- Antigens are molecules that elicit the formation and release of antibodies from the immune system. The reaction between the antibodies and the antigens leads to agglutination i.e. the cells clump together.
- Blood group systems can be ABO, Rhesus factor, Lewis, Duffy, etc. Here we will discuss the ABO system which is classifying the blood based on the carbohydrate antigens on the surface of the red blood cells.
It is important to remember that the ABO blood group gene is a polymorphic one. This is because it has more than two alleles (three) in the general population.
Genetics and Inheritance of the ABO blood group system
As mentioned above, the ABO gene has three alleles in the population:
- The IA allele that codes for the 'A' antigen
- The IB allele that codes for the 'B' antigen
- The i allele that doesn't code for any antigen
It is important to note that the IA and IB alleles are codominant and both of them are dominant to the i allele which is recessive(details on dominance and codominance are discussed in the Transmission Genetics: Mendelian Genetics: Introduction and important terms course). Fortunately (or otherwise), this is not the end of the story.
The ABO gene is under recessive epistasis from the H gene(details on recessive epistasis are discussed in the Transmission Genetics: Mendelian Genetics: Epistasis course). This means that expression of the ABO gene is dependent on the expression of the H gene i.e. if the H gene is expressed, the ABO gene will be expressed and vice versa. The H gene is non-polymorphic and has two alleles; a dominant H and a recessive h. Out of the two H alleles present in a person, one of them must