Biology • Year 12 • Module 6 • Lesson 6
Fertilisation, Meiosis and Mutation as Causes of Genetic Variation
Lock in the three distinct sources of genetic variation — mutation (new alleles), meiosis (reshuffling), and fertilisation (random combination) — and the exam vocabulary that keeps their roles separate.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: genetic variation, mutation, meiosis, fertilisation, independent assortment, crossing over, gene pool, gamete, zygote, allele. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A change in DNA sequence that can create a new allele. | |
| 1.2 | The total collection of alleles in a population. | |
| 1.3 | Cell division producing haploid gametes that reshuffles alleles through independent assortment and crossing over. | |
| 1.4 | One of two or more alternative forms of a gene at the same locus. | |
| 1.5 | The fusion of two gametes, combining alleles from each parent in a new offspring. | |
| 1.6 | Differences in genetic makeup between individuals in a population. | |
| 1.7 | Random separation of homologous chromosomes during meiosis I. | |
| 1.8 | A haploid sex cell (e.g. sperm, egg) that fuses during fertilisation. | |
| 1.9 | Exchange of segments between homologous chromosomes during prophase I that generates recombinant chromatids. | |
| 1.10 | The diploid cell formed when two gametes fuse. |
2. Complete the comparison table
Fill in the blanks (one short phrase per cell). The structure follows Card 1 of the lesson. 9 marks
| Process | What it does (mechanism) | Creates new alleles? | Main variation role |
|---|---|---|---|
| Mutation | 2.1 ____________________ | 2.2 ____________________ | 2.3 ____________________ |
| Meiosis | 2.4 ____________________ | 2.5 ____________________ | 2.6 ____________________ |
| Fertilisation | 2.7 ____________________ | 2.8 ____________________ | 2.9 ____________________ |
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 10 marks (1 for T/F, 1 for the correction where needed)
3.1 Mutation is the only source of new alleles in a population. T / F
3.2 Meiosis routinely creates new alleles by changing DNA sequence. T / F
3.3 Fertilisation combines gametes randomly, generating new allele combinations in offspring. T / F
3.4 Siblings can only differ genetically if a new mutation has occurred in one of them. T / F
3.5 Independent assortment and crossing over both occur during meiosis and contribute to variation in gametes. T / F
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 What is the function of mutation in producing genetic variation in a population?
4.2 What is the function of independent assortment during meiosis I?
4.3 What is the function of crossing over during prophase I?
4.4 What is the function of fertilisation in producing genetic variation in offspring?
4.5 What is the function of genetic variation in a population that natural selection acts upon?
5. Sort the source
Each item below describes a scenario that produces or maintains genetic variation. In the right-hand column write whether the main source is mutation, meiosis, or fertilisation. 8 marks
| # | Scenario | Main source |
|---|---|---|
| 5.1 | A single base substitution introduces a new allele at the BRCA1 locus. | |
| 5.2 | Homologous chromosomes line up randomly in metaphase I, so different combinations of maternal and paternal chromosomes enter different gametes. | |
| 5.3 | One particular sperm out of millions fuses with one particular egg. | |
| 5.4 | Chromatids exchange segments at chiasmata during prophase I, producing recombinant chromatids. | |
| 5.5 | A frameshift insertion in a gametic cell creates a brand-new allele passed to the next generation. | |
| 5.6 | Two siblings inherit different combinations of the same parental alleles because their parents produced genetically different gametes. | |
| 5.7 | A zygote contains a new combination of maternal and paternal alleles that exists in neither parent's somatic cells. | |
| 5.8 | UV radiation alters a thymine dimer in a germ-line cell, producing an allele that did not previously exist in the gene pool. |
Q1 — Term–definition matches (10 marks)
1.1 mutation • 1.2 gene pool • 1.3 meiosis • 1.4 allele • 1.5 fertilisation • 1.6 genetic variation • 1.7 independent assortment • 1.8 gamete • 1.9 crossing over • 1.10 zygote.
Marking: 1 mark per correct match (max 10).
Q2 — Comparison table (9 marks)
Mutation: 2.1 changes the DNA sequence (substitution, insertion, deletion etc.); 2.2 yes; 2.3 introduces new alleles into the gene pool.
Meiosis: 2.4 independent assortment of homologous chromosomes and crossing over between homologues; 2.5 not usually; 2.6 reshuffles existing alleles into genetically different gametes.
Fertilisation: 2.7 random fusion of two haploid gametes to form a diploid zygote; 2.8 no; 2.9 combines gametes randomly to create new allele combinations in offspring.
Marking: 1 mark per correctly completed cell (max 9). Accept biologically equivalent phrasing.
Q3 — True / false with correction (10 marks)
3.1 True. [1] Mutation is the only direct source of genuinely new alleles; meiosis and fertilisation only reshuffle alleles that already exist.
3.2 False. [1] Correction: meiosis does not routinely create new alleles. It produces variation by independently assorting homologous chromosomes and by crossing over, both of which only reshuffle existing alleles. [1]
3.3 True. [2]
3.4 False. [1] Correction: siblings differ routinely because meiosis produces genetically different gametes and fertilisation combines gametes randomly — no new mutation is required to explain ordinary sibling difference. [1]
3.5 True. [2]
Q4.1 — Function of mutation (2 marks)
Mutation introduces new alleles into the gene pool by changing the DNA sequence [1]. This is the only direct source of genuinely new genetic variation, on which natural selection can later act [1].
Q4.2 — Function of independent assortment (2 marks)
Independent assortment ensures that homologous chromosomes separate randomly in meiosis I, so different gametes receive different combinations of maternal and paternal chromosomes [1]. This generates variation between gametes from the same parent without changing any allele [1].
Q4.3 — Function of crossing over (2 marks)
Crossing over exchanges segments between homologous chromosomes during prophase I [1], producing recombinant chromatids that carry new combinations of maternal and paternal alleles on the same chromosome [1].
Q4.4 — Function of fertilisation (2 marks)
Fertilisation fuses one gamete from each parent at random [1], combining one set of maternal alleles with one set of paternal alleles in a new diploid genotype — generating new combinations of alleles without creating new alleles [1].
Q4.5 — Function of genetic variation (2 marks)
Genetic variation provides the raw material on which natural selection acts [1]: when the environment changes, populations with more variation are more likely to contain some individuals whose allele combinations suit the new conditions, supporting long-term continuity [1].
Q5 — Sort the source (8 marks)
5.1 Mutation • 5.2 Meiosis (independent assortment) • 5.3 Fertilisation • 5.4 Meiosis (crossing over) • 5.5 Mutation • 5.6 Meiosis (the gametes differ before fertilisation) • 5.7 Fertilisation • 5.8 Mutation.
Marking: 1 mark per correct identification (max 8).