Think of the alleles of a given gene as different versions of that gene. For example, the ABO blood types are coded for by three different alleles of the same gene: IA, IB, and i. They code for antigenic substances, on the surface of RBCs that are slightly different from one another, resulting in different blood types. In other words, different alleles of the same gene code for the same class of substances (In this case antigenic substances), but different types of it.
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