Codominance occurs when two alleles are both expressed simultaneously in a heterozygote, giving a phenotype that shows contributions from both. Multiple alleles means a gene has more than two possible allelic forms in the population. Together, these inheritance patterns explain blood group genetics and other traits that simple dominant-recessive models cannot predict.
What is codominance and how does it differ from dominance?
In simple dominance, one allele masks the other. In codominance, both alleles are expressed equally — neither is masked. The heterozygous phenotype is distinct from either homozygous phenotype.
Notation: codominant alleles are written as superscripts to a capital letter, e.g. C^R and C^W for red and white flower colour in carnations.
| Genotype | Phenotype (carnation flowers) | Explanation |
|---|---|---|
| C^R C^R | Red | Only red pigment produced |
| C^W C^W | White | Only white (no pigment) |
| C^R C^W | Pink | Both red and white alleles expressed; both pigments present |
Note: the pink heterozygote is not a blend — individual petals contain both red and white pigment-producing cells. This distinguishes codominance from incomplete dominance (where the phenotype really is an intermediate).
What are multiple alleles?
Most genes have exactly two alleles discussed at GCSE, but multiple alleles means that more than two variants of a gene exist in the population. Each individual still carries only two alleles (one on each homologous chromosome), but there are more than two options in the gene pool.
The ABO blood group system is controlled by a single gene with three alleles:
| Allele | Symbol | Antigen produced |
|---|---|---|
| I^A | Dominant | Antigen A on red blood cells |
| I^B | Dominant | Antigen B on red blood cells |
| I^O (or i) | Recessive (to both I^A and I^B) | No antigen |
I^A and I^B are codominant with each other (both expressed when both present); both are dominant over I^O.
How do you work out ABO blood groups from genotypes?
Possible genotypes and their blood groups:
| Genotype | Blood group | Reasoning |
|---|---|---|
| I^A I^A or I^A I^O | A | A antigen present (I^O recessive) |
| I^B I^B or I^B I^O | B | B antigen present (I^O recessive) |
| I^A I^B | AB | Both antigens expressed (codominance) |
| I^O I^O | O | No antigens; only genotype giving group O |
Worked example — Punnett square:
Parents: mother blood group A (genotype I^A I^O) × father blood group B (genotype I^B I^O)
| I^B | I^O | |
|---|---|---|
| I^A | I^A I^B (AB) | I^A I^O (A) |
| I^O | I^B I^O (B) | I^O I^O (O) |
Predicted ratio: 1 AB : 1 A : 1 B : 1 O (25% each)
This means that two parents — one blood group A, one blood group B — can have a child with blood group O, which might seem surprising without knowledge of the genotypes.
How does sickle-cell disease illustrate codominance?
Sickle-cell disease arises from a mutation in the haemoglobin gene. The normal allele is H^A (produces normal haemoglobin); the sickle allele is H^S (produces sickle haemoglobin). The two alleles show codominance in heterozygotes:
| Genotype | Phenotype | Clinical effect |
|---|---|---|
| H^A H^A | Normal | Healthy; fully susceptible to malaria |
| H^A H^S | Sickle-cell trait (carrier) | Some sickle + some normal haemoglobin; mild symptoms; partial malaria resistance |
| H^S H^S | Sickle-cell disease | Red blood cells sickle under low oxygen; pain crises; anaemia |
The heterozygote makes both types of haemoglobin simultaneously (codominance). This is why sickle-cell trait is distinct from sickle-cell disease — the carrier phenotype is genuinely intermediate and represents both alleles being expressed, not one masking the other.
How do you identify codominance from data?
A cross is codominant if:
- The heterozygous offspring has a third, distinct phenotype that is not shown by either parent.
- A 1:2:1 ratio (homozygous A : heterozygous : homozygous B) is observed, rather than the 3:1 of simple dominance.
- Back-crossing the heterozygote with either homozygote gives a 1:1 ratio of the two parent phenotypes, not all of one.
Frequently asked questions
Why can two people with blood group A have a child with blood group O?
Blood group A can result from either I^A I^A or I^A I^O genotypes. If both parents are I^A I^O (group A carriers of the recessive I^O allele), there is a 1 in 4 chance (25%) that a child inherits I^O from each parent, giving genotype I^O I^O — blood group O. This illustrates why genotype cannot always be inferred from phenotype alone when multiple alleles are involved.
What is the difference between codominance and incomplete dominance?
In codominance, both alleles produce their respective products in the heterozygote — for example, a person with I^A I^B blood group produces both A and B antigens on their red blood cells. In incomplete dominance, the heterozygous phenotype appears to be a blend — for example, a cross between red (R^R) and white (R^W) snapdragons gives pink offspring that produce neither full red nor white pigment but a diluted amount. At GCSE, both patterns require using appropriate notation (superscripts or C^R/C^W style); the key distinction is whether both products are simultaneously present (codominance) or a diluted version appears (incomplete dominance).
How many possible blood group genotypes exist?
With three alleles (I^A, I^B, I^O), the number of possible genotypes is given by the formula n(n+1)/2, where n = 3. This gives 6 genotypes: I^A I^A, I^A I^O, I^B I^B, I^B I^O, I^A I^B, and I^O I^O. These six genotypes produce only four blood groups (A, B, AB, O), which is why blood group testing alone cannot always identify a person's genotype — I^A I^A and I^A I^O both appear as group A in a simple blood test.
Why is the H^A H^S sickle-cell carrier phenotype beneficial in malaria-endemic regions?
The Plasmodium falciparum parasite that causes malaria invades red blood cells. Sickle cells, when they form, make a less hospitable environment for the parasite — and the carrier's immune system clears infected cells more effectively. This gives H^A H^S individuals partial resistance to severe malaria, providing a survival advantage in sub-Saharan Africa and other malaria-endemic regions. This is an example of heterozygous advantage, where the heterozygous genotype has higher fitness than either homozygote in a particular environment.
For Socratic GCSE biology with Professor Darwin — connecting allele-level codominance to population-level blood group distributions and natural selection — visit aitutors.me.