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Genetics of ABO and Rh Blood Groups — 2024 Paper I
Question · 2024 · Paper I · 15 Marks
Describe the genetics and inheritance patterns of the ABO and Rh blood groups in man.
Model Answer
VAID ICSIntroduction
Blood groups are inherited antigenic variations present on red blood cells. The ABO system, discovered by Karl Landsteiner in 1900–01, and the Rh system, described by Landsteiner and Wiener in 1940, are important in transfusion medicine, pregnancy, population genetics and medico-legal studies.
ABO Blood-Group System
The ABO system is controlled by the ABO gene on chromosome 9. It has three principal alleles:
IA, IB and i
IA and IB are codominant, while both are dominant over i. Thus, the system illustrates multiple allelism, codominance and Mendelian segregation.
The ABO gene codes for glycosyltransferase enzymes acting on the precursor H substance:
- IA adds N-acetylgalactosamine, producing A antigen.
- IB adds galactose, producing B antigen.
- i produces an inactive enzyme, leaving H substance unchanged.
| Phenotype | Genotype | RBC antigen | Plasma antibody |
|---|---|---|---|
| A | IAIA or IAi | A | Anti-B |
| B | IBIB or IBi | B | Anti-A |
| AB | IAIB | A and B | None |
| O | ii | Neither A nor B | Anti-A and Anti-B |
ABO Inheritance
Each parent transmits one allele.
For example:
IAi × IBi
may produce:
- IAi — Group A
- IBi — Group B
- IAIB — Group AB
- ii — Group O
Therefore, all four ABO phenotypes may occur among the children of heterozygous A and B parents.
An AB parent and an O parent normally produce only A or B children, while two O parents normally produce only O children.
Bombay Phenotype
Expression of A and B antigens requires H substance, controlled by the FUT1 gene on chromosome 19. Individuals with genotype hh cannot express A or B antigens even when they possess IA or IB alleles.
This Bombay phenotype (Oh) illustrates recessive epistasis, in which hh masks the ABO genotype.
Rh Blood-Group System
The Rh system contains several antigens, particularly D, C, c, E and e. These are mainly controlled by the closely linked RHD and RHCE genes on chromosome 1.
Routine Rh status depends chiefly on the D antigen:
- D antigen present — Rh-positive
- D antigen absent — Rh-negative
In the simplified Mendelian model:
| Genotype | Phenotype |
|---|---|
| DD or Dd | Rh-positive |
| dd | Rh-negative |
Thus:
- Dd × dd gives approximately 1 Rh-positive : 1 Rh-negative.
- Dd × Dd gives approximately 3 Rh-positive : 1 Rh-negative.
- dd × dd produces only Rh-negative children.
However, molecularly, Rh-negative status commonly results from deletion or alteration of the RHD gene, rather than from a universal recessive d allele.
Rh Incompatibility
Unlike ABO antibodies, anti-D is not normally present naturally. It develops after exposure to Rh-positive red cells.
An Rh-negative mother carrying an Rh-positive foetus may become sensitised. In a later Rh-positive pregnancy, maternal IgG anti-D antibodies may cross the placenta and cause haemolytic disease of the foetus and newborn. It is largely prevented by administering anti-D immunoglobulin.
Combined Inheritance
ABO and Rh genes lie on different chromosomes and generally assort independently. Their combination produces eight common phenotypes: A+, A−, B+, B−, AB+, AB−, O+ and O−.
Conclusion
The ABO system demonstrates multiple allelism, codominance and epistasis, whereas the Rh system demonstrates inheritance of closely linked antigen-producing genes, particularly RHD. Their study has major significance in transfusion compatibility, maternal–foetal health, population variation and parentage exclusion.
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