Fertilisation, Meiosis and Mutation as Causes of Genetic Variation

Q1 — Sample Band 6 response (8 marks), annotated

Genetic variation refers to differences in the alleles carried by individuals within a population — that is, differences in the gene pool. [1 — definition + alleles/gene-pool link]

The three processes contribute to variation in fundamentally different ways. Mutation changes the DNA sequence (e.g. base substitution, insertion, deletion, frameshift) and is the only process that produces new alleles — alleles that did not previously exist in the gene pool. [1 — mutation = new alleles + mechanism] Meiosis does not change DNA sequence; instead, homologous chromosomes assort independently in metaphase I, and crossing over during prophase I exchanges segments between homologues, producing recombinant chromatids. The result is genetically different gametes from a single parent without any new allele being created. [1 — meiosis = reshuffle + named mechanisms] Fertilisation then fuses one gamete from each parent at random, combining one set of maternal alleles with one set of paternal alleles in a new diploid zygote. This adds another layer of new combinations, but no new alleles. [1 — fertilisation = random fusion → new combinations]

The three processes therefore play complementary, non-substitutable roles. Mutation supplies the genetic novelty — without it, the gene pool would only ever contain the alleles it started with. Meiosis and fertilisation distribute that novelty (and the pre-existing alleles) into new combinations every generation, so that natural selection has many distinct genotypes to act on. [1 — complementary roles framed correctly]

This is why sibling difference can occur with no new mutation: meiosis reshuffles the parents' existing alleles into different gametes, and random fertilisation pairs them differently for each conception. [1 — applies framework to sibling difference] Equally, long-term population change cannot be explained by meiosis and fertilisation alone, because they cannot produce alleles that the population's ancestors did not carry. Mutation supplies that long-term novelty. [1 — long-term role of mutation]

Together, mutation generates new alleles, while meiosis and fertilisation generate and maintain the new combinations of alleles on which natural selection acts — both the generation and the maintenance of variation in a sexually reproducing population require all three processes. [1 — explicit overall judgement linking all three to "generation + maintenance"]

Marking criteria.

• 1 mark — Defines genetic variation as differences in alleles in a gene pool.
• 1 mark — Correctly identifies mutation as the source of new alleles and names a sequence-changing mechanism.
• 1 mark — Correctly identifies meiosis as a reshuffling source and names both independent assortment and crossing over.
• 1 mark — Correctly identifies fertilisation as random fusion of gametes producing new allele combinations.
• 1 mark — Explicitly frames the three processes as complementary (new alleles vs new combinations).
• 1 mark — Applies the framework to a worked example (e.g. sibling difference, breed variation).
• 1 mark — Explains why mutation is essential for long-term variation (meiosis/fertilisation cannot create novel alleles).
• 1 mark — Reaches an explicit overall judgement linking all three processes to both generation and maintenance of variation.

Q2 — Sample Band 6 response (8 marks)

The claim is partly correct but ultimately wrong. [1 — judgement]

What is defensible: The claim is correct that much of the genetic variation visible within a population at any one time — including sibling difference, breed-to-breed combinations of existing alleles, and the variation that natural selection sorts among at a given generation — can be accounted for by meiosis (independent assortment + crossing over) and random fertilisation, without invoking new mutation. These two processes do the day-to-day work of reshuffling alleles into the genotypes natural selection acts on. [1 — concedes correct element; 1 — names mechanisms]

What is wrong:

• "Mutation is essentially irrelevant." Meiosis and fertilisation cannot create an allele that did not already exist in the gene pool. They are exclusively reshuffling and recombining mechanisms. Any allele present in a contemporary population must, at some point in its history, have arisen by mutation. [1 — refutes "irrelevant" with the new allele vs new combination distinction]
• "Too rarely to matter." Although individual mutations are rare per locus per generation, large populations and long timescales mean a steady supply of new alleles enters the gene pool. Even rare alleles can become significant once meiosis and fertilisation distribute them into new combinations and natural selection acts on the resulting phenotypes. [1 — refutes "too rarely" using population-scale reasoning]
• Sibling difference example. The claim correctly notes that meiosis + fertilisation can explain ordinary sibling difference, but this is a within-generation observation. It does not extend to long-term variation in populations, where mutation is essential. [1 — distinguishes within-generation from long-term variation]

Defensible reformulation: "Most of the genetic variation visible within a single sexually reproducing population at any one time is produced by meiosis and random fertilisation, which reshuffle and combine the alleles already present in the gene pool. Mutation, however, remains essential, because it is the only process that introduces genuinely new alleles into that gene pool — without it, meiosis and fertilisation would have nothing new to combine in the long term." [1 — biologically defensible reformulation distinguishing 'new alleles' from 'new combinations']

Marking criteria.

• 1 mark — States an overall evaluative judgement (partly correct, ultimately wrong / similar).
• 1 mark — Identifies the defensible element (meiosis + fertilisation do explain much within-generation variation).
• 1 mark — Names the mechanisms by which meiosis and fertilisation generate combinations (independent assortment, crossing over, random fusion).
• 1 mark — Refutes "mutation is irrelevant" by appealing to the principle that only mutation can introduce new alleles.
• 1 mark — Refutes "too rarely to matter" by appealing to population-scale supply of new alleles over time.
• 1 mark — Explicitly distinguishes within-generation variation from long-term population variation.
• 1 mark — Uses precise lesson terminology throughout (new allele vs new combination, gene pool, gamete, zygote).
• 1 mark — Reformulates the claim into a biologically defensible statement that integrates all three sources of variation.