A-Level Biology May/June 2025 Q4(a): In most species of plants and animals, the cell that is formed as a result of fertilisa…

1 answer

• ▲0
  accepted ✓

  Fertilisation is the fusion of two haploid gametes, one from each parent, to form a zygote.

  1. Diploid Nature: Each haploid gamete (e.g., sperm and egg) contains one set of chromosomes (n). When they fuse during fertilisation, the resulting zygote contains two complete sets of chromosomes ($2n$), one set from each gamete. This is why the cell is diploid.

  2. Homologous Chromosomes: Since one set of chromosomes is contributed by the maternal parent and the other set by the paternal parent, the zygote contains chromosomes from both. A homologous pair of chromosomes consists of one maternal and one paternal chromosome. These chromosomes are described as homologous because they are the same size and length, have the same centromere position, and carry the same genes at the same corresponding loci.

  ---

  How the marks are awarded

  • MP1 — The answer correctly states that the diploid cell is formed from the fusion of two haploid gametes.
  • MP2 — This mark is for explaining that the diploid cell contains two sets of chromosomes, with one set originating from each gamete.
  • MP3 — The identification of homologous chromosomes as being the maternal and paternal chromosomes in a pair earns this mark. The model answer also includes details for MP4, which would also be credited.

  Common mistakes

  • Confusing genes and alleles; stating that homologous chromosomes have the same alleles is incorrect, as alleles can differ between the maternal and paternal chromosome.
  • Providing a vague definition of diploid, such as 'has two chromosomes' instead of the correct 'has two sets of chromosomes'.
  • Failing to answer both parts of the question; for example, explaining why the cell is diploid but not explaining why it contains homologous chromosomes.
  • Using circular reasoning, e.g., 'The cell is diploid because it contains homologous chromosomes', which does not explain the origin of the diploid state.

  Examiner tip: Always break down 'explain' questions into their component parts and define any key biological terms precisely.

  ---

  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

  @markscheme_answers

Your answer

Sign in to answer this question.