Explain how multiple alleles and codominance together can produce more phenotypes in a population than would be possible with a single gene having only two alleles showing simple dominance.

IB DP Biology Higher 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).

The ABO blood group system in humans is controlled by a single gene (the I gene) that exists as three alleles: I^A, I^B, and i. I^A and I^B are codominant to each other, and both are dominant over i.

Model answer (4 marks)

Multiple alleles mean that more than two allelic forms of the I gene are present in the population, so there are more possible genotypic combinations.

With simple dominance and only two alleles, a heterozygote shows the same phenotype as the dominant homozygote, giving only two phenotypes.

Codominance allows both alleles in a heterozygote to be expressed simultaneously, giving a distinct phenotype that is not seen in either homozygote.

Thus, the combination of three alleles and codominance produces four phenotypes (A, B, AB, O) – more than the two phenotypes that could arise from a single gene with only two alleles and simple dominance.

Examiner tips

  • Use the exact terms ‘multiple alleles’, ‘codominance’, ‘phenotype’, ‘homozygote’, ‘heterozygote’
  • Show the logical progression: more alleles → more genotypes → codominance → extra phenotype

Common mistakes

  • Confusing codominance with incomplete dominance
  • Saying ‘three alleles give three phenotypes’ instead of four

Mark scheme (4 marks)

  1. Multiple alleles means more than two allelic forms of a gene exist in the population, increasing the number of possible genotypic combinations.
  2. With simple dominance and only two alleles, only two phenotypes are possible, because heterozygotes express the same phenotype as the dominant homozygote.
  3. Codominance means that when both codominant alleles are present in a heterozygote, both are expressed simultaneously, producing a distinct/additional phenotype not seen in either homozygote.
  4. The combination of multiple alleles and codominance therefore produces four phenotypes (A, B, AB, O) from three alleles, which is more than the maximum of two phenotypes possible with two alleles and simple dominance.

Key terms in this question

multiple alleles · codominance · simple dominance · phenotype

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