Serological Markers: ABO and Rh Blood Group Polymorphisms

Every time you check your blood group card, you are looking at the output of a few genes working through Mendelian inheritance. The ABO and Rh systems are the most studied of these genetic polymorphisms, and they do far more than decide who can donate blood to whom. They act as serological markers that geneticists use to trace population history, study migration patterns, and understand why some genetic traits are more common in certain regions than others. Let’s break down how these two systems work and why they matter so much in a genetics classroom and in a hospital blood bank alike.

What makes a blood group a serological marker 🔗

A serological marker is simply a genetic trait detected through blood serum reactions rather than direct DNA sequencing. Antigens sit on the surface of red blood cells, and antibodies in the plasma react with antigens they don’t recognise as “self”. Because these antigens are inherited in predictable Mendelian patterns, blood groups became one of the earliest tools population geneticists had for studying human variation, decades before DNA sequencing existed. The ABO and Rh systems remain the two most clinically and academically significant examples.

The ABO blood group system: a classic case of tri-allelic inheritance 🔗

The ABO system was discovered by Karl Landsteiner in 1901, a finding that later earned him the Nobel Prize. The system is controlled by a single gene located on the long arm of chromosome 9, at position 9q34. This gene has three major alleles, commonly written as I^A, I^B, and i (or O).

What makes ABO a favourite teaching example is its inheritance pattern. The A and B alleles are co-dominant, meaning if you inherit both, your red cells express both antigens. The O allele is recessive, since it results from a single nucleotide deletion that produces a non-functional enzyme, so it only shows up phenotypically when inherited from both parents. This combination of co-dominance and recessiveness gives rise to four distinct phenotypes: A, B, AB, and O.

At the molecular level, the A and B alleles differ by a handful of nucleotide substitutions that change the specificity of the enzyme each produces. The A allele’s enzyme adds one type of sugar to the red cell surface, the B allele’s enzyme adds a different one, and the O allele’s enzyme is essentially switched off due to a frameshift mutation. That is the entire molecular story behind four visibly different blood types.

ABO blood group polymorphism in the Indian subcontinent 🔗

ABO is one of the most extensively surveyed polymorphisms in Indian population genetics, largely because blood banks routinely record this data. A large pan-India study covering 10,000 donors across five geographical regions found that blood group O was the most common at 37.12 percent, closely followed by B at 32.26 percent, with A at 22.88 percent and AB the rarest at 7.74 percent. The calculated allele frequencies followed the order O > B > A.

Interestingly, regional and community-level studies often show B and O trading places for the top spot. A study of blood donors in northern India found the phenotype order B > O > A > AB, with allele frequencies following I^O > I^B > I^A. This is consistent with a broader pattern across South and Central Asia, where the B allele tends to sit at unusually high frequency compared to European or East Asian populations, where A or O typically dominate.

Endogamy adds another layer of variation. Tribal and caste-based populations in India often show sharper differences from the national average because they have historically married within defined groups. A study of six endogamous populations in Jammu and Kashmir found the Gujjar and Bakarwal tribal population had the highest B phenotype frequency at over 42 percent, while the Mughal population sampled in the same region showed the lowest. This kind of variation is exactly why a systematic, India-wide review of ABO and Rh(D) blood group distribution across dozens of regional studies is useful; it captures how genetic drift and founder effects have shaped different communities differently, even within one country.

These frequencies follow standard Mendelian and Hardy-Weinberg expectations, and they carry direct clinical weight. Blood banks use this data to plan inventory, since a population with unusually high B frequency needs a different stocking strategy than one dominated by O.

The Rh blood group system and the D antigen 🔗

The Rh system was identified almost four decades after ABO, when Philip Levine and Rufus Stetson described an unusual case in 1939. A woman who had just delivered a stillborn child suffered a severe reaction when transfused with her own husband’s blood, despite both being ABO compatible. Her serum was found to agglutinate red cells from the vast majority of ABO-matched donors tested, pointing to an entirely separate antigen system. A year later, Karl Landsteiner and Alexander Wiener identified a related antibody through experiments with rhesus macaques, which is where the system gets its name.

Genetically, the Rh system is more complex than ABO. It is controlled by two closely linked genes, RHD and RHCE, both located on chromosome 1. RHD codes for the D antigen, while RHCE codes for the C/c and E/e antigen pairs on the same red cell surface protein family. Whether someone is Rh-positive or Rh-negative depends entirely on the presence or absence of the D antigen, which is by far the most immunogenic antigen in the system. People who carry a functional RHD gene are Rh-positive; those with a deleted or non-functional gene are Rh-negative.

Understanding Rh haplotypes: CDe and beyond 🔗

Because RHD and RHCE sit so close together on the chromosome, they are usually inherited as a block, or haplotype, rather than as independently assorting genes. The DCe haplotype is the most common Rh haplotype in Caucasian, Asian, and Native American populations, while the Dce haplotype is slightly more frequent in populations of African descent. The frequency gap between common and rare haplotypes can be enormous. One classic dataset found the Cde haplotype at roughly 41 percent frequency compared to just 0.08 percent for CDE in the population studied.

In Indian donor populations, the D allele frequency is consistently high. A pan-India donor study reported that 94.61 percent of donors were Rh-positive, and other regional studies calculating allele frequencies through Hardy-Weinberg equilibrium have found the D allele frequency ranging from roughly 0.75 to 0.80, several times higher than the frequency of the recessive d variant. This mirrors global patterns, where D-negative frequency is generally low across Asian and African populations and considerably higher in parts of Europe.

Why serological markers matter beyond the classroom 🔗

Blood group data is not just a genetics exercise. It has three overlapping uses that make ABO and Rh some of the most practically important polymorphisms geneticists study.

Population genetics and ancestry: Because blood group frequencies vary predictably between populations, researchers have long used them alongside other genetic markers to reconstruct population history. Large-scale genetic studies of Indian population structure have confirmed earlier findings from blood group and protein polymorphism surveys that showed India’s population is structured along both geographic and social lines, shaped by endogamy over thousands of years. More recent regional studies, such as work on tribal populations in Nagaland, continue to use ABO polymorphism data to study genetic affinities between different ethnic groups and their historical relationships.

Blood transfusion safety: This is the most immediate clinical application. Mismatched ABO or Rh transfusions can trigger severe, sometimes fatal, hemolytic reactions, which is exactly the phenomenon Levine and Stetson stumbled onto in 1939. In India, the National Blood Transfusion Council under the Ministry of Health and Family Welfare sets the guidelines that blood banks follow for compatibility testing, ensuring that ABO and Rh(D) typing is confirmed before every transfusion.

Rh and pregnancy: Rh incompatibility carries a specific risk during pregnancy. If an Rh-negative mother carries an Rh-positive fetus, she can develop antibodies against the D antigen after exposure to fetal blood, putting subsequent Rh-positive pregnancies at risk of hemolytic disease of the newborn. This is precisely the clinical scenario that led to the discovery of the entire Rh system in the first place.

Organ transplantation: While tissue typing (HLA matching) is the primary compatibility test for organ transplants, ABO compatibility between donor and recipient is still checked as a first screening step, since a major ABO mismatch can trigger hyperacute rejection.

What do you think? 🔗

What do you think? Given how strongly ABO and Rh frequencies vary between Indian communities separated by geography or endogamy, what does this tell us about how social practices like caste and regional marriage patterns have shaped human genetic diversity over generations? And if a genetic trait as simple as blood group can show this much variation, what does that suggest about traits that are harder to observe directly?

References

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10563568/
2. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/abo-blood-group-system
3. https://www.ncbi.nlm.nih.gov/books/NBK2267/
4. https://journals.lww.com/ajts/fulltext/2014/08020/abo_and_rh__d__group_distribution_and_gene.13.aspx
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5958589/
6. https://www.sciencedirect.com/science/article/abs/pii/S1473050214000160
7. https://onlinelibrary.wiley.com/doi/abs/10.1111/voxs.12576
8. https://www.ncbi.nlm.nih.gov/books/NBK594252/
9. https://www.sciencedirect.com/topics/immunology-and-microbiology/rh-blood-group-system
10. https://www.ncbi.nlm.nih.gov/books/NBK2269/
11. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/blood-group-rh
12. https://reich.hms.harvard.edu/sites/reich.hms.harvard.edu/files/inline-files/2009_Nature_Reich_India.pdf
13. https://www.sciencedirect.com/science/article/pii/S3050474025000266
14. https://naco.gov.in/national-blood-transfusion-council-nbtc-0