. Allele Y first arises at generation 10. Both alleles are then redistributed by meiosis and fertilisation each generation. 6 marks
Stylised model — illustrative of how the three sources of variation interact in a population over time.
2.1 Which of the three sources of variation (mutation, meiosis, fertilisation) is the only one that can explain the appearance of allele Y at generation 10? 1 mark
2.2 After generation 10, allele Y spreads from near 0 to about 0.45 by generation 50, while no new mutation events occur. Which two processes from this lesson are responsible for that spread, and what is the role of each? 3 marks
2.3 A student says: "Meiosis and fertilisation alone could have produced this graph." Refute this in one sentence using lesson terminology. 2 marks
3. Diagram critique — what's wrong with this student's diagram?
A Year 12 student has drawn the diagram below to explain "how sibling difference arises." There are three biological errors. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
3.1 Error 1: What is wrong?
Correction:
3.2 Error 2: What is wrong?
Correction:
3.3 Error 3: What is wrong?
Correction:
4. Apply to a new scenario — wheat-breeding program
A plant breeder wants to maximise genetic variation in a wheat population so that some plants survive a future, currently unknown stressor. She considers three actions: (i) treat seeds with a mutagen to induce point mutations; (ii) hand-cross plants from genetically distinct parental lines so that gametes from each line meet at fertilisation; (iii) propagate one elite plant clonally (no meiosis, no fertilisation) to fill the field. 5 marks
4.1 Which of the three actions could plausibly introduce new alleles that did not exist before in the population? Justify in one sentence. 2 marks
4.2 Which of the three actions would generate new allele combinations in offspring without changing any DNA sequence? Explain how, naming the two processes from this lesson. 2 marks
4.3 The breeder argues: "If I do action (iii) properly, I don't need actions (i) or (ii) — clonal propagation is enough to maintain variation." Briefly refute this claim. 1 mark
Q1.1 — Two processes responsible (2 marks)
Meiosis (in each parent) reshuffles the parents' existing alleles into genetically different gametes via independent assortment and crossing over [1]; fertilisation then combines one of those gametes from each parent at random, producing different allele combinations in each child [1]. No new mutation is required.
Q1.2 — Locus A across three children (3 marks)
Parent 1 is A1A2, so meiosis produces some gametes carrying A1 and others carrying A2 (independent assortment of the homologous pair) [1]. Parent 2 is A3A3, so every gamete from Parent 2 carries A3 [1]. The genotype of each child depends on which Parent 1 gamete was involved at fertilisation: Children 1 and 3 received an A1 gamete (giving A1A3), while Child 2 received an A2 gamete (giving A2A3). The difference between siblings is therefore explained by meiosis and the random gamete fusion of fertilisation, with no mutation required [1].
Marking: 1 for explaining the two parental gamete types from Parent 1 (meiosis); 1 for noting Parent 2 produces only A3 gametes; 1 for linking the difference to random fertilisation.
Q1.3 — A novel A4 allele (2 marks)
Mutation. [1] Neither meiosis nor fertilisation can produce an allele that did not previously exist in either parent — they only reshuffle and recombine existing alleles. A genuinely new allele (A4) must have arisen by a change to the DNA sequence in a germ-line cell of one parent [1].
Q2.1 — Origin of allele Y (1 mark)
Mutation — it is the only direct source of a new allele in the population [1].
Q2.2 — Spread of allele Y from generation 10 to 50 (3 marks)
Meiosis places allele Y into a subset of gametes each generation by independent assortment and crossing over, alongside allele X gametes [1]. Fertilisation then combines those gametes randomly into the next generation's zygotes [1]. As Y-carrying gametes participate in fertilisation each generation, the proportion of zygotes carrying Y rises and the frequency of Y in the population climbs toward 0.45 — without any new mutation event being required after generation 10 [1].
Q2.3 — Refute "meiosis and fertilisation alone" (2 marks)
Meiosis and fertilisation only reshuffle and combine existing alleles [1]; they cannot produce allele Y in the first place, so the graph requires a mutation event at generation 10 to create the new allele that meiosis and fertilisation then spread [1].
Q3 — Diagram critique (6 marks)
3.1 Error 1 (arrow labelled "mutation creates the difference"): Sibling difference does not usually require new mutation [1]. Correction: relabel the arrow to "meiosis (independent assortment + crossing over) and random fertilisation produce different allele combinations in each sibling" [1].
3.2 Error 2 ("Meiosis = creates new alleles by changing DNA sequence"): Meiosis does not change DNA sequence and does not normally create new alleles [1]. Correction: relabel the box as "Meiosis = reshuffles existing alleles into genetically different gametes via independent assortment and crossing over" [1].
3.3 Error 3 ("Fertilisation produces identical zygotes from identical gametes"): Fertilisation combines genetically different gametes at random [1]. Correction: replace the caption with "Fertilisation combines one (genetically distinct) gamete from each parent at random, producing zygotes with new allele combinations" [1].
Q4.1 — New alleles (2 marks)
Only action (i), mutagen treatment, can introduce new alleles [1], because mutagens change the DNA sequence and so create alleles that did not previously exist in the population — actions (ii) and (iii) can only reshuffle existing alleles or copy them [1].
Q4.2 — New combinations without new alleles (2 marks)
Action (ii) [1]: when plants from genetically distinct parental lines are crossed, meiosis produces gametes with reshuffled allele combinations (independent assortment + crossing over), and fertilisation combines a gamete from each line at random — generating new allele combinations in the offspring without altering any DNA sequence [1].
Q4.3 — Refute "clonal propagation maintains variation" (1 mark)
Clonal propagation involves neither meiosis nor fertilisation, so existing alleles are not reshuffled or recombined, and mutation alone is too rare to maintain meaningful variation generation-to-generation — the field would be near-uniform and vulnerable to a single stressor [1].