Codominance: When Both Alleles Show — Genetics & Inheritance | IGCSE Biology

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Past paper frequency (2018 to 2024)

This topic accounts for approximately 17% of your exam marks.

increasing

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Increasing17%

Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.

In most monohybrid crosses, one allele is dominant and the other is recessive. But sometimes both alleles in a heterozygote show their effects in the phenotype. This is called codominance.

The ABO blood group system

The classic example is the inheritance of human blood groups. The ABO gene has three alleles (not two):

Allele	What it codes for
I^A	Production of antigen A on red blood cells
I^B	Production of antigen B on red blood cells
I^O	No antigen produced (no A and no B)

The rules of dominance are:

I^A and I^B are codominant with each other. If you have one of each, both antigens appear on your red blood cells.
Both I^A and I^B are dominant over I^O.

This gives four possible phenotypes (blood groups A, B, AB, O) from six possible genotypes:

Genotype	Phenotype (blood group)
I^A I^A	A
I^A I^O	A
I^B I^B	B
I^B I^O	B
I^A I^B	AB (codominance: both A and B antigens)
I^O I^O	O

Example — A father with blood group A and a mother with blood group B have a child with blood group O. Show how this is possible using a genetic diagram.

For the child to have blood group O, the child must be I^O I^O.
This means both parents must carry at least one I^O allele.
So the father's genotype must be I^A I^O and the mother's must be I^B I^O.

Punnett square:

	I^A	I^O
I^B	I^A I^B (AB)	I^B I^O (B)
I^O	I^A I^O (A)	I^O I^O (O)

So the four possible offspring genotypes are:

I^A I^B: AB (25%)
I^A I^O: A (25%)
I^B I^O: B (25%)
I^O I^O: O (25%)

From an A-parent and a B-parent, the child can have any of the four blood groups (A, B, AB, or O), each with a 25% chance.