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There are four blood groups in the ABO system - A, B, AB, and O, and classification is based on the presence or absence of antigenic substances that appear on the surface of blood cells. Both parents contribute to a child's blood type and the alleles that contribute to this are O, A, and B.
Knowing the blood phenotypes of a couple, a person is able to predict the blood type of their child using the Punnet square. For example, if a blood type BO woman has a child with a man who is blood type O or at least carries an O allele (AO, BO, or OO genotypes) then they could have a blood type O child.
However, there are rare instances when a couple produces a type O child even if they don’t possess any O allele. If this situation occurs, the child possibly carries the Bombay blood type, a blood type first discovered among three unrelated individuals in Bombay (now Mumbai) India in 1952 by Dr. Bhende and his colleagues. The major characteristic of the red blood cells of the Bombay blood group is the absence of the H antigen.
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Rare Blood Type
The H antigen is located on the surface of red blood cells and is the precursor of A and B antigens. The A allele is needed to produce a transferase enzyme to modify the H antigen into A antigen. Likewise, the B allele is needed to make the transferase enzyme that would transform the H antigen into B antigen. For type O individuals, the H antigen cannot be transformed further because no functional transferase enzyme is produced to modify the antigen. Note that antigen modification is through the addition of complex carbohydrates into the H antigen by the transferase enzyme.
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Bombay, or h/h, Blood Group
A person of the Bombay blood group inherited the recessive form of the allele for the H antigen from each of his parents. He carries the homozygous recessive (hh) genotype instead of the homozygous dominant (HH) or heterozygous (Hh) genotypes of the ABO blood group. As a result, the H antigen is not expressed in the red blood cell surfaces; consequently, the A and B antigens are not formed.
The h allele is a result of the mutation of the H gene (FUT1) that would express the H antigen in the red blood cells of ABO blood group. Scientists found that people of the Bombay phenotype are homozygous (hh) for the T725G mutation (Leucine 242 is changed to Arginine) in the FUT1 coding region. The consequence of this mutation is the production of an inactivated enzyme that is incapable of producing the H antigen.
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People of the Bombay blood group produce antibodies against H, A, and B antigens to protect themselves. Since they have antibodies against H, A, and B antigens, they can only receive blood donations from other people with Bombay blood type. Receiving blood transfusions from the ABO blood group can be fatal. The antibodies of the Bombay blood react to the red blood cells of the donor and causes cell death. In the past, some patients who were classified as type O by the ABO test died because doctors failed to test them for the Bombay blood type.
Because the Bombay blood type is very rare, it is very hard to find blood donors for patients who need it. The probability of finding a person with Bombay blood type is 1 for every 250,000 people. India has the highest number of people with the Bombay blood group where there is one Bombay blood type per 7,600 people. Geneticists believe that the high number of Bombay blood group people in India is the result of consanguineous marriage among members of a caste class. Higher caste class allows consanguineous marriage to maintain their position in the society and to protect their wealth.
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James S. O’Donnell, Thomas A.J. McKinnon, James T.B. Crawley, David A. Lane and Michael A. Laffan. 2005. Bombay Phenotype is Associated with Reduced Plasma VWFLevels and an Increased Susceptibility to Adamts13 Proteolysis. From www.bloodjournal.org.
R. S. Balgir. 2005. Detection of a Rare Blood Group “Bombay (Oh) Phenotype" Among the Kutia Kondh Primitive Tribe of Orissa, India. Int J Hum Genet, 5(3): 193-198.