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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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?
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.
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.
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.
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.
Produces recombinant chromosomes with new allele combinations.
Distributes chromosomes into many possible gamete combinations.
Combines different gametes into many possible zygote genotypes.
Creates new alleles by altering DNA sequence.
Genetic variation arises from reshuffling existing alleles and from mutation creating new alleles.
Crossing over, independent assortment and random fertilisation produce new allele combinations, while mutation creates new alleles.
Do not say crossing over creates new alleles. That is the role of mutation.
Genetic variation is produced when meiosis and fertilisation reshuffle existing alleles, while mutation...
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
For each example, identify whether it is best explained by crossing over, independent assortment, random fertilisation or mutation.
1. A gamete receives a different mix of maternal and paternal chromosomes from another gamete made by the same parent.
2. A new DNA sequence change creates a new allele.
3. Homologous chromosomes exchange corresponding segments.
4. One sperm fuses with one egg out of many possible gamete combinations.
Explain why ordinary siblings can resemble each other genetically but are usually not genetically identical. Use meiosis and fertilisation in your answer.
1. Which process is the source of genuinely new alleles?
2. What is the main effect of crossing over?
3. Which event best illustrates independent assortment?
4. Why are ordinary siblings usually not genetically identical?
5. Which statement best distinguishes reshuffling from mutation?
6. Distinguish between crossing over and mutation as sources of variation. 3 marks
7. Explain how meiosis and fertilisation together generate genotype variation in offspring. 4 marks
8. Evaluate the statement: "Sibling variation can be explained mainly by reshuffling existing alleles, while mutation is important because it introduces new alleles." 5 marks
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.
1. Independent assortment.
2. Mutation.
3. Crossing over.
4. Random fertilisation.
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.
1. C - Mutation is the source of genuinely new alleles.
2. A - Crossing over creates new combinations of existing alleles.
3. D - Independent assortment is the random separation of homologous chromosome pairs.
4. B - Meiosis plus random fertilisation explains most sibling genetic differences.
5. C - Reshuffling and mutation contribute to variation in different ways.
Q6 (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].
Q7 (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].
Q8 (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].
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