ABO blood groups are controlled by the gene I. The gene I has three alleles and i . The alleles and produce a slightly different form of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possess any two of the three 'I' gene alleles. IN and are completely dominant over , but when and are present together, they both express their own types of sugar, thus, behaving as codominant alleles.

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