Multiple allelism and co-dominance are associated with

ABO blood group system in humans is controlled by an (I) gene having multiple alleles (${\mathrm{I}}^{\mathrm{A} },{\mathrm{I}}^{\mathrm{B}}$and i) and the genotype ${\mathrm{I}}^{\mathrm{A}}{\mathrm{I}}^{\mathrm{B}}$ shows co-dominance.

Starch branching gene in pea is an example of pleiotropic gene.

Phenylketonuria is an example of pleiotropy.

Flower colour in Snapdragon show incomplete dominance.