A codominant allele is expressed alongside another codominant allele in a heterozygote, so both effects appear distinctly in the phenotype. A recessive allele does not determine the phenotype when paired with a dominant allele and usually affects the phenotype only in the homozygous state.
The central difference is therefore what happens in a heterozygous genotype. Codominant alleles both contribute visibly, whereas the phenotypic effect of a recessive allele is masked by a dominant allele. The human ABO blood group system is especially useful because it demonstrates codominance, recessiveness, and multiple alleles within the same gene.
Codominant vs recessive allele at a glance
Dominance relationships describe how alleles at the same gene locus affect the phenotype when they occur together. They do not tell you which allele is more common, more advantageous, or physically stronger.
FeatureCodominant allelesRecessive alleleExpression in a heterozygoteBoth alleles have distinct effects on the phenotypeIts phenotypic effect is masked by a dominant alleleTypical phenotypeA dual phenotype showing both characteristicsThe dominant phenotypeExpression with two identical copiesEach allele produces its associated phenotype when homozygousThe recessive phenotype is usually expressedStandard exampleIᴬIᴮ produces blood group ABii produces blood group OCommon notationSuperscripts, such as Cᴿ and CᵂA lowercase letter, such as a, when simple Mendelian notation is appropriateExpected phenotype ratio from two heterozygotesOften 1:2:1Often 3:1 under complete dominance
The most precise comparison is not that one allele is “codominant” and another is “recessive” in isolation. Dominance is a relationship between alleles. For example, Iᴬ and Iᴮ are codominant with each other, but both are dominant over i.
Essential genetics terminology
An allele is an alternative form of a gene found at a particular locus. In a diploid organism, an individual normally has two alleles at each autosomal locus, one inherited from each parent, even though many different alleles may exist in the wider gene pool.
The genotype is the combination of alleles inherited by an organism. The phenotype consists of observable or measurable characteristics resulting from the genotype, often together with environmental influences.
A genotype is:
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Homozygous when it contains two identical alleles, such as
AAoraa. -
Heterozygous when it contains two different alleles, such as
AaorIᴬIᴮ. -
Homozygous recessive when both alleles are recessive, such as
aa.
These distinctions matter because dominance relationships are identified by comparing the phenotypes produced by different genotypes, especially the heterozygous genotype.
What is a recessive allele?
A recessive allele is an allele whose phenotypic effect is masked in a heterozygote by a dominant allele. In a straightforward autosomal dominant-recessive model, the recessive phenotype appears when the individual has two copies of the recessive allele.
Suppose A produces purple flowers and is dominant over a, which produces white flowers:
GenotypePhenotypeAAPurpleAaPurpleaaWhite
The a allele is still present in an Aa plant and can be transmitted to offspring. It has not been removed, weakened, or converted into the dominant allele. Its effect is simply not distinguishable in the stated phenotype when A is present.
Why can an allele be recessive?
Dominance often reflects how gene products contribute to a phenotype. One functional allele may produce enough protein or enzyme for the dominant phenotype, so the effect of a second, non-functional allele is not observable at that level.
This is a useful explanation, but it is not a universal rule that every recessive allele produces a non-functional protein. Molecular mechanisms differ among genes, and dominance must ultimately be defined from the observed relationship between genotype and phenotype.
A recessive allele is also not necessarily rare or harmful. Allele frequency describes how common an allele is in a population, whereas dominance describes its phenotypic relationship with another allele.
What are codominant alleles?
Codominance occurs when two different alleles are both expressed in a heterozygote, producing a dual phenotype. Neither allele masks the other, and their effects remain distinguishable rather than combining into an intermediate form.
Consider two hypothetical alleles affecting coat colour:
GenotypePhenotypeCᴮCᴮBlack coatCᵂCᵂWhite coatCᴮCᵂDistinct black and white hairs
The heterozygote displays both black and white features. It is not uniformly grey, which would instead suggest an intermediate phenotype associated with incomplete dominance.
The current IB Biology guide places this distinction in D3.2 Inheritance. For codominance, students should understand that the heterozygote has a dual phenotype; for incomplete dominance, the heterozygote has an intermediate phenotype. Both SL and HL students should be able to distinguish these patterns at the phenotypic level.
How the ABO blood group demonstrates both patterns
The ABO blood group gene has three principal alleles used in IB Biology notation:
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Iᴬ, which results in the production of the A antigen -
Iᴮ, which results in the production of the B antigen -
i, which does not produce a functional A- or B-modifying enzyme
Although three alleles occur in the population, a diploid individual inherits only two. This makes ABO blood groups an example of multiple alleles, not an example of an individual carrying three alleles.
GenotypeAntigen or antigens on red blood cellsBlood groupIᴬIᴬAAIᴬiAAIᴮIᴮBBIᴮiBBIᴬIᴮA and BABiiNeither A nor BO
The Iᴬ and Iᴮ alleles are codominant with each other. In an IᴬIᴮ heterozygote, both A and B antigens are produced, giving the distinct AB phenotype.
The i allele is recessive to both Iᴬ and Iᴮ. An Iᴬi person has blood group A, while an Iᴮi person has blood group B. Blood group O appears in the basic ABO model only with the homozygous genotype ii.
This single system shows why dominance terminology must always identify the allele comparison. Iᴬ is codominant with Iᴮ, but dominant over i.
Comparing Punnett square outcomes
A Punnett square predicts probabilities by combining the alleles that may occur in parental gametes. It does not guarantee the exact distribution in a small family because each fertilization is an independent event.
Complete dominance with a recessive allele
For the cross Aa × Aa, each parent produces A and a gametes:
AaAAAAaaAaaa
The expected genotypic ratio is 1 AA : 2 Aa : 1 aa. Because AA and Aa have the same dominant phenotype, the expected phenotypic ratio is 3 dominant : 1 recessive.
Codominance
For a hypothetical cross CᴿCᵂ × CᴿCᵂ, each parent produces Cᴿ and Cᵂ gametes:
CᴿCᵂCᴿCᴿCᴿCᴿCᵂCᵂCᴿCᵂCᵂCᵂ
The genotypic ratio is again 1:2:1. However, all three genotypes produce distinguishable phenotypes, so the phenotypic ratio is also 1:2:1.
This comparison is highly useful in exam questions. The segregation of alleles has not changed; what changes is how the heterozygous genotype contributes to the phenotypic categories.
A cross combining codominance and recessiveness
Consider an Iᴬi parent and an Iᴮi parent:
IᴮiIᴬIᴬIᴮIᴬiiIᴮiii
The predicted offspring are:
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25% blood group AB, showing codominance
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25% blood group A
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25% blood group B
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25% blood group O, showing the recessive phenotype
This cross directly tests whether you can apply both relationships rather than merely define them.
Codominance is not incomplete dominance
Students often group codominance and incomplete dominance together because neither follows a simple dominant-recessive phenotype pattern. Their heterozygous phenotypes are nevertheless different.
PatternHeterozygous phenotypeExampleComplete dominanceSame phenotype as the dominant homozygoteAa has the dominant phenotypeCodominanceBoth parental characteristics appear distinctlyIᴬIᴮ produces both A and B antigensIncomplete dominanceAn intermediate phenotype occursRed and white four o'clock flowers producing pink heterozygotes
AB blood is not a blend between blood groups A and B. Red blood cells carry both antigen types, so the effects remain separately identifiable. The phrase both expressed distinctly is safer than saying the alleles “mix.”
Common misconceptions to avoid
“Dominant means stronger or better”
Dominant does not mean healthier, more adaptive, or more frequent. It only means that the allele determines the stated phenotype in a heterozygous genotype when paired with the relevant recessive allele.
“A recessive allele is not expressed at all”
At the phenotype level being studied, its effect is masked in the heterozygote. The allele remains in the genotype and may still be transcribed or have molecular effects that are not visible in the simplified phenotype.
“Each codominant allele produces half a phenotype”
Codominance means both effects are detectable, not that each is weakened to 50%. An IᴬIᴮ individual produces both A and B antigens rather than half of a blended antigen.
“A heterozygote always has a dominant and recessive allele”
A heterozygote simply has two different alleles. Those alleles may show complete dominance, codominance, incomplete dominance, or another relationship.
“A 3:1 ratio identifies every dominant-recessive cross”
The classic 3:1 phenotype ratio applies to a specific monohybrid cross between two heterozygotes under complete dominance. Different parental genotypes produce different ratios, and small observed samples may deviate from predicted probabilities.
How to write an exam-ready answer
For a short “distinguish” or “state the difference” question, compare allele expression in the heterozygote directly:
Codominant alleles both affect the phenotype when present together in a heterozygote, whereas the phenotypic effect of a recessive allele is masked in a heterozygote containing a dominant allele and is usually observed in the homozygous state.
If the question asks for an example, add that Iᴬ and Iᴮ are codominant and produce blood group AB in IᴬIᴮ, while i is recessive and produces blood group O in ii.
Other useful exam practices include:
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State whether you are discussing genotype or phenotype.
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Use
Iᴬ,Iᴮ, andifor ABO alleles, as specified by the IB guide. -
Include all gametes before completing a Punnett square.
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Give both genotypic and phenotypic ratios when requested.
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Use probabilities rather than claiming that four predicted offspring must include one of each type.
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Explain codominance as a dual phenotype, not an intermediate phenotype.
For broader context, review IB Biology inheritance explained with an exam-focused D3.2 overview. The focused RevisionDojo pages on dominant and recessive allele effects, ABO blood groups and multiple alleles, and incomplete dominance and codominance can then be used for targeted revision.
Conclusion
The difference between a codominant and a recessive allele is most clearly seen in a heterozygote. Two codominant alleles both make distinguishable contributions to the phenotype, whereas the effect of a recessive allele is masked when an appropriate dominant allele is present.
ABO inheritance connects both ideas: Iᴬ and Iᴮ are codominant with each other, while i is recessive to both. Once this relationship is understood, Punnett square ratios and genotype-to-phenotype reasoning become much easier to interpret.
After reviewing the concept, use the D3.2 Inheritance Questionbank to practise identifying inheritance patterns. RevisionDojo Flashcards can reinforce the definitions, while Jojo AI can provide feedback on explanations and Punnett square reasoning.