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Biology› Year 12› Module 6› Lesson 06

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.

35 min IQ1: Mutation Variation synthesis Lesson 6 of 18
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Meiosis Mutation

Meiosis Mutation

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Prediction

Think First

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.

Key Terms
Genetic variationDifferences in genetic makeup between individuals in a population.
MutationA change in DNA sequence that can create a new allele.
MeiosisCell division that produces haploid gametes and reshuffles alleles through independent assortment and crossing over.
FertilisationFusion of two gametes, combining alleles from each parent in a new offspring.
Independent assortmentRandom separation of homologous chromosomes during meiosis.
Gene poolThe total collection of alleles in a population.

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 Learning

Misconceptions to Fix

✗

Wrong: The immune system always remembers every pathogen it encounters.

✗

Right: Immunological memory is specific; the body remembers previously encountered antigens, not all pathogens.

1
Main Comparison

Mutation, meiosis and fertilisation all generate variation, but not in the same way

The most important distinction is this: mutation creates new alleles, while meiosis and fertilisation mostly create new combinations of alleles that already exist.

Sources of genetic variation: mutation, meiosis and fertilisation

Sources of genetic variation: mutation, meiosis and fertilisation

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
2
Module 5 Link

Meiosis generates variation before fertilisation even occurs

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.

Anchor
Brothers and sisters are genetically different largely because each parent produces a different set of gametes through meiosis. They are not different only because mutation happens every generation in the relevant genes.
3
Random Combination

Fertilisation adds another layer of variation by combining gametes at random

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.

4
Variation Framework

Think of variation as a three-step system rather than one process

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.

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Population consequence

  • Mutation supplies novelty.
  • Meiosis and fertilisation spread combinations of novelty and pre-existing alleles.
  • Together they generate the variation on which selection can act.
Variation is strongest understood as interacting processes with distinct jobs.

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”.

Copy Into Your Books ▼

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.

Revisit Your Initial Thinking

Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?

Interactive: Nondisjunction Explorer Interactive
id="phase-questions" class="phase">

Activities

Activity 1 - Match the process to the role

Match each role to the correct process: mutation, meiosis, or fertilisation.

1. Produces recombinant gametes through crossing over.

2. Introduces a genuinely new allele into the gene pool.

3. Randomly combines one maternal and one paternal gamete.

4. Separates homologous chromosomes independently.

Activity 2 - Explain sibling difference

Write a short paragraph explaining why siblings can differ genetically even when no new mutation is involved in the relevant genes. Then add one sentence explaining why mutation still matters in the long term for populations.

Multiple Choice

UnderstandBand 3

1. Which process is the direct source of new alleles?

A
Fertilisation
B
Meiosis
C
Mutation
D
Random mating only
UnderstandBand 3

2. What is the main contribution of fertilisation to genetic variation?

A
It changes DNA sequence to create new alleles.
B
It combines gametes randomly to create new allele combinations.
C
It duplicates chromosomes before meiosis.
D
It causes crossing over between homologous chromosomes.
ApplyBand 4

3. Two siblings differ genetically because each parent produced different gametes and different gametes fused at fertilisation. This best shows the role of

A
meiosis and fertilisation reshuffling existing alleles.
B
purposeful mutation caused by the environment.
C
chromosomal deletion only.
D
gene flow between populations.
AnalyseBand 4

4. Which statement correctly compares mutation with meiosis?

A
Both usually create new alleles by changing DNA sequence.
B
Meiosis creates all new alleles, while mutation combines them.
C
Mutation is unimportant if meiosis occurs.
D
Mutation creates new alleles, while meiosis reshuffles existing alleles into different gametes.
EvaluateBand 5

5. Which statement is the best evaluation of the role of mutation in variation?

A
Mutation is unnecessary because meiosis and fertilisation already create all possible variation.
B
Mutation matters only to one individual and never to populations.
C
Mutation is essential because it introduces new alleles, even though meiosis and fertilisation generate much of the visible variation between siblings.
D
Mutation has the same function as fertilisation.

Short Answer

UnderstandBand 3

6. Explain how meiosis contributes to genetic variation. 3 marks

AnalyseBand 4

7. Compare the roles of mutation, meiosis and fertilisation in producing genetic variation. 4 marks

EvaluateBand 5

8. Evaluate the claim: "Meiosis and fertilisation are enough to explain all genetic variation, so mutation is not very important." 5 marks

Rapid Review

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.

Revisit Your Thinking

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.

Answers and Explanations

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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.

Multiple Choice

1. C - Mutation is the direct source of new alleles.

2. B - Fertilisation combines gametes randomly to create new allele combinations.

3. A - This describes variation from meiosis and fertilisation, not necessarily new mutation.

4. D - Mutation creates new alleles, while meiosis reshuffles existing ones.

5. C - Meiosis and fertilisation generate major variation, but mutation remains essential for new alleles.

Short Answer Model Responses

Q6 (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].

Q7 (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].

Q8 (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].

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