The blood group characteristic in humans is controlled by three set of alleles,
namely, ${\mathrm{I}}^{\mathrm{A}}$, ${\mathrm{I}}^{\mathrm{B}}$, and i. The alleles, ${\mathrm{I}}^{\mathrm{A}}$ and ${\mathrm{I}}^{\mathrm{B}}$, are equally dominant whereas
allele, i, is recessive to the other alleles. The individuals with genotype, ${\mathrm{I}}^{\mathrm{A}}$ ${\mathrm{I}}^{\mathrm{A}}$ and ${\mathrm{I}}^{\mathrm{A}}$ i, have blood group A whereas the individuals with genotype, ${\mathrm{I}}^{\mathrm{B}}$ ${\mathrm{I}}^{\mathrm{B}}$ and ${\mathrm{I}}^{\mathrm{B}}$ i, have blood group B. The persons with genotype ${\mathrm{I}}^{\mathrm{A}}$${\mathrm{I}}^{\mathrm{B}}$, have blood group AB while those with blood group O have genotype ii.
Hence, if the father has blood group A and the mother has blood group B, then
the possible genotype of the parents will be
Father Mother
${\mathrm{I}}^{\mathrm{A}}{\mathrm{I}}^{\mathrm{A}} \mathrm{or} {\mathrm{I}}^{\mathrm{A}} \mathrm{i} {\mathrm{I}}^{\mathrm{B}} \mathrm{or} {\mathrm{I}}^{\mathrm{B}} \mathrm{i}$

A cross between homozygous parents will produce progeny with AB blood group.

A cross between heterozygous parents will produce progenies with AB
blood group (${\mathrm{I}}^{\mathrm{A}}$${\mathrm{I}}^{\mathrm{B}}$) and O blood group (ii).