CO-DOMINANCE
The two alleles are able to express themselves independently when present together.
For example: ABO blood grouping in humans is controlled by gene I. It has three alleles IA, IB and i.
IA, IB are dominant over i. If IA and I are present, only IA expresses. IA and IB are present both of them express each other.
ABO blood grouping is also a good example of multiple alleles.
