The alleles which do not show dominance-recessive relationship and are able to express themselves independently when present together are called codominant alleles. Independent expression of two alleles in an individual is termed codominance.
Example :- 1
Coat colour in short horned cattle
There are two types of pure short horned cattle, red and white. On cross breeding the individuals of F1 generation are found to have roan colour. The effect is produced due to juxtaposition of small patches of red and white colour. It is also known as mosaic inheritance. As both the alleles form small patches side by side, they are codominant. It was previously considered to be a case of incomplete dominance because the colour effect is intermediate between red and white, on in breeding the roan hybrids produce three type of cattle-red, roan and white in the ratio of 1 : 2 : 1.
Example :- 2
AB Blood Group
Alleles for blood group A (IA) and blood group B (IB) are codominant so that when they come together in an individual they produce blood group AB. It is characterised by the presence of both antigen A (from IA) and antigen B (from IB) over the surface of erythrocytes.
Example :- 3
MN Blood Group
This phenomenon has been observed in MN blood group in humans. The red blood cells can carry two types of native antigens M and N, and an individual can be MM, MN or NN, exhibiting either one or both of them.
Example :- 4
Sickle Cell Haemoglobin
The allele for sickle cell haemoglobin Hbs is codominant with allele for normal haemoglobin HbA.