AP Syllabus focus:
‘In codominance, both alleles are fully expressed, so heterozygotes show a distinct phenotype from either homozygote.’
Codominance is a common inheritance pattern in which two different alleles in a heterozygote are both expressed in the phenotype. Recognising codominance helps explain non-Mendelian trait patterns and molecular evidence of allele expression.
Core idea: both alleles are expressed
In codominance, heterozygotes display traits from both alleles at the same time, rather than one allele masking the other. This produces a phenotype that is distinct from either homozygote, consistent with the syllabus focus.
Codominance: An inheritance pattern in which both alleles in a heterozygote are fully expressed, producing a phenotype that shows both allele products.
Codominance is best understood by focusing on gene products:
An RFLP (restriction fragment length polymorphism) gel-and-blot diagram demonstrates codominance in molecular markers: each allele produces a characteristic set of DNA fragment bands. Homozygotes show one banding pattern, while the heterozygote lane contains bands from both alleles simultaneously, making the genotype visually identifiable from the phenotype. Source
If each allele encodes a functional (but different) protein or cell-surface marker, a heterozygote can produce both products.
The phenotype reflects simultaneous expression, not blending into one intermediate outcome.
What “fully expressed” means in practice
“Fully expressed” often means that in a heterozygote:
Both alleles are transcribed and translated.
Both protein forms are detectable (for example, as distinct antigens or protein bands).
Schematic alkaline hemoglobin electrophoresis illustrates how different hemoglobin variants separate into distinct bands based on charge and migration. In a heterozygous state (e.g., sickle cell trait), both hemoglobin forms are present, producing a two-band/dual-signal pattern that directly supports codominance at the protein level. Source
The organism’s observable trait shows both contributions (for example, two antigen types on red blood cells).
Codominant genotypes and phenotypes
Codominance changes how you interpret genotype-to-phenotype relationships because the heterozygote is not phenotypically identical to a homozygote.
Heterozygote: An individual with two different alleles at a gene locus.
A key implication is that phenotype categories may map more directly to genotype:
Homozygote 1 (allele A / allele A) → phenotype showing only allele A’s product
Heterozygote (allele A / allele B) → phenotype showing both products
Homozygote 2 (allele B / allele B) → phenotype showing only allele B’s product
This is why codominant traits can show three distinguishable phenotypes in a simple two-allele system.
Classic AP Biology example: ABO blood group
The ABO blood group illustrates codominance at the level of red-blood-cell antigens:
Alleles and encode different enzymes that add different sugars, creating A and B antigens.
In the heterozygote, both antigens are present on the cell surface, producing blood type AB.
Codominant inheritance in the ABO system: the and alleles are both expressed, so an individual displays both A and B antigen phenotypes (blood type AB). The diagram also links parental genotypes to possible offspring genotypes and phenotypes, emphasizing that heterozygotes are not phenotypically identical to either homozygote. Source
Allele: An alternative version of a gene at a specific locus.
In codominance, the heterozygous phenotype (AB) is distinct from either homozygous phenotype (A or B), matching the syllabus description.
How codominance appears in data
Codominance is often supported by observable evidence that both allele products are present:
Cell-surface markers: two antigen types present simultaneously
Protein assays: two distinct protein variants detected
Phenotypic scoring: heterozygotes consistently form their own category rather than grouping with a dominant homozygote
When analysing inheritance patterns, codominance is suggested when:
Heterozygotes are phenotypically distinct from both homozygotes
Offspring phenotypes include a category that shows both parental traits
Common pitfalls and clarifications
Students often confuse codominance with other non-Mendelian patterns; keep the focus on the heterozygote.
Codominance is not “one allele stronger”; it is co-expression of both.
Codominance does not contradict allele segregation; alleles still separate during gamete formation, but both are expressed together in heterozygotes.
Codominance can be most obvious at the molecular level (detecting both products), even if the organism-level phenotype seems subtle.
FAQ
No. Codominance can be detectable only with molecular tests (e.g., identifying two protein variants), even when the visible phenotype difference is subtle.
Common mechanisms include both alleles producing:
different functional enzymes that generate distinct cell-surface markers
structurally different proteins detectable as separate variants
By showing both allele products in heterozygotes using methods such as:
immunoassays for distinct antigens
protein separation techniques (e.g., electrophoresis) demonstrating two bands
Because codominance refers to $I^A$ and $I^B$ being jointly expressed in $I^A I^B$, while “multiple alleles” refers to more than two alleles ($I^A$, $I^B$, $i$) existing in the population.
Yes. If heterozygotes express both allele products, selection can act on heterozygotes differently than either homozygote, potentially maintaining both alleles in a population under certain conditions.
Practice Questions
Define codominance and state what is observed in the phenotype of a heterozygote. (2 marks)
Correct definition: both alleles are fully expressed in a heterozygote (1)
States heterozygote shows a distinct phenotype that includes both allele products/traits (1)
A gene has two alleles, and . Homozygotes produce one detectable protein type (A-protein or B-protein). Heterozygotes produce both detectable protein types. Explain how this supports codominance and predict the number of phenotypic classes expected from an cross. (5 marks)
Links detection of both proteins in heterozygotes to expression of both alleles (1)
States heterozygote phenotype is distinct from both homozygotes (1)
Identifies this pattern as codominance (1)
Predicts three phenotypic classes (, , ) (1)
Justifies three classes by mapping each genotype to protein detection pattern (1)
