Explain how multiple alleles at a single gene locus can produce more than two phenotypes in a population, using the ABO blood group system as an example.

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)

A gene can have more than two allelic forms in a population, but each individual carries only two alleles. In the ABO blood group system there are three alleles: I^A, I^B and i. I^A and I^B are both dominant over i and codominant with each other. Therefore the six possible genotypes (I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B and ii) give four phenotypes: A, B, AB and O. The extra phenotypes arise because the different combinations of the three alleles produce distinct antigen patterns that would not occur if only two alleles existed.

Examiner tips

  • Mention the key point that each individual has two alleles but the population can have more than two.
  • State the three alleles and their dominance relationships.
  • Show the six genotypes and link them to the four phenotypes.
  • Use the correct notation (I^A, I^B, i) and UK spelling.

Common mistakes

  • Confusing codominance with dominance; writing I^A and I^B as recessive.
  • Omitting the genotype–phenotype table or the fact that each individual has only two alleles.
  • Using incorrect allele symbols or misspelling "codominant".

Mark scheme (4 marks)

  1. A gene can exist in more than two allelic forms (multiple alleles) in a population, even though each individual carries only two alleles.
  2. In the ABO system there are three alleles: I^A, I^B, and i (or equivalent correct notation), controlling blood group.
  3. I^A and I^B are both dominant over i and codominant with each other, so different combinations of two alleles from three possible alleles produce four distinct phenotypes: A, B, AB, and O.
  4. The greater number of phenotypes arises because different combinations (genotypes) of the three alleles — such as I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B, and ii — produce distinct phenotypic outcomes that would not be possible with only two alleles.

Key terms in this question

multiple alleles · ABO blood group · locus · phenotype

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