Genetics
Patients who test as type O may have the Bombay phenotype if they have inherited two recessive alleles of the H gene, (their blood group is Oh and their genotype is hh), and so they do not produce the H carbohydrate that is the precursor to the A and B antigens. It then no longer matters whether the A or B enzymes are present or not, as neither A nor B antigen can be produced since the precursor antigen H is not present. Despite the designation O, Oh negative is not a sub-group of any other group. Because both parents must carry this recessive allele to transmit this blood type to their children, the condition mainly occurs in small closed-off communities where there is a good chance of both parents of a child either being of Bombay type, or being heterozygous for the h gene allele and so carrying the Bombay characteristic as recessive. Other examples may include noble families, which are inbred due to custom rather than local genetic variety.
Read more about this topic: Hh Antigen System