In humans, type A blood is caused by the A antigen in red blood cells resulting from an allele A of the ABO gene. Type B is caused by a B allele and leads to the production of antigen B. Type O blood results from the absence of either allele while the presence of both alleles causes both antigens to be formed resulting in type AB blood. If a person of blood type AB has children with someone belonging to blood group O, what will be the possible blood groups of their children?