Why allele is represented with the letter I?

The letter "I" is used to represent the allele for the ABO blood group system, specifically for the gene that determines blood type. The "I" stands for "isoagglutinogen," which refers to the antigens present on the surface of red blood cells. The different alleles, such as IA, IB, and i, correspond to the various blood types (A, B, AB, and O). This notation helps in understanding inheritance patterns and blood compatibility.