Fertilisation, Meiosis and Mutation as Causes of Genetic Variation
Siblings can be genetically different even when no new mutation occurs, because meiosis reshuffles parental alleles and fertilisation combines gametes randomly. Mutation plays a different role: it introduces genuinely new alleles into the population. This lesson brings those sources of variation together and keeps their jobs separate.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Meiosis reshuffles existing alleles; mutation introduces new ones.
Two siblings look different from each other, but neither parent has experienced any new mutation in the relevant genes.
Explain how that can still happen. Then add one sentence explaining what mutation contributes that meiosis and fertilisation usually do not.
Know
- Mutation, meiosis and fertilisation all contribute to variation.
- Mutation creates new alleles.
- Meiosis and fertilisation mainly reshuffle existing alleles.
Understand
- These processes have different roles, not interchangeable roles.
- Sibling difference can arise without new mutation.
- Population change depends on new alleles entering and then being combined or spread.
Apply
- Compare sources of variation in exam language.
- Explain why meiosis/fertilisation are not the source of all new alleles.
- Link Module 5 heredity to Module 6 population change.
Core Content
Main comparison · new alleles vs new combinations
The most important distinction is this: mutation creates new alleles, while meiosis and fertilisation mostly create new combinations of alleles that already exist.
| Process | What it does | Creates new alleles? | Main variation role |
|---|---|---|---|
| Mutation | Changes DNA sequence | Yes | Introduces new alleles into the gene pool |
| Meiosis | Independent assortment and crossing over | Not usually | Reshuffles existing alleles into different gametes |
| Fertilisation | Fusion of gametes | No | Combines gametes randomly to create new allele combinations in offspring |
Sources of genetic variation: mutation, meiosis and fertilisation.
What to write in your book
- Mutation → new alleles (changes DNA sequence).
- Meiosis → reshuffles existing alleles into gametes (independent assortment + crossing over).
- Fertilisation → combines gametes randomly → new genotypes.
- Key distinction: new alleles vs new combinations.
Mutation creates new alleles; meiosis and fertilisation create new _____ of existing alleles.
Module 5 link · independent assortment + crossing over
Meiosis contributes to variation in two major ways. First, homologous chromosomes assort independently, so different combinations of maternal and paternal chromosomes enter different gametes. Second, crossing over exchanges segments between homologous chromosomes, producing recombinant chromatids.
This means one parent can produce many genetically different gametes even without any new mutation. Module 5 established this as the basis of inheritance variation, and Module 6 now places it beside mutation so students do not confuse reshuffling with new allele creation.
What to write in your book
- Meiosis variation = independent assortment + crossing over.
- One parent can make many genetically different gametes (no new mutation needed).
- This is the Module 5 basis of inheritance variation.
- Reshuffling ≠ new allele creation.
Which two meiosis processes generate genetic variation?
Random combination · selection acts on phenotypes
Even after meiosis has produced varied gametes, fertilisation creates additional variation because which sperm meets which egg is largely random. This combines one set of maternal alleles with one set of paternal alleles in a new genotype.
Fertilisation therefore does not create new alleles by itself, but it creates new combinations of alleles in offspring. That matters biologically because natural selection acts on whole phenotypes produced by these combinations, not just on isolated alleles.
What to write in your book
- Fertilisation: random fusion of gametes → new genotype each time.
- It does NOT create new alleles — only new combinations.
- Selection acts on whole phenotypes from these combinations.
- Adds a layer of variation on top of meiosis.
Fertilisation by itself creates brand-new alleles.
Random fertilisation contributes to genetic variation by combining gametes from two genetically different parents.
Independent assortment creates new alleles that were not present in either parent.
Variation framework · distinct jobs
1. Mutation
New allele enters the gene pool when DNA sequence changes.
2. Meiosis
Existing alleles are reshuffled into genetically different gametes.
3. Fertilisation
Random gamete fusion creates new allele combinations in offspring.
Population consequence
Mutation supplies novelty; meiosis and fertilisation spread combinations — together generating the variation selection acts on.
If students say "genetic variation is caused by mutation, meiosis and fertilisation", that is correct but incomplete. High-quality HSC answers explain how each process contributes and clearly separate "new allele" from "new combination".
What to write in your book
- Step 1 Mutation: new allele enters the gene pool.
- Step 2 Meiosis: existing alleles reshuffled into varied gametes.
- Step 3 Fertilisation: random gamete fusion → new combinations.
- Explain HOW each contributes; separate "new allele" from "new combination".
Which process supplies genuinely new genetic novelty (new alleles)?
Activities
Match the Process to the Role
Name the process (mutation, meiosis or fertilisation) responsible for each effect.
- Different maternal and paternal chromosomes enter different gametes.
- A new allele appears because the DNA sequence changed.
- One sperm fuses with one egg out of many possible combinations.
- Homologous chromosomes exchange segments by crossing over.
Explain Sibling Difference
Explain why siblings can differ genetically without any new mutation, then add why mutation still matters in the long term.
Core biological claim
- Mutation, meiosis and fertilisation all contribute to variation, but they contribute in different ways.
Mechanism or process
- Mutation creates new alleles, meiosis reshuffles existing alleles into gametes, and fertilisation combines gametes randomly into new genotypes.
Common exam error
- Saying meiosis or fertilisation creates all new alleles.
Evaluative sentence starter
- "Although meiosis and fertilisation generate extensive genetic variation, mutation remains essential because it is the source of genuinely new alleles."
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Explain how meiosis contributes to genetic variation.
AnalyseBand 4(4 marks) 2. Compare the roles of mutation, meiosis and fertilisation in producing genetic variation.
EvaluateBand 5–6(5 marks) 3. Evaluate the claim: "Meiosis and fertilisation are enough to explain all genetic variation, so mutation is not very important."
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Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Match the process to the role
1. Meiosis.
2. Mutation.
3. Fertilisation.
4. Meiosis.
Activity 2 — Explain sibling difference
Siblings differ genetically because each parent produces genetically different gametes through meiosis, including independent assortment and crossing over. Fertilisation then combines one gamete from each parent at random, producing different allele combinations in each child. Mutation still matters in the long term because it introduces genuinely new alleles into the population.
Short Answer Model Responses
Q1 (3 marks): Meiosis contributes to genetic variation by producing genetically different gametes [1]. It does this through independent assortment of homologous chromosomes [1] and crossing over between homologous chromosomes [1].
Q2 (4 marks): Mutation creates new alleles by changing DNA sequence [1]. Meiosis contributes to variation by reshuffling existing alleles through independent assortment and crossing over [1]. Fertilisation contributes by combining gametes randomly [1]. Therefore mutation provides new genetic novelty, while meiosis and fertilisation mainly create new combinations of alleles already present [1].
Q3 (5 marks): The claim is incomplete because meiosis and fertilisation do explain much of the variation seen between siblings [1]. Meiosis reshuffles existing alleles into different gametes [1]. Fertilisation combines those gametes randomly in offspring [1]. However, mutation is still essential because it introduces genuinely new alleles into the gene pool [1]. Therefore meiosis and fertilisation are major sources of variation, but mutation remains critical for long-term genetic change in populations [1].
Mutation
Source of genuinely new alleles.
Meiosis
Reshuffles existing alleles into different gametes.
Fertilisation
Combines gametes randomly into new genotypes.
Exam trap
Using "variation" as if all sources do the same job.
Rapid-fire questions on mutation, meiosis, fertilisation and new alleles vs new combinations. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
Return to the sibling example. You should now be able to explain sibling difference using meiosis and fertilisation alone, and then separately explain why mutation still matters as the long-term source of new alleles in populations.