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D3.2.9—ABO blood groups

ABO blood groups depend on three alleles, with IA and IB codominant and both dominant over i in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.9
Level
HL

Exam analysis

Chance of appearing9%of analysed past papers
Latest appearanceNovember 2023
Most common paperPaper1
Typical marks1

Common command terms

  • Describe
  • State
  • Identify
  • Outline

Recent exam appearances

November 2023Paper1 ["HL"] · TZ236[ 1 ]D3.2.9—ABO blood groups
May 2023Paper1 ["HL"] · TZ134[ 1 ]D3.2.9—ABO blood groups
November 2022Paper1 ["HL"] · TZ011[ 1 ]D3.2.9—ABO blood groups
May 2021Paper2 ["HL"] · TZ17(a)[ 4 ]D3.2.9—ABO blood groups
November 2020Paper1 ["HL"] · TZ011[ 1 ]D3.2.9—ABO blood groups

Coverage 2013–2023 · Updated 16 Jul 2026

ABO Combines Three Alleles into Four Blood-Group Phenotypes

Phenotype Possible genotype(s) Red-cell antigen(s)
A IᴬIᴬ or Iᴬi A
B IᴮIᴮ or Iᴮi B
AB IᴬIᴮ A and B
O ii neither A nor B

Iᴬ and Iᴮ are codominant because both antigen products appear in IᴬIᴮ. Each is dominant to i, which produces neither A nor B antigen.

Blood group is a phenotype; Iᴬ, Iᴮ and i are alleles. A person with group A or B may hide an i allele, so phenotype does not always identify genotype.

Red blood cells show A antigen for blood group A, B antigen for group B, both antigens for AB and neither antigen for O, alongside the corresponding ABO genotypes.

ABO Crosses Must Resolve the Parents' Hidden Genotypes First

A group-A parent could be IᴬIᴬ or Iᴬi, and a group-B parent could be IᴮIᴮ or Iᴮi. State the genotype assumption before predicting children.

Iᴬi × Iᴮi Iᴮ gamete i gamete
Iᴬ gamete IᴬIᴮ, group AB Iᴬi, group A
i gamete Iᴮi, group B ii, group O

With these heterozygous parents, A, B, AB and O phenotypes each have probability 1/4. If either parent is homozygous, the outcome changes.

ABO data can sometimes exclude a proposed parent–child relationship, but common blood groups cannot uniquely prove parentage because many people share the same phenotype and genotype.

Inheritance Predictions Move from Chromosomes to Alleles to Phenotypes

homologous chromosomes carry two alleles in a diploid parent → meiosis separates one allele into each gamete → fertilization restores a two-allele genotype → allele relationship and environment shape phenotype → offspring data test the prediction

For any genetic cross:

  • define allele symbols and their relationship
  • write parental genotypes, not only phenotypes
  • derive gametes before drawing the grid
  • combine gametes, then translate genotype to phenotype
  • state probabilities and the assumptions behind them
If the simple 3:1 expectation fails First question to ask
heterozygote has a third phenotype incomplete dominance?
both products appear codominance?
phenotype changes with conditions environmental or plastic response?
more than two alleles occur in the population multiple-allele locus such as ABO?

ABO blood groups

[ 9 ]

Describe the inheritance of ABO blood groups.

Concept essentials

  • IA and IB are codominant in blood group AB.
  • The i allele is recessive to both IA and IB.
  • Blood group phenotype may correspond to more than one genotype.