The Bombay Phenotype is a genetic trait in which the given indivdual expresses no A or B antibodies in their blood resulting in the O blood type. This is due to another third antigen H on the surface of the blood cell which effects the ability of the A and B antigens from forming. If a individual is double recessive for this H antigen, hh, it will result in none of the H antigen being produced. Since this H antigen is required for the formation of the A and B antigens, if no H antigen is produced regardless of the genetype the cell will be unble to generate any A or B antigens resulting in the O blood type. This means that it is possible for an AB parent to have offspring O blood type as long both parents are at least heterozygous for this h antigen.
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