Explain the role of meiosis in maintaining the chromosome number of a species during sexual reproduction.

IB DP Biology Standard Level (2023 syllabus) — D3.1 Reproduction · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Meiosis reduces the chromosome number by half, producing haploid gametes from diploid parent cells.
Fertilisation fuses two haploid gametes, restoring the diploid chromosome number in the zygote.
Without meiosis, fertilisation would double the chromosome number each generation, which would not be sustainable.
Thus meiosis and fertilisation alternate in the sexual life cycle, keeping the chromosome number constant across generations.

Examiner tips

  • Use the word ‘halves’ to show understanding of chromosome reduction. Mention both gamete formation and fertilisation. Show the cycle of reduction and restoration. Keep the answer concise and to the point.

Common mistakes

  • Confusing meiosis with mitosis. Forgetting to mention fertilisation. Using vague terms like ‘reduction’ without explaining it halves the number.

Mark scheme (4 marks)

  1. Meiosis halves the chromosome number, producing haploid gametes (from diploid parent cells).
  2. Fertilisation (fusion of two gametes) restores the diploid chromosome number in the zygote.
  3. Without meiosis, fertilisation would double the chromosome number each generation, which would not be sustainable.
  4. Meiosis and fertilisation therefore alternate in the sexual life cycle, ensuring the chromosome number remains constant across generations.

Key terms in this question

meiosis · chromosome number

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