Revision summary
ABO lies on chromosome 9 and encodes glycosyltransferases acting on H antigen. Iᴬ and Iᴮ are codominant; both dominate common O. Bombay phenotype hh lacks H antigen and demonstrates epistasis. Rh is a chromosome-1 complex centred on RHD and RHCE, not literally one allele pair. Maternal IgG anti-D can cause haemolytic disease after sensitisation; prophylaxis prevents most cases. Blood-group frequencies trace populations but cannot define races or prove unique paternity.
Model answer
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Introduction
ABO and Rh are inherited red-cell antigen systems and classic markers of human variation. ABO illustrates multiple alleles and codominance; Rh, especially the D antigen, illustrates a genetically complex system with major maternal–foetal significance.
Body
ABO system
Karl Landsteiner discovered ABO agglutination. The ABO locus on chromosome 9 encodes glycosyltransferases. Iᴬ adds N-acetylgalactosamine to the H antigen; Iᴮ adds galactose; common i/O alleles produce inactive enzyme. Iᴬ and Iᴮ are codominant and both dominate O:
- A phenotype: IᴬIᴬ or Iᴬi; anti-B in plasma.
- B: IᴮIᴮ or Iᴮi; anti-A.
- AB: IᴬIᴮ; both antigens and neither antibody.
- O: ii; neither A nor B antigen and both antibodies.
A and B parents can therefore have an O child only if both are heterozygous; paternity can sometimes be excluded, never uniquely proved, by ABO alone. Bombay phenotype (hh), first reported in Mumbai, lacks H antigen and can type deceptively as O—an important epistatic exception.
Rh system
The Rh complex lies on chromosome 1, principally RHD and RHCE. Presence of D usually defines Rh-positive; absence or inactivation defines Rh-negative. “Positive dominant over negative” is a useful pedigree shorthand, but C/c and E/e antigens, variant D alleles and gene structure make Rh more complex than one two-allele locus.
Unlike naturally occurring ABO antibodies, anti-D usually follows sensitisation. An Rh-negative mother carrying an Rh-positive foetus may form IgG anti-D; in a later incompatible pregnancy it can cross the placenta and cause haemolytic disease of the foetus/newborn. Anti-D immunoglobulin prophylaxis greatly reduces this risk.
ABO and Rh frequencies helped early population anthropology map clines and affinity, including Indian caste and tribal samples. Drift, founder effect, selection and gene flow shape frequencies, but no blood group defines a race. DNA markers now provide finer histories.
Flow diagram
Conclusion
ABO is a clean example of multiple allelism, codominance and epistasis; Rh is a multi-antigen system whose D incompatibility matters in pregnancy. Both remain valuable teaching and clinical markers, but population frequencies are not racial essences.
Quick related
Students also ask
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Polygenic Inheritance
Next question on this syllabus topic (2023 · Q5(a)). View answer →
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Is Rh inheritance simply D dominant and d recessive?
That shorthand predicts many pedigrees, but the real RHD/RHCE system includes several antigens and variant alleles.
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Can ABO prove paternity?
It can exclude some alleged fathers but is far too common to establish unique paternity; STR DNA is required.
Same topic · past papers
UPSC has asked this before
These previous-year questions sit on the same topic. Open one to practise the earlier ask.
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2023 · Q5(a) · Anthropology GS 1 · 10 marks
Polygenic Inheritance
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