Explain how sexual reproduction in animals results in offspring that are genetically different from their parents.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 2.3 Sexual reproduction in animals and plants · Explain · 5 marks · View as Markdown

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

Sexual reproduction in animals involves the production of gametes by meiosis and their fusion during fertilisation.

Model answer (5 marks)

1. Gametes are haploid, each containing one copy of every chromosome.
2. During meiosis, homologous chromosomes separate and cross‑over occurs, producing gametes with different chromosome combinations.
3. Fertilisation fuses a male and a female gamete, forming a diploid zygote.
4. The zygote restores the diploid chromosome number, with one set from each parent.
5. The offspring inherit a unique mix of parental chromosomes and genes, giving genetic diversity.

Examiner tips

  • Use the exact terms: haploid, meiosis, fertilisation, diploid, genetic variation.
  • Show the sequence: gamete → meiosis → fertilisation → zygote.
  • Mention cross‑over as a source of variation.
  • Keep each point brief and directly linked to the mark scheme.

Mark scheme (5 marks)

  1. Gametes are haploid / contain a single copy of each chromosome
  2. Meiosis produces gametes with different combinations of chromosomes / genetic variation is introduced during meiosis
  3. Fertilisation is the fusion of a male and female gamete
  4. Fertilisation restores the diploid number / the fertilised egg contains two copies of each chromosome
  5. The offspring inherit a unique combination of chromosomes / genes from both parents, resulting in genetic variation

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