Introduction
Blood groups in humans are determined by specific antigens present on the surface of red blood cells (RBCs). These antigens are inherited according to Mendelian principles. The two most important blood group systems are the ABO system and the Rh factor system. Understanding the genetics of blood groups is essential for blood transfusion, paternity tests, and population genetics.
1. The ABO Blood Group System
Genetic Basis
The ABO system is controlled by a single gene I (isoagglutinin) located on chromosome 9. It has three alleles:
- IA → produces antigen A on RBCs.
- IB → produces antigen B on RBCs.
- i → produces no antigen.
Allelic Relationship:
- IA and IB are codominant (both expressed when present together).
- i is recessive to both IA and IB.
Genotypes and Phenotypes
| Genotype | Phenotype (Blood Group) | Antigens on RBCs | Antibodies in Plasma |
|---|---|---|---|
| IAIA or IAi | A | A antigen | Anti-B |
| IBIB or IBi | B | B antigen | Anti-A |
| IAIB | AB | A and B antigens | None |
| ii | O | No antigen | Anti-A and Anti-B |
Significance: In transfusion, blood group compatibility must be checked to avoid agglutination (clumping) caused by antigen-antibody reactions.
2. The Rh Factor System
Genetic Basis
The Rh system is determined by a gene located on chromosome 1. The most important antigen is the D antigen.
- Presence of D antigen → Rh positive (Rh+).
- Absence of D antigen → Rh negative (Rh-).
Inheritance:
- Rh+ is dominant over Rh-.
- Genotypes: Rh+Rh+ or Rh+Rh- → Rh positive.
- Rh-Rh- → Rh negative.
Medical Importance
- Hemolytic Disease of the Newborn (Erythroblastosis Fetalis): Occurs when an Rh-negative mother carries an Rh-positive fetus. Maternal antibodies may attack fetal RBCs in subsequent pregnancies, leading to anemia and jaundice in the newborn.
- Prevented by injecting the mother with Rho(D) immunoglobulin (RhoGAM).
3. Other Blood Group Systems
Although ABO and Rh are most important, there are over 30 additional systems such as MNS, Kell, Duffy, and Lewis. These also have clinical significance in transfusions and organ transplantation.
- The ABO system is based on multiple allelism (IA, IB, i) and codominance.
- The Rh system is based on the presence or absence of D antigen, with Rh+ being dominant.
- Both systems are critical for safe blood transfusion and understanding genetic inheritance patterns.
