Bombay Phenotype

It is important to be cautious in predicting the ABO blood type of children based on the phenotypes of their parents.  This is due to the fact that a third antigen (H) on the surface of red cells can prevent the expected ABO blood type from occurring. 

Normally, if 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 (OO, AO, or BO genotype).

schematic drawings of phenotypes and genotypes of an expected ABO inheritance pattern

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.

schematic drawings of phenotypes and genotypes of an ABO Bombay inheritance pattern

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.  It is the same for the A allele.  However, if only recessive alleles for the H antigen are inherited (hh), as in the case above, the H antigen will not be produced.  Subsequently, the A and B antigens also will not be produced.  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 pronounce word because it was first described in that Indian city.

The ABO blood system is further complicated by the fact that there are two subtypes of type A and two of AB.  These are referred to as A1,  A2,  A1B,  and A2B.

For a more in-depth biochemical explanation of what the ABO alleles are actually doing to cause the ABO blood types, refer to the Related Internet Sites section of this tutorial.


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