Multiple Alleles: the ABO Blood Group
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A gene with multiple alleles has more than two versions in a population, though any individual carries only two, as with ABO's three alleles.
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ABO's three alleles, A, B, and O, combine so A and B are each dominant to O and codominant to each other.
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Multiple alleles exist at the population level — an individual's genotype holds two at that locus.