Codominance and ABO blood groups

Inheritance · Monohybrid inheritance · note 4 of 5

Spec 17.4.14, 17.4.15

Supplement

Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Download all Monohybrid inheritance notes (PDF)Download all notes (PDF)8 pages

Codominance and ABO blood groupsSpec 17.4.14, 17.4.15

In short

Codominance is when both alleles in a heterozygous organism contribute to the phenotype. The ABO blood group is controlled by one gene with three alleles, IA, IB and Io. IA and IB are codominant, so a person with IA IB has blood group AB. Io is recessive to both, so group O needs two Io alleles.

In codominance, both alleles in heterozygous organisms contribute to the phenotype. Neither allele is hidden and the two do not blend.

The ABO blood group is controlled by one gene with three alleles: IA, IB and Io. Exam papers print the A, B and o as small raised letters after the I. A person inherits two alleles, one from each parent. IA and IB are codominant. Io is recessive to both.

ABO blood groups
Phenotype (blood group)	Possible genotypes
A	IA IA or IA Io
B	IB IB or IB Io
AB	IA IB
O	Io Io

Group A mother and group B father

A mother is IA Io and a father is IB Io. Which blood groups could their children have?

Mother's gametes: IA or Io. Father's gametes: IB or Io.
Punnett square boxes: IA IB, IA Io, IB Io, Io Io.

Answer: Group AB, A, B and O, each with probability 1 in 4 (25%), a ratio of 1 : 1 : 1 : 1.

Exam tip:

IA IB gives group AB because both alleles are expressed. Group O needs two Io alleles.

Punnett square for a group A mother (IA Io) and a group B father (IB Io): gametes IA and Io across the top and IB and Io down the side, giving IA IB (group AB), IB Io (group B), IA Io (group A) and Io Io (group O). — IA Io × IB Io can give children of all four blood groups, each with a probability of 1 in 4.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is a genotype and phenotype?

A genotype is the genetic make-up of an organism in terms of the alleles present, such as Tt. A phenotype is the observable features of an organism, such as being tall. A heterozygous Tt pea plant has the tall phenotype, because the dominant allele T is expressed even though a recessive allele is present.

What is a Punnett square used for in genetics?

A Punnett square is used to show the possible offspring of a genetic cross. The gametes of one parent go along the top and the other along the side, and the boxes show the possible offspring genotypes. From these you work out the phenotypes and the ratio, such as 3 : 1 or 1 : 1.

How do you find out if an organism is homozygous or heterozygous?

Supplement You use a test cross: cross the organism showing the dominant phenotype with a homozygous recessive individual. If any offspring show the recessive phenotype, the unknown parent must be heterozygous. If all the offspring show the dominant phenotype, it is probably homozygous dominant. A large number of offspring gives a more reliable result.

All 5 questions on Monohybrid inheritance