ABO blood group is controlled by gene I and its types are IA, IB and IO.

As mother is A and the father is B, the possible genotypes for parents are - Mother - ${\mathrm{I}}^{\mathrm{A}}{\mathrm{I}}^{\mathrm{A}}/{\mathrm{I}}^{\mathrm{A}}{\mathrm{I}}^{\mathrm{O}}$ and Father - ${\mathrm{I}}^{\mathrm{B}}{\mathrm{I}}^{\mathrm{B}}/{\mathrm{I}}^{\mathrm{B}}{\mathrm{I}}^{\mathrm{O}}$ .

Therefore, the child's blood group genotype possibilities are -

${\mathrm{I}}^{\mathrm{A}}{\mathrm{I}}^{\mathrm{A}}/{\mathrm{I}}^{\mathrm{A}}{\mathrm{I}}^{\mathrm{O}}-\mathrm{A}$

${\mathrm{I}}^{\mathrm{B}}{\mathrm{I}}^{\mathrm{O}}/{\mathrm{I}}^{\mathrm{B}}{\mathrm{I}}^{\mathrm{O}}-\mathrm{B}$

${\mathrm{I}}^{\mathrm{o}}{\mathrm{I}}^{\mathrm{O}}-\mathrm{O}$

${\mathrm{I}}^{\mathrm{A}}{\mathrm{I}}^{\mathrm{B}}-\mathrm{A}\mathrm{B}$