Some alleles are both expressed in the same phenotype, this is called codominance.
Both codominant alleles are shown with upper case letters in genetic diagrams, but the letters used are different. For example, feather colour in hens may be white, black or speckled (it has both white feathers and black feathers).
The alleles can be shown as W for white and B for black.
There are three possible genotypes: WW, BB and BW.
There are also three possible phenotypes: WW = white, BB = black, and BW = speckled.
The gene controlling human ABO blood groups has three alleles, not just two:
This table shows the possible genotypes (alleles present) and phenotypes (blood group):
|IA IA or IA IO||A|
|IB IB or IB IO||B|
So to inherit blood group A you must receive an IA allele from one parent and either an IA or an IO allele from the other. To inherit blood group B you must receive an IB allele from one parent and either an IB or an IO allele from the other. To inherit blood group AB you must receive an IA allele from one parent and an IB allele from the other. To inherit blood group O you must receive an Io from both parents.