ABO blood group system in human beings is an example of codominance and multiple alleles. Codominance is a phenomenon in which alleles which do not show dominance-recessive relationship are able to express themselves independently when present together.
More than two alternate forms of a gene present on the same locus are called as multiple alleles and the mode of inheritance in these alleles is called as multiple allelism. Human beings have six genotypes and four blood group phenotypes A, B, AB and O.
