Sources of Genetic Variation, Meiosis, Crossing Over, Fertilisation, Mutation
Between 1977 and 2012, Peter and Rosemary Grant measured beak depth in the medium ground finch Geospiza fortis on Daphne Major Island in the Galápagos. They calculated a heritability of 0.79, meaning 79% of the variation in beak depth between individuals was heritable. Their long-term study documented how crossing over, independent assortment, and random fertilisation continuously reshuffle existing alleles to generate the heritable variation on which natural selection acts. Grant and Grant's work remains one of the most rigorous direct observations of the sources of genetic variation in a wild population.
Practise this lesson
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
When Peter and Rosemary Grant weighed and measured beak depths across hundreds of individual Geospiza fortis finches on Daphne Major between 1977 and 2012, they found continuous variation, not discrete categories. Some birds had deep beaks, some shallow, most somewhere between. Most of that variation was heritable (h² = 0.79), yet it had not all arisen from recent mutations. The variation existed because meiosis, through crossing over and independent assortment, continuously creates new combinations of the existing alleles in the population. Understanding where that diversity comes from requires examining the three major sources of allele combination change.
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.
Four sources of genetic variation: crossing over, independent assortment, random fertilisation (first three → new COMBINATIONS of existing alleles) and mutation (→ genuinely NEW alleles by changing DNA sequence). Crossing over does NOT create new alleles.
Pause, copy the highlighted four-source summary into your book, noting which creates new alleles vs new combinations.
Mutation is the source of genuinely _____ alleles.
Meiosis · two reshuffling mechanisms
We just saw that there are four sources of variation, with mutation being the only one that creates genuinely new alleles. That raises a question: how exactly do crossing over and independent assortment reshuffle alleles during meiosis? This card answers it → crossing over exchanges chromosome segments; independent assortment randomly distributes whole chromosomes to gametes.
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
Crossing over: homologous chromosomes exchange segments during meiosis → new allele combinations on chromosomes. Independent assortment: random orientation of homologous pairs → different chromosome mixes in each gamete. Both produce gametes from the same individual that are usually genetically different.
Add the highlighted crossing-over and independent-assortment definitions to your notes.
Which process exchanges corresponding segments between homologous chromosomes?
Gamete fusion · many possible zygotes
We just saw that crossing over and independent assortment create varied gametes through meiosis. That raises a question: does variation end at gamete formation, or does the fertilisation step add even more diversity? This card answers it → random fertilisation multiplies the combinations further because any gamete from one parent can fuse with any gamete from the other.
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.
Random fertilisation: any gamete from one parent may fuse with any gamete from the other → many possible zygote genotypes. This adds further variation on top of meiotic shuffling. It explains why siblings are similar but not identical (identical twins require a special developmental event).
Pause, write the highlighted random fertilisation point into your book.
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
We just saw that random fertilisation adds variation by combining already-varied gametes. That raises a question: where do genuinely new alleles, not just new combinations, come from? This card answers it → mutation changes the actual DNA sequence, creating an allele that did not exist before.
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.
Mutation = a change in DNA sequence → can create a genuinely NEW allele. Meiosis and fertilisation only reshuffle existing alleles. Precise wording: mutation → new alleles; crossing over/independent assortment/fertilisation → new combinations. Do not call every variation source a "mutation".
Add the highlighted mutation definition and the new-alleles vs new-combinations distinction to your notes.
Which process creates a genuinely new allele by changing the DNA sequence?
Model · three reshuffle, one creates
We just saw that mutation is the only source that creates new alleles; the other three sources reshuffle existing ones. That raises a question: how can we visualise all four sources together in one clear diagram? This card answers it → a concept map with four branches from "Genetic Variation", distinguishing reshuffle vs new-allele sources.
Concept map: Genetic Variation branches into crossing over (recombinant chromosomes), independent assortment (chromosome mixes), random fertilisation (zygote combinations), all reshuffle existing alleles, and mutation (changes DNA sequence → new alleles).
Pause, reproduce the concept map structure in your book from memory.
Three major sources reshuffle existing variation; mutation introduces new alleles.
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..."
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. 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).
Grant and Grant's 1977–2012 Daphne Major study, calculating heritability of beak depth at 0.79 in Geospiza fortis, demonstrates that most heritable variation in a population comes from three non-mutational processes: crossing over (new allele combinations on individual chromosomes), independent assortment (random orientation of 23 homologous pairs producing up to 2²³ gamete combinations), and random fertilisation (any sperm combining with any egg). Together these three processes can produce theoretically trillions of distinct genotype combinations from the alleles already present in a population, without any new mutation. Mutation remains important as the ultimate source of new alleles, but most of the variation visible between siblings or between individuals in a wild population results from reshuffling, not from new sequence change.