Explain why brothers and sisters from the same two parents can look different from each other.

WJEC GCSE Biology (Wales) — 2.3.2 Inheritance · Explain · 4 marks · View as Markdown

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

Humans inherit characteristics from both of their parents. Although siblings share the same mother and father, they are not identical to each other (unless they are identical twins).

Model answer (4 marks)

Each parent carries two alleles for every gene. During fertilisation only one allele from each parent is transmitted to the zygote. The allele chosen is random, so the combination of alleles that a child receives is different from that of a sibling. Consequently each child inherits a unique set of alleles, giving rise to different phenotypes.

Examiner tips

  • State that each parent has two alleles per gene.
  • Explain that only one allele from each parent is passed on.
  • Mention that the allele chosen is random and that different combinations occur.
  • Show how this leads to different characteristics in siblings.

Common mistakes

  • Forgetting to mention that each parent has two alleles.
  • Confusing the random choice with a non‑random process.
  • Not linking the different allele combinations to the resulting phenotypic differences.

Mark scheme (4 marks)

  1. Each parent has two alleles for each gene / characteristic
  2. Only one allele from each parent is passed on to the offspring (during reproduction)
  3. Which allele is passed on is random / different combinations of alleles are possible
  4. So each child receives a different combination of alleles, leading to different characteristics / phenotypes

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