Felix Bernstein in 1924-25, discovered multiple alleles, codominant and dominant-recessive relationships in the determination of human blood groups.
He proposed the 'three allele model'. The ABO blood group antigens are encoded by one genetic locus, the ABO locus, which has three alternatives (allelic) forms A, B and O. A child receives one of the three alleles from each parent, giving rise to six possible genotypes and four possible blood types (phenotype).
Therefore, the correct answer is option C.