Sources of Genetic Variation — Meiosis, Crossing Over, Fertilisation, Mutation

Q1 — Sample Band 6 response (7 marks), annotated

Genetic variation is the set of differences in genetic makeup between individuals in a population. A useful distinction within that idea is the difference between a new allele combination (a rearrangement of alleles already present in the population) and a new allele (a sequence variant that did not previously exist). [1 — definition + new combination vs new allele]

Crossing over occurs during prophase I of meiosis: paired homologous chromosomes physically exchange segments, producing recombinant chromosomes carrying new mixes of maternal and paternal alleles. Independent assortment occurs at metaphase I of meiosis: each homologous pair orients independently of every other pair, so each gamete receives a different combination of whole maternal and paternal chromosomes. Random fertilisation occurs after meiosis, when any one of a parent's many possible gametes can fuse with any one of the other parent's many possible gametes, producing many possible zygote genotypes. Mutation is a change in DNA sequence — for example a single base substitution — that can occur at any time in any cell, but is heritable only when it occurs in a germ-line cell. [1 — four mechanisms; 1 — cellular stage of each]

Critically, crossing over, independent assortment and random fertilisation can only rearrange alleles that are already present in the parents — they generate new combinations. Mutation is the only one of the four that can change a DNA base and therefore introduce a new allele into the population. [1 — three reshufflers; 1 — mutation as the new-allele source]

A population needs both. Reshuffling generates the genetic diversity that natural selection acts on in the short term: every generation, varied gametes plus random fertilisation produce many different offspring genotypes from the same parental allele pool. But reshuffling alone cannot expand that pool — without mutation, the same fixed set of alleles would simply be permuted forever, and the population could not evolve novel traits in response to genuinely new environmental pressures. [1 — why reshuffling is needed]

Therefore the four sources are not interchangeable. The three reshuffling sources are the engine of short-term variation; mutation is the long-term source of evolutionary novelty. Both are required for continued adaptation — neither alone is sufficient. [1 — explicit judgement linking both]

Q2 — Sample Band 6 response (8 marks), annotated

A and B (~50% shared). Full siblings receive half their genome from each parent. Each parental gamete has been shaped by crossing over (recombinant chromosomes carrying new mixes of grandparental alleles) and independent assortment (each chromosome pair sorted independently), so A and B inherit a different reshuffle of M's and F's allele pool. Random fertilisation then determined which particular gamete-pair formed each sibling. On average, this combination of reshuffling sources delivers ~50% identity-by-descent — high but not identical. [1 — meiotic reshuffling; 1 — random fertilisation]

A and C (~25% shared). Half-siblings share only one parent (M). A's other 50% comes from F and C's other 50% comes from C's different father, so only the M-derived half of each genome is even eligible to match. Within that M-derived half, the same crossing-over and independent-assortment processes mean that A and C each received a different reshuffle of M's alleles, dropping the expected match to ~25%. [1 — only one shared parent; 1 — reshuffling within that half]

A and D (~100% shared). Monozygotic twins arise from a single fertilised zygote that split into two embryos after fertilisation. They share the same single gamete-pair (same products of M's and F's meiosis combined by the same fertilisation event), so crossing over, independent assortment and random fertilisation did not produce any difference between them. The only routes to genetic difference between A and D are post-zygotic somatic mutations — small in number and detectable but tiny compared to inter-sibling differences. [1 — same gamete-pair / same fertilisation; 1 — only post-zygotic mutation remains]

A's de novo G→A variant. Sharing percentages count alleles inherited from the parents' existing pool. They cannot capture variation that did not exist in the parents at all. The G→A variant is present in neither M nor F, so it cannot have arisen by crossing over, independent assortment or random fertilisation — those three sources can only reshuffle alleles already in the parental pool. It must therefore be a mutation, almost certainly in a parental germ-line cell, because only mutation changes the DNA sequence itself and so introduces a genuinely new allele. This is exactly the distinction between "new allele combinations" (the three reshufflers) and "new alleles" (mutation alone) from the lesson's Precision callout. [1 — new combinations vs new alleles applied; 1 — justifies mutation as the only source of genuinely new genetic material]

Q3 — Sample Band 6 response (6 marks)

The claim is almost entirely wrong, though it does contain one tiny element of truth. [1 — overall judgement]

What is (barely) defensible: Mutation does contribute to differences between siblings — for example, A's de novo G→A variant in question 2. Over a human lifetime each person accumulates a small number of new germ-line and somatic mutations, so mutation is not zero. [1 — concedes the small role of mutation]

What is wrong:

• "Entirely caused by mutation." The vast majority of sibling variation is reshuffling of existing parental alleles — crossing over produces recombinant chromosomes, independent assortment scrambles whole chromosomes into gametes, and random fertilisation pairs different gametes. A typical sibling pair differs at millions of sites already present in the parents, far more than the few new mutations either sibling has. [1 — refutes "entirely mutation"]
• "Identical twins prove this." Identical twins are similar because they came from the same gamete-pair (same products of meiosis combined by the same fertilisation event) — they isolate-out exactly the three reshuffling sources. Their similarity therefore highlights how much variation those three sources normally contribute; it does not show that mutation is the cause. [1 — refutes the twin argument]
• "Crossing over etc. are minor details." They are in fact the dominant source of sibling differences. Mutation is the source of new alleles in the population, but it is a minor source of this-generation differences between full siblings. [1 — refutes "minor details"]

Defensible reformulation: "Sibling differences are mostly explained by reshuffling of existing parental alleles through crossing over, independent assortment and random fertilisation. Mutation contributes a smaller number of new alleles per generation, and is essential for long-term evolutionary change, but it is not the main source of routine sibling variation." [1 — defensible reformulation linking reshuffling and mutation correctly]