Blood typing is an example of this where there is one gene but one gene can have several different characteristics

Blood typing is an example of codominance.

The blood type is controlled by a single gene (I gene) which has three alleles A, B and i.

The IA allele gives type A,

IB gives type B, and

i gives type O.

Individuals with IAIA or IAi have type A blood, and individuals with IBIB or IBi have type B. IAIB people have both phenotypes, they express codominance (characteristics of both alleles expressed). ii genotype is for O blood group.