Edexcel IGCSE Biology · Spec 3.21B
Codominance (Triple)
Codominance and how to work out genetic crosses, using human blood groups as an example.
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Codominance (Triple)
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Explained
When neither allele hides the other
In ordinary inheritance one allele is dominant and masks the other, so a heterozygote shows only the dominant characteristic. Codominance is different: both alleles are expressed, and both show in the phenotype.
Neither is dominant and neither is recessive, so the usual capital and lower case notation will not work. Instead both are written as capitals of the same letter with different superscripts or superscript letters.
The standard example
In some cattle, one allele gives red hair and the other white. A heterozygote is not pink. It has both red hairs and white hairs together, which is called roan.
That is what codominance means in practice: both alleles produce their effect, side by side, rather than blending.
The cross
Cross two roan cattle, each heterozygous, and the offspring are one red to two roan to one white.
Notice the ratio. In a normal monohybrid cross of two heterozygotes you get 3 to 1, because two of the four genotypes look identical. Here all three genotypes look different, so the phenotype ratio is 1 to 2 to 1, matching the genotype ratio exactly.
Recognising that a 1 to 2 to 1 phenotype ratio means codominance is often the quickest route into a question.
Blood groups
Human ABO blood groups combine codominance with ordinary dominance, which is why they are the harder example.
The A allele and the B allele are codominant with each other, so someone with both is group AB and produces both antigens. The O allele is recessive to both, so group O requires two copies of it.
Group A can therefore be two A alleles or an A and an O, and the same for group B. Group AB has one of each, and group O has two O alleles. That is the whole system.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for a genetic cross awards four separate marks: the parent genotypes, the gametes, the offspring genotypes, and the phenotype ratio. Setting out the cross properly is therefore worth as much as getting the answer.
Its guidance is generous about notation, allowing any symbols and different letters, and accepting the genotypes and ratio taken from a Punnett square rather than written as a list. It also allows an error carried forward, so correct offspring produced from incorrect parents still earn credit.
What it will not accept is the gametes written wrongly. It states that there must not be four alleles in a gamete line, giving examples of the incorrect combinations.
The reason is biological rather than presentational. A gamete is haploid and carries one allele of each gene, so writing two alleles in a gamete shows a misunderstanding of meiosis rather than a slip of the pen.
The safest approach is a Punnett square with the two parental gametes along each edge. It makes the haploid gametes explicit and produces the offspring genotypes automatically.
Codominance is not incomplete dominance
These are often confused. In codominance both alleles are fully expressed, so a roan animal has both red and white hairs and you could point to each.
In incomplete dominance the effects blend into an intermediate, as in a pink flower from a red and a white parent. There is no red and no white to point at.
The IGCSE specification asks for codominance, so the roan and blood group examples are the ones to have ready, and describing a heterozygote as a blend would be describing the wrong thing.
Spec 3.21B
What you need to know
- Explain what codominance means
- Use a cross to show codominant alleles
- Predict the ratio from a codominant cross
Active recall
Quick check
Answer each question before opening the answer.
What is codominance?
When both alleles are expressed in the phenotype because neither is dominant over the other.
Give an example of codominance.
The ABO blood group system, where the I^A and I^B alleles are codominant (giving blood group AB).
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