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3.3.6 Inheritance of ABO blood groups

Codominance

Definition

Codominance

Where two different alleles of a gene are both fully expressed in the phenotype of a heterozygous organism.

  1. In codominance, neither allele is dominant over the other, so both show in the phenotype.
  2. A heterozygote then shows both features at once, not just one.
  3. Codominant alleles are written as capital letters with a raised label, rather than a capital and a lower-case.
Example

Worked cross: group A and group B parents

  • Parent genotypes: IAi×IBiI^A i \times I^B iIAi×IBi. The alleles IAI^AIA and IBI^BIB are codominant, while iii is recessive.
  • Gametes: the group A parent produces IAI^AIA or iii gametes; the group B parent produces IBI^BIB or iii gametes.
  • Offspring genotypes: IAIBI^A I^BIAIB, IAiI^A iIAi, IBiI^B iIBi and iiiiii.
  • Phenotypes: group AB, group A, group B and group O.
  • Each blood group has a probability of 14=25%\dfrac{1}{4}=25\%41​=25% for each child.
Note

Codominance is not a blend: both alleles are fully shown at the same time.

ABO alleles

Definition

Codominance

Where two different alleles of a gene are both fully expressed in the phenotype of a heterozygous organism.

  1. The ABO blood group is controlled by a gene with three alleles: IAI^AIA, IBI^BIB and iii.
  2. IAI^AIA and IBI^BIB are codominant, so both show if they are present together.
  3. Both IAI^AIA and IBI^BIB are dominant over iii.
  4. Each person carries only two of the three alleles.
Hint

Only IAI^AIA and IBI^BIB are codominant; iii is recessive to both.

ABO genotypes and phenotypes

Definition

Allele

A different version of a gene.

  1. Genotypes IAIAI^A I^AIAIA and IAiI^A iIAi give group A.
  2. Genotypes IBIBI^B I^BIBIB and IBiI^B iIBi give group B.
  3. Genotype IAIBI^A I^BIAIB gives group AB, because both alleles are shown.
  4. Genotype iiiiii gives group O.

A table showing the four ABO blood groups (A, B, AB, and O), the antigens present on the red blood cells for each group, and the possible genotypes (IAIA, IAi, IBIB, IBi, IAIB, and ii).

Example

A group A parent (IAiI^A iIAi) and a group B parent (IBiI^B iIBi) can have a child of any of the four groups.

Common Mistake

Do not forget that group O is the homozygous recessive genotype iiiiii.

Self review
  • What are the three ABO alleles?
  • Which two alleles are codominant?
  • What genotype gives group O?
  • What genotype gives group AB?
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Table showing blood groups A, B, AB and O, the antigens on their red blood cells, and their possible genotypes

The ABO blood group gene has three alleles: IAI^AIA, IBI^BIB and iii. Although three alleles exist in the population, each person inherits only two, one from each parent.

The alleles IAI^AIA and IBI^BIB produce A and B antigens on red blood cells. Allele iii does not produce either antigen.

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How many alleles control the ABO blood group system?

3.3.6 Inheritance of ABO blood groups Revision Guide

  1. GCSE
  2. /Biology
  3. /3.3.6 Inheritance of ABO blood groups

Revision notes for Edexcel GCSE Biology 3.3.6 Inheritance of ABO blood groups: explanations and worked examples.

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