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The ABO blood group system, is the classification of human blood based on the inherited properties of red blood cells (determined by the presence or absence of the antigens A and B, which are carried on the surface).
Because of this, an individual may have type A, type B, type O, or type AB blood.
The ABO blood group system in humans is an example of co-dominance. It is of great medical importance: before blood is transfused, it is vital to find out the blood group of a patient and ensure that it is matched. Unless this is done, there may be complications due to coagulation of red blood cells.
When representing blood group alleles, the letter i is used to represent the different antigenic forms (isoantigens)
A allele = I^A ; B allele = I^B ; O allele = i (recessive)
The genotypes for the different blood groups can be summarized as follows:
Neither I^A
nor I^B
is dominant over the other
allele (resulting in blood type AB, and co-dominance).
The reasons for two
alleles being co-dominant and the other allele
being recessive:
--> All three alleles produce a glycoprotein in the membrane of red blood
cells.
--> I^A alters the glycoprotein by addition of acetyl-
galactosamine. This altered glycoprotein is
absent from people who do not have the allele I^A
so if exposed to it they make anti-A antibodies.
--> I^B alters the glycoprotein by addition of
galactose. This altered glycoprotein is not
present in people who do not have the allele I^B
so if exposed to it they make anti-A antibodies.