Blood grouping system
ABO blood grouping
The most common blood grouping system is the ABO blood grouping. Blood groups are classified depending on the presence or absence of A or B antigen on red blood cells. The resulting blood groups are A, B, AB, or O as shown in Figure 1.
Figure 1: ABO blood grouping system
The genotypes and resulting phenotypes of each blood type are shown in the table below. Notice that the phenotype is decided by the dominant allele of the genotype. A and B are co-dominant, whereas O is recessive.
| Phenotype (ABO) | Genotype |
|---|---|
| A | AA, AO |
| B | BB, BO |
| AB | AB |
| O | OO |
Rhesus group system
Besides the ABO system, the second most widely used blood system is the Rhesus (Rh) group system. The Rh status is typically denoted with a positive or negative sign after ABO type of a person. Positive relates to the presence of the Rhesus factor, or D antigen, and negative relates to the absence of this antigen.