It can be difficult to predict the ABO blood type of children based on the phenotypes of their parents, due to the fact that a third antigen (H) on the surface of red cells can prevent the expected ABO blood type options from occurring.
Usually, when an A blood type mother has an O type child, the father is expected to be type O or at least to carry the O allele recessively (OO, AO, or BO genotype). The child has inherited an O allele from both parents. However, an O blood type child can also be born to parents who do not have the O allele if a recessive form of the allele for the H antigen also is inherited from both parents.
The H antigen is a precursor to the A and B antigens. For instance, the B allele must be present to produce the B enzyme that modifies the H antigen to become the B antigen. As well, the A allele must be present to produce the A enzyme that modifies the H antigen to become the A antigen.
However, when only recessive alleles for the H antigen are inherited (hh), as in the case above, the H antigen will not be produced and thus, the A and B antigens will not be produced either . The result is an O phenotype, by default since a lack of A and B antigens is the O type. This seemingly impossible phenotype result has been referred to as a Bombay phenotype.
The ABO blood system is further complicated by the fact that there are two subtypes of type A and two subtypes of type AB. These are A1, A2, A1B, and A2B.