A, B, & O blood - (Jan/15/2019 )

Hi,

          The genotypic term for a type of blood is I^A , I^B, or i. Where does this letter "I" come from?

Thanks

Use Google, Luke

"The ABO blood type is controlled by a single gene (the ABO gene) with three types of alleles inferred from classical genetics: i, I^A, andI^B. The I designation stands for isoagglutinogen, another term for antigen."

https://en.wikipedia.org/wiki/ABO_blood_group_system

Thank you. I got confused because it looked like the genotype reference in sex-linked traits (X^allele).