With the help of one example, explain the phenomenon of co-dominance and multiple allelism in human population.

Co-dominance: The phenomenon of independent expression of both the alleles in heterozygotes is called co-dominance. The alleles do not show the dominant recessive relationship.Multiple allelism: Presence of more than 2 alleles for a gene is called multiple allelism.Human blood group shows co-dominance as well as multiple allelism. ABO blood group is controlled by I gene. I gene has 3 alleles- IA, IBand i. When alleles IAand IBare present together both are able to express its effect independently. It produces blood group AB.