Lesson 9 of 9 / Mutation, cell division and cancer
Halve the sets and vary the combinations
Why does sexual reproduction need a reduction division?
In this lesson: Explain meiosis in chromosome-number maintenance and genetic variation without detailed stage descriptions.
About 5 min
The key ideaMeiosis produces haploid cells with varied genetic combinations. Fertilisation restores the diploid number and adds further variation through random gamete combinations.
Explore the idea
Reduce before fertilisation
This summary shows the significance of reduction and varied combinations, not the detailed stages of meiosis. Mutation supplies new alleles; meiosis rearranges them.
Explanation
A diploid cell has two chromosome sets; a haploid gamete has one. Meiosis reduces the number of sets before fertilisation. When two haploid gametes combine, the diploid number is restored rather than doubling at each generation.
The products of meiosis are not generally genetically identical. Recombination and the assortment of maternal and paternal chromosome contributions generate different combinations of alleles. H1 requires this significance, not a detailed account of every meiotic stage or its membrane and centriole behaviour.
Random fertilisation adds another source of combination: which sperm and egg unite affects the offspring's genotype. Variation among gametes is therefore combined with variation in possible pairings.
Meiosis reshuffles existing alleles; mutation is the ultimate source of new alleles. These processes play different roles. Mitosis, in contrast, normally preserves chromosome number and produces genetically very similar daughter cells for growth and repair.
Step by step
- 1
Reduce before combining
Meiosis produces n cells; fertilisation combines n + n to restore 2n.
- 2
Generate combinations
Meiotic processes produce varied allele combinations in gametes.
- 3
Add random fertilisation
Different gamete pairings create further combinations in offspring.
Worked example
Maintain a species' chromosome number
A species has diploid number 12. State the normal gamete number and the number after fertilisation, and explain why meiosis matters.
One way to explain it
Gametes have 6 chromosomes; fertilisation restores 12. Reduction before fusion prevents the chromosome number doubling in every generation of sexual reproduction.
Why this answer works
- Distinguish one set from two sets.
- Connect reduction and restoration as a cycle.
Is this true? "Meiosis creates all new alleles because every gamete differs."
Meiosis mainly rearranges existing alleles into new combinations. Mutations generate new sequence variants.