A flowering plant reproduces sexually. Explain why the seeds produced by sexual reproduction in this plant are not genetically identical to each other or to the parent plant.

Pearson Edexcel A-Level Biology A (9BI0) — 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).

A gardener collects seeds from a single flowering plant and grows them the following year. She notices that the new plants show a range of different characteristics, even though they all came from the same parent plant.

Model answer (5 marks)

Sexual reproduction involves the fusion of two haploid gametes during fertilisation. The gametes are produced by meiosis, so each contains only one copy of every chromosome and therefore a different combination of alleles. In this plant the pollen grain (male gamete) and the egg cell (female gamete) are produced by two different parent organisms – or, if self‑fertilisation occurs, by two gametes that have undergone independent assortment and crossing‑over. When the two gametes fuse, the resulting zygote contains a unique combination of chromosomes and alleles from both parents. Consequently each fertilisation event produces a genetically distinct zygote, so the seeds that develop are not genetically identical to each other or to the parent plant.

Examiner tips

  • Mention both meiosis and fertilisation as sources of variation; include ‘haploid’ and ‘different parents’ to hit all points.
  • Show the chain: meiosis → haploid gametes → different parents → fertilisation → unique zygote → variation.
  • Use the exact wording ‘genetic variation’ and ‘not genetically identical’ to match the mark scheme.

Common mistakes

  • Forgetting to state that gametes are haploid or that they come from different parents.
  • Confusing mitosis with meiosis – claiming the gametes are produced by mitosis.
  • Failing to mention that each fertilisation event gives a different combination, so seeds differ from each other and the parent.

Mark scheme (5 marks)

  1. Sexual reproduction involves the fusion of two gametes (fertilisation)
  2. Gametes are haploid / contain only one copy of each chromosome
  3. Gametes are produced from two different parent organisms (pollen from one plant, egg cell from another / or self-fertilisation combines differently formed gametes), so each gamete carries different genetic information / alleles
  4. The zygote formed contains a new / unique combination of chromosomes / alleles from both parents
  5. Because each fertilisation event produces a different combination, the seeds / offspring show genetic variation / are not genetically identical to each other or the parent

Key terms in this question

sexual reproduction

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