In multiple allelism problems , the character is determined by more than two alleles for a gene. The most typical example is the AB0 system blood group. Each individual presents in his genotype only two of the possible alleles. To make this idea clear, a nomenclature will be used in which all the alleles for that gene are represented using the same letter (I), and differing in the superscripts (IA, IB, I0) or subscripts IA, IB, I0). The allele I0 can also be represented by i to express its recessiveness with respect to the other two.
Remember that the number 0 is not the letter O.