The blood types (A, B, and O) are controlled by the multiple alleles kind of genes.
Two O blood type parents can produce a child with only O blood type. Two parents with A blood type can produce a child with either A or O blood types. Two parents with B blood type can produce a child with either B or O blood type. One parent with A and another with B can produce a child with A, B, AB or O blood types.
If a child belonged to blood type o, he or she could not have been produced by Type AB mother and type O father.
