What is multiple alleleism? Can a person have three alleles that determine the blood group (according to the AB0 system)? Why?

Multiple allelism is a phenomenon of the existence of a gene in more than two allelic forms. For example, for a gene that defines a person’s blood group, three allelic forms are known: IA, IB, and I0.
Human somatic cells have a diploid (2n) chromosome set. Therefore, a person cannot have simultaneously three alleles that determine the blood group (there are only two of them).

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