Remember that a person has two copies of those genes (one from each parent). So A could be AA or AO, and B could be BB or BO.
The resulting recombinations allow for all of A (AO), B (BO), AB and O (OO).
q10.To add on to what everyone else has already said, it actually is possible (contrary to popular belief) for the parents to have offspring with O blood. In some instances, the mother may have a rare mutation and have cis-AB blood where the alleles for A and B are located on the same chromosome, with a remaining O allele on the chromosome. If we assume that the father is heterozygous for type B blood (B allele and O allele), then it's possible for the offspring to inherit both O alleles, leaving them with type O blood. In other words, a mother with type AB blood and a father with type B blood can have offspring with A, B, AB, or O blood.
