Codominance

On occasion, both alleles within a genotype are expressed in the phenotype of an individual – this is known as codominance
Inheritance of blood group is an example of codominance
There are three alleles of the gene governing this instead of the usual two
- I represents the gene and the superscript A, B and O represent the alleles
Alleles I^A and I^B are codominant, but both are dominant to I^O
I^A results in the production of antigen A in the blood
I^B results in the production of antigen B in the blood
I^O results in no antigens being produced in the blood
These three possible alleles can give us the following genotypes and phenotypes

Blood Phenotypes Table

We can use genetic diagrams to predict the outcome of crosses that involve codominant alleles:

‘Show how a parent with blood group A and a parent with blood group B can produce offspring with blood group O’

Punnett square showing the inheritance of Blood Group

The parent with blood group A has the genotype I^AI^O
The parent with the blood group B has the genotype I^BI^O
We know these are their genotypes (as opposed to both being homozygous) as they are able to produce a child with blood group O and so the child must have inherited an allele for group O from each parent
Parents with these blood types have a 25% chance of producing a child with blood type O