Co-dominance IGCSE Biology 0610

Co-dominance: When Both Alleles Are Expressed

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

In co-dominance, both alleles are expressed in the phenotype of a heterozygous individual. Neither allele is dominant over the other. The notation uses a gene letter with superscript allele letters (e.g. C^R and C^W for red and white).

Co-dominance is a Supplement-only concept that changes the notation system. Students must switch from uppercase/lowercase to the superscript system and recognise that the heterozygote shows a different phenotype from either homozygote.

What co-dominance means

In co-dominance, both alleles in a heterozygous individual contribute to the phenotype. Neither allele is dominant over the other. The heterozygote has a phenotype that is different from both homozygotes, often showing both characteristics simultaneously.

This is different from simple dominance where the heterozygote looks the same as the homozygous dominant. In co-dominance, you can distinguish all three genotypes by their phenotypes.

Notation for co-dominance

Because neither allele is dominant, the standard uppercase/lowercase system cannot be used. Instead, a gene letter is written with a superscript for each allele. For example, for flower colour: C^RC^R = red, C^WC^W = white, C^RC^W = pink (or red and white together).

The gene letter (C) stays the same for both alleles to show they are forms of the same gene. The superscript letters (R and W) distinguish the two alleles. This convention must be followed in all exam answers.

Sickle cell anaemia as an example

Sickle cell anaemia is a classic IGCSE example of co-dominance. The haemoglobin gene has two alleles: Hb^A (normal haemoglobin) and Hb^S (sickle haemoglobin).

Genotype	Phenotype
Hb^AHb^A	Normal haemoglobin; no sickle cells
Hb^AHb^S	Sickle cell trait; both normal and sickle haemoglobin produced
Hb^SHb^S	Sickle cell anaemia; most haemoglobin is abnormal

The heterozygous individual (Hb^AHb^S) produces both types of haemoglobin. This is co-dominance because both alleles are expressed. Individuals with sickle cell trait have some resistance to malaria, which explains why the Hb^S allele is common in malaria-affected regions.

Genetic crosses with co-dominance

Setting up a Punnett square for co-dominance follows the same steps as for simple dominance, but with the different notation. Crossing two heterozygotes (C^RC^W x C^RC^W) produces offspring in the ratio 1 C^RC^R: 2 C^RC^W: 1 C^WC^W.

Because each genotype produces a distinct phenotype, the phenotype ratio is 1:2:1, not 3:1. This is the key difference from simple dominance and is the ratio examiners expect to see in co-dominance crosses.

Try a focused question (4 marks)

In a plant, red flower colour (C^R) and white flower colour (C^W) are co-dominant. Heterozygous plants have pink flowers. Cross two pink-flowered plants and give the expected phenotype ratio of the offspring.

Show the answer and marking guidance

Parental genotypes: C^R C^W x C^R C^W.
Gametes: C^R or C^W from each parent.
Offspring genotypes: 1 C^R C^R: 2 C^R C^W: 1 C^W C^W.
Phenotype ratio: 1 red: 2 pink: 1 white.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Using uppercase/lowercase notation instead of the superscript co-dominance notation.
Giving a 3:1 ratio instead of 1:2:1.
Calling co-dominance "incomplete dominance" or "blending" (both alleles are expressed, not blended).
Forgetting to show that the heterozygote has a distinct phenotype.