There are three alleles or versions of the blood type gene: A, B, and O. Since everybody has two copies of these genes, there are six possible combinations; AA, BB, OO, AB, AO, and BO. In genetic terms, these combinations are called genotypes, and they describe the genes you got from your parents.
In addition to the proteins (agglutinogens) existing on your red blood cells, other genes make proteins called agglutinins (a-GLOO-tin-ins) that circulate in your blood plasma. Agglutinins are responsible for ensuring that only the blood cells of your blood type exist in your body.
Your genotype determines your blood type and statue
The agglutinogen produced by the O allele has no special enzymatic activities. However, the agglutinogens produced by the A and B alleles do have enzymatic activities, which are different from each other. Therefore people whose genotype is OO are said to have type O blood, meaning the agglutinogen on their red blood cells doesn't have any enzymatic activity. People with Type O blood have agglutinins a and b in their blood plasma. Agglutinin a helps the body destroy any type A blood cells that might enter the circulation system. Agglutinin b helps the body destroy any type B blood cells that might enter the circulation system.
The ABO Blood system
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Blood type (genotype) |
Type A (AA, Ai) |
Type B (BB, Bi) |
Type AB (AB) |
Type O (ii) |
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Red Blood Cell Surface Antigens (Phenotype) |
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Plasma Antibodies (Phenotype) |
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People who have an AA genotype are said to have type A blood because the agglutinogen on their red blood cells has the enzyme activity associated with the A allele. It is important to recognize that people with the AO genotype also have the enzyme activity associated with the A allele, so they are also said to have type A blood. (Remember the O allele doesn't have any enzyme activity associated with it!) People with Type A blood have agglutinin b in their blood plasma. Agglutinin b helps the body destroy any type B blood cells that might enter the circulation system.
