Explain how codominance differs from complete dominance, using an example from genetics to support your answer.

IB DP Biology Standard Level (2023 syllabus) — D3.2 Inheritance · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

In complete dominance one allele completely masks the other, so a heterozygote (Aa) shows the same phenotype as the homozygous dominant (AA). In codominance neither allele is masked; both are expressed in the heterozygote, giving a phenotype that is a blend of the two homozygotes. A classic example is the human ABO blood system: the I^A and I^B alleles are codominant, so an I^A I^B individual has both A and B antigens on their red cells (blood group AB). In contrast, the I^A allele is dominant over the i allele, so I^A i gives blood group A.

Key points: complete dominance – one allele masks the other; codominance – both alleles expressed; example – ABO blood groups; notation – codominant alleles use two uppercase letters, dominant/recessive use uppercase/lowercase.

Examiner tips

  • Use the word ‘mask’ for complete dominance and ‘expressed simultaneously’ for codominance.
  • Give a clear example (AB blood group) and explain the phenotype of the heterozygote.
  • Show the allele notation difference (I^A I^B vs I^A i).

Common mistakes

  • Confusing codominance with incomplete dominance; describing a blended phenotype rather than both alleles expressed.
  • Using the wrong notation (e.g., I^A i for codominance).
  • Failing to explain that the heterozygote phenotype differs from both homozygotes.

Mark scheme (4 marks)

  1. In complete dominance, one allele masks the expression of the other allele entirely, so the heterozygote has the same phenotype as the dominant homozygote.
  2. In codominance, both alleles are expressed simultaneously in the heterozygote, producing a phenotype distinct from either homozygote.
  3. A valid example of codominance is given, such as ABO blood groups where the I^A and I^B alleles are codominant, producing blood group AB in a heterozygote (I^A I^B) with both A and B antigens present on red blood cells.
  4. Codominant alleles are represented by two uppercase letters (or superscripts), reflecting that neither allele is dominant over the other, whereas in complete dominance uppercase (dominant) and lowercase (recessive) letters are used.

Key terms in this question

codominance · complete dominance

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