The ABO blood group system is a standard method of determining the blood types among individuals. This system is based on the presence of specific antigens that are presented on the cell surface of the red blood cells or erythrocytes. In human beings, there are two types of antigens for cell-to-cell communication, namely the A and the B antigen. The presence of a particular type of antigen in the body of an individual is very restrictive because there are specific antibodies that are present in the immune system that are ready to combat any mismatches that are detected in the body.
For example, if an individual carries red blood cells with the A antigen, then his body will carry antibodies that are specifically against the B antigen. The role of genetics in the ABO blood system is important to understand because it allows a better understanding of how blood transfusions work. Extra caution should thus be performed in blood typing before an individual is given supplementary blood through transfusion because it is possible that an extreme allergic reaction may occur as soon as an incompatible blood type is introduced to a patient.
It is also important to understand the in every gene, there are at least two alleles that confer different forms of a gene. Thus for the ABO system, the A antigen can be presented as a dominant (A) allele or a recessive (a) allele. An individual with a blood type A may thus carry at least one A or one a allele. At the same time, the B antigen is presented as a dominant (B) allele or a recessive (b) allele. An individual with blood type B may thus carry at least one B or one b allele. An individual with the blood type O means that he does not carry any of the A or B antigens, which are coded by the A, a, B, or b alleles.
Another important use of the ABO blood group system is for paternity testing. Although it is not possible to determine the actual father of a child, the ABO system allows to determine what blood types are impossible to obtain when given a combination of alleles from the mother and the father. For example, a parent with the blood type A can only have the following possible genotypes and phenotypes: On the other hand, a parent with the blood type B can only have the following possible genotypes and phenotypes:
Thus, in order to determine the whether a father is a possible parent of a child with blood type O, it is necessary to determine the blood types of the mother and the father and perform a Punnett square analysis of the possible blood that may be observed in the offspring. Given a case wherein a father has the A blood type and the mother has a B blood type, the following scenarios are generated: Thus for Scenario 1, if the father has a genotype of AA and the mother has a genotype of BB, then all their children should have the AB blood type.
If any child carries any other blood type, then the father can not be the biological parent of the child. In Scenario 2, wherein the father has the genotype Aa and the mother has a genotype Bb, their children can have any of the following bloodtypes—A, B andAB. If a child has the blood type O, then the father could not be the biological father. In Scenario 3, the father has the genotype AO while the mother has the genotype BO, and the children can have any of the possible phenotypes, such as A, B, AB and O and thus it is difficult to rule out the father as the wrong biological parent.