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.
Practice Questions
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.
