A pea plant produces both male gametes and female gametes. Explain how fertilisation in this flowering plant differs from fertilisation in a mammal, and why the offspring produced by each organism are genetically different from both 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).

Flowering plants and mammals both reproduce sexually, but the process of fertilisation differs between these groups. In both cases, gametes are haploid cells that are formed by meiosis.

Model answer (5 marks)

In a pea plant fertilisation is external to the plant’s body – the pollen grain lands on the stigma and a pollen tube grows through the style to the ovule, where the male gamete fuses with the female gamete inside the ovary. In a mammal fertilisation is internal; sperm are deposited in the female reproductive tract and swim to the egg in the fallopian tube.

In both organisms the gametes are haploid, produced by meiosis. At fertilisation the haploid male gamete fuses with the haploid female gamete to form a diploid zygote, restoring the full chromosome number.

The offspring are genetically different from both parents because meiosis generates new combinations of alleles through independent assortment and crossing‑over, so each gamete carries a unique set of genes; fertilisation then combines two distinct sets of genetic information.

Examiner tips

  • Use the word ‘external’ vs ‘internal’ to show the key difference. Include the pollen tube route for plants. Mention meiosis and haploid gametes. Explain genetic variation via independent assortment/crossing‑over. Keep answer concise and use exam‑specific terminology.

Common mistakes

  • Confusing the location of fertilisation (e.g. saying it is inside the ovary for plants but not specifying external to the plant body).
  • Forgetting to state that gametes are haploid or that meiosis produces genetic variation.

Mark scheme (5 marks)

  1. In the pea plant, fertilisation is external to the parent body / occurs inside the ovule/ovary of the flower, whereas in a mammal fertilisation is internal (inside the female's body/reproductive tract).
  2. In the plant, the male gamete (in pollen) travels to the female gamete (in the ovule) via a pollen tube, whereas in the mammal sperm swim to the egg / travel through the female reproductive tract.
  3. Gametes are haploid / contain half the normal number of chromosomes, formed by meiosis.
  4. At fertilisation, a male gamete fuses with a female gamete to form a diploid zygote / restoring the full chromosome number.
  5. Offspring are genetically different from both parents because meiosis produces genetic variation (through independent assortment / crossing over), so each gamete has a unique combination of chromosomes/alleles, and fertilisation combines two different sets of genetic information.

Key terms in this question

fertilisation · gametes

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