The blood group characteristic in humans is controlled by three set of alleles, namely, $I^{\mathrm{A}}, {\mathrm{I}}^{\mathrm{B}},$ and $\mathrm{i}$. 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,$ I$$^{\mathrm{A}}$ $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{A}} \mathrm{i}$, have blood group $\mathrm{A}$ whereas the individuals with genotype, $\mathrm{I}^{\mathrm{B}} \mathrm{I}^{\mathrm{B}}$ and $I^{B}$ $i$, have blood group $B$. The persons with genotype $I^{A} I^{B}$ have blood group $AB$ while those with blood group $O$ have genotype $ii$.

Hence, if the father has blood group $A$ and mother has blood group $\mathrm{B}$, then the possible genotype of the parents will be

Father Mother

$\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{A}}$ or $\mathrm{I}^{\mathrm{A}} \mathrm{i} \mathrm{I}^{\mathrm{B}} \mathrm{I}^{\mathrm{B}}$ or $\mathrm{I}^{\mathrm{B}}$$ i$

A cross between homozygous parents will produce progeny with $AB$ blood group.(figure)

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