According to the ABO system, the commonest blood group is the human population is O while the rarest blood group is AB. ABO blood group is controlled by gene I and its types are ${\mathrm{I}}^{\mathrm{A}},\mathrm{ }{\mathrm{I}}^{\mathrm{B,}}$ and ${\mathrm{I}}^{\mathrm{O}}$.

Wife has AB blood group, so her genotype is ${\mathrm{I}}^{\mathrm{A}}\mathrm{ }{\mathrm{I}}^{\mathrm{B}}$

Husband has O blood group, so his genotype is ${\mathrm{I}}^{\mathrm{O}} {\mathrm{I}}^{\mathrm{O}}$

Thus their children could have either ${\mathrm{I}}^{\mathrm{A}}\mathrm{ }{\mathrm{I}}^{\mathrm{O}}$ (A blood group) or ${\mathrm{I}}^{\mathrm{B}}\mathrm{ }{\mathrm{I}}^{\mathrm{O}}$ (B blood group)