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Lesson 9 of 9 / Mutation, cell division and cancer

Halve the sets and vary the combinations

Why does sexual reproduction need a reduction division?

I can explain meiosis in chromosome-number maintenance and genetic variation without detailed stage descriptions.

  • I can explain meiosis, random fertilisation, chromosome number and variation at H1 depth.Syllabus

    Syllabus 8876, Core 2(k). Explain meiosis, random fertilisation, chromosome number and variation at H1 depth.

About 5 min

Make a guess. You are not marked.

Does meiosis create brand-new alleles?

  1. Each gamete gets new alleles.
  2. It does, during crossing over.
  3. It shuffles existing alleles.
Show the answer

It shuffles existing alleles.

Crossing over and independent assortment make new combinations. New alleles come from mutation.

The key idea

Meiosis 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

Diploid cell: 2nHaploid, varied combinations

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.

Two events in meiosis I produce most of this variation, and multiple-choice questions use their names. In prophase I, homologous chromosomes pair up and non-sister chromatids exchange matching segments at points called chiasmata (singular: chiasma). This is crossing over. It produces chromatids with new combinations of alleles. In metaphase I, each homologous pair lines up at the equator independently of every other pair. This is independent assortment. With n pairs there are 2^n possible combinations of chromosomes in a gamete; for 23 pairs in humans that is 2^23 = 8,388,608, before crossing over adds more.

Crossing over happens between non-sister chromatids, one from each homologue. Sister chromatids are identical copies, so an exchange between them would not change any combination of alleles. H1 does not require the other stages of meiosis.

Step by step
  1. 1

    Reduce before combining

    Meiosis produces n cells; fertilisation combines n + n to restore 2n.

  2. 2

    Generate combinations

    Meiotic processes produce varied allele combinations in gametes.

  3. 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.

Watch out for this

A student says: "Meiosis creates all new alleles because every gamete differs." What is wrong with this?

Show the answer

Meiosis mainly rearranges existing alleles into new combinations. Mutations generate new sequence variants.

Check your understanding

Which comparison is correct?

  1. Meiosis reduces chromosome number but random fertilisation removes the resulting variation.
  2. Mitosis always halves chromosome number, while meiosis preserves it.
  3. Meiosis reduces chromosome sets and generates varied combinations.
  4. Crossing over between sister chromatids in prophase II is the main source of variation.
Show the answer

Meiosis reduces chromosome sets and generates varied combinations.

These are its key significance for sexual reproduction.

Exam-style questions

Check your understanding

Which processes produce genetic variation among offspring? 1: Crossing over in meiosis I. 2: Independent assortment in meiosis I. 3: Mitosis in the zygote. 4: Random fertilisation.

  1. 1, 2 and 4 only
  2. 1, 2 and 3 only
  3. 1, 2, 3 and 4
  4. 1 and 2 only
Show the answer

1, 2 and 4 only

Crossing over, independent assortment and random fertilisation all create new combinations. Mitosis copies the genome exactly.

Quick recall

Card 1 of 3. Answer in your head, then check.

Why does sexual reproduction need a reduction division?

You can return to this lesson any time.