In ABO blood group system, an individual can be blood group A, B, AB or O based on the types of antigens on the surfaces of red blood cells.
A cross between an individual with blood group A and an individual with blood group B
=> A and B are codominant whereas O is recessive
=> For blood group A: IAIA or IAIO
=> For blood group B: IBIB or IBIO
=> For blood group AB: IAIB
=> For blood group O: IOIO
Parent phenotypes: Blood group A X Blood group B
Parent genotypes: IAIO X IBIO
Gametes: IA IO IB IO
F1 genotypes: IAIB IAIO IBIO IOIO
F1 phenotypes: AB A B O