Sources of Genetic Variation — Meiosis, Crossing Over, Fertilisation, Mutation
Variation in offspring is not caused by a single process. Some variation comes from reshuffling existing alleles during meiosis and fertilisation, while mutation creates entirely new alleles.
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Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Two siblings have the same parents, but they are not genetically identical. A student says, "That must mean one of them got mutations everywhere across the genome. Otherwise siblings should be genetically the same."
Before reading on, explain why that reasoning is weak. How can meiosis and fertilisation create genetic differences between siblings even without new mutations in every gene?
Know
- Major sources of genotype variation in sexually reproducing organisms.
- That mutation is the source of new alleles.
Understand
- Why crossing over and independent assortment reshuffle existing alleles.
- How random fertilisation increases possible genotype combinations.
Can Do
- Distinguish new allele combinations from genuinely new alleles.
- Explain sibling similarity without identity using meiosis and fertilisation.
Core Content
Core idea · combinations vs new alleles
Not all genetic variation is produced in the same way. Some processes rearrange existing alleles, while mutation creates new alleles.
In this lesson, four major sources of variation matter: crossing over, independent assortment, random fertilisation and mutation. The first three mainly create new combinations of already existing alleles. Mutation is different because it can create a genuinely new allele by changing the DNA sequence itself.
What to write in your book
- Four sources: crossing over, independent assortment, random fertilisation, mutation.
- First three → new COMBINATIONS of existing alleles.
- Mutation → genuinely NEW alleles (DNA sequence change).
- Crossing over does not create new alleles.
Mutation is the source of genuinely _____ alleles.
Meiosis · two reshuffling mechanisms
During meiosis, homologous chromosomes pair and can exchange segments by crossing over. This produces chromosomes containing new combinations of existing maternal and paternal alleles.
Independent assortment also contributes to variation. Homologous chromosome pairs line up randomly, so each gamete receives a different mix of maternal and paternal chromosomes. Together, crossing over and independent assortment mean gametes produced by the same individual are usually not genetically identical.
Crossing Over
- Occurs between homologous chromosomes
- Exchanges chromosome segments
- Creates new allele combinations
Independent Assortment
- Occurs when homologous pairs separate
- Randomly distributes whole chromosomes
- Changes chromosome combinations in gametes
What to write in your book
- Crossing over: homologous chromosomes exchange segments → new allele combinations.
- Independent assortment: random orientation of pairs → different chromosome mixes.
- Both occur in meiosis.
- Result: gametes from the same individual are usually genetically different.
Which process exchanges corresponding segments between homologous chromosomes?
Gamete fusion · many possible zygotes
Even after meiosis has produced varied gametes, another source of variation comes from random fertilisation. Any one gamete from one parent may fuse with any one gamete from the other parent, producing many possible zygote genotype combinations.
This is why siblings can be genetically similar but not identical. They inherit alleles from the same parents, but the specific gamete combinations that formed each sibling are different.
What to write in your book
- Random fertilisation: any gamete may fuse with any other → many possible zygotes.
- Adds variation on top of varied gametes from meiosis.
- Explains why siblings are similar but not identical.
- Identical siblings need a special developmental circumstance.
Random fertilisation can only ever combine genetically identical gametes.
Crossing over during prophase I of meiosis creates new combinations of alleles on chromatids.
Fertilisation doubles the genetic variation because it creates new allele combinations from two identical parents.
New alleles · the precise distinction
Mutation is different from crossing over, independent assortment and fertilisation because it can create a genuinely new allele by changing the DNA sequence. This means mutation is the source of new alleles, not just new combinations of existing ones.
In contrast, meiosis and fertilisation mostly reshuffle alleles that already exist in the population. This distinction matters because many exam errors come from calling every source of variation a "mutation" or saying that crossing over creates new alleles.
What to write in your book
- Mutation = a change in DNA sequence → can create a NEW allele.
- Meiosis + fertilisation reshuffle EXISTING alleles.
- Precise wording: mutation → new alleles; the others → new combinations.
- Don't call every variation source a "mutation".
Which process creates a genuinely new allele by changing the DNA sequence?
Model · three reshuffle, one creates
Three major sources reshuffle existing variation; mutation introduces new alleles.
What to write in your book
- Crossing over → recombinant chromosomes with new allele combinations.
- Independent assortment → chromosomes into many possible gamete combinations.
- Random fertilisation → many possible zygote genotypes.
- Mutation → new alleles by altering DNA sequence.
Activities
Sort the Source
For each example, identify whether it is best explained by crossing over, independent assortment, random fertilisation or mutation.
| Item | Answer | Justification |
|---|---|---|
| A gamete receives a different mix of maternal and paternal chromosomes from another gamete made by the same parent. | ||
| A new DNA sequence change creates a new allele. | ||
| Homologous chromosomes exchange corresponding segments. | ||
| One sperm fuses with one egg out of many possible gamete combinations. |
Sibling Similarity Without Identity
Explain why ordinary siblings can resemble each other genetically but are usually not genetically identical. Use meiosis and fertilisation in your answer.
Core idea
- Genetic variation arises from reshuffling existing alleles and from mutation creating new alleles.
Mechanism / process
- Crossing over, independent assortment and random fertilisation produce new allele combinations, while mutation creates new alleles.
Common mistake
- Do not say crossing over creates new alleles. That is the role of mutation.
Exam sentence starter
- "Genetic variation is produced when meiosis and fertilisation reshuffle existing alleles, while mutation..."
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UnderstandBand 3(3 marks) 1. Distinguish between crossing over and mutation as sources of variation.
AnalyseBand 4(4 marks) 2. Explain how meiosis and fertilisation together generate genotype variation in offspring.
EvaluateBand 5–6(5 marks) 3. Evaluate the statement: "Sibling variation can be explained mainly by reshuffling existing alleles, while mutation is important because it introduces new alleles."
Show all answers
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 — Sort the Source
1. Independent assortment.
2. Mutation.
3. Crossing over.
4. Random fertilisation.
Activity 2 — Sibling Similarity Without Identity
Siblings inherit alleles from the same parents, so they are genetically similar. However meiosis produces genetically varied gametes through crossing over and independent assortment, and fertilisation combines these gametes randomly. This means siblings are usually not genetically identical.
Short Answer Model Responses
Q1 (3 marks): Crossing over exchanges segments between homologous chromosomes during meiosis and creates new combinations of existing alleles [1]. Mutation is a change in DNA sequence [1]. Mutation can create a new allele, whereas crossing over mainly reshuffles alleles that already exist [1].
Q2 (4 marks): Meiosis generates genotype variation because crossing over creates new allele combinations and independent assortment distributes chromosomes randomly into gametes [1]. This means gametes produced by the same parent are usually genetically different [1]. Fertilisation then adds variation because any one gamete from one parent may fuse with many possible gametes from the other parent [1]. Together, meiosis and fertilisation produce many possible offspring genotypes [1].
Q3 (5 marks): The statement is valid because most variation between ordinary siblings is explained by reshuffling of existing alleles rather than new mutation in every gene [1]. Crossing over and independent assortment during meiosis produce genetically varied gametes [1]. Random fertilisation then combines these gametes in many possible ways [1]. Mutation remains important because it introduces new alleles by changing DNA sequence [1]. Therefore sibling variation is largely explained by reshuffling, while mutation is the source of genuinely new alleles in a population [1].
Crossing over
Reshuffles existing alleles by exchanging chromosome segments.
Independent assortment
Randomly distributes chromosomes into gametes.
Random fertilisation
Combines different gametes into many possible zygotes.
Mutation
Creates new alleles by changing DNA sequence.
Rapid-fire questions on crossing over, independent assortment, random fertilisation and mutation. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
You should now be able to reject the idea that sibling differences require mutation everywhere. Most sibling variation is explained by meiosis reshuffling existing alleles and fertilisation combining gametes randomly, while mutation introduces genuinely new alleles.