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Every hospital in India matches blood before a transfusion, and every NEET aspirant learns the ABO table. Both go back to one simple experiment in Vienna around 1900. This post explains what Karl Landsteiner did, why it mattered, and how his idea turns into the genetics questions you will see in your exam.
Why did early blood transfusions fail?
Doctors had tried transfusion for centuries. Sometimes the patient recovered; sometimes the patient went into shock, became jaundiced or passed haemoglobin in the urine, and died. Nobody knew why. The Nobel Prize biography of Landsteiner notes that in 1875 Landois had already shown that red cells from animals clumped and broke up when transfused into humans. The puzzle was why blood from one human could harm another.
What was Landsteiner's experiment?
Landsteiner described it himself in his Nobel Lecture of 11 December 1930. He wanted to know whether individuals within one species differ chemically, as species differ from each other. So he chose "the simplest experimental arrangements available": he took the blood serum and the red cells of different people and let each serum react with each set of cells.
The result surprised him. Many mixtures showed no change. In others, the serum made the red cells of the other person clump together, a reaction called agglutination, and it was as strong as the reactions between blood of different animal species. The reactions followed a regular pattern that held for all human blood.
The Nobel Prize facts page sums it up: he explained in 1901 that people have different types of red blood cells, that is, different blood groups, and the discovery led to safe transfusions between people with compatible blood. He received the 1930 Nobel Prize in Physiology or Medicine "for his discovery of human blood groups".
Why are there exactly four groups?
In his lecture Landsteiner gave the logic clearly. Red cells carry substances of two different structures (we call them antigens A and B). A person can have neither, one or both, which gives four groups. And every serum contains the antibodies against whichever antigens that person's own cells lack.
The NCERT Class 11 chapter on Body Fluids and Circulation sets out the same rule:
Group A: antigen A on red cells, anti-B in plasma.
Group B: antigen B, anti-A.
Group AB: both antigens, no antibodies. Can receive from all groups.
Group O: no antigens, both antibodies. Can give to all groups.
How do you turn this into a genetics problem?
The NCERT Class 12 chapter on Principles of Inheritance and Variation uses ABO as its example of both co-dominance and multiple alleles. One gene, I, has three alleles: I^A, I^B and i. I^A and I^B are both dominant over i, and when they occur together both are expressed, giving group AB.
Worked example. A mother is group A with genotype I^A i, and a father is group B with genotype I^B i. What blood groups can their children have?
Mother's gametes: I^A or i, each with probability 1/2.
Father's gametes: I^B or i, each with probability 1/2.
Combine them: I^A I^B (AB), I^A i (A), I^B i (B), ii (O). Each has probability 1/2 × 1/2 = 1/4.
So this couple can have children of all four groups, each with a chance of 1 in 4. Landsteiner himself suggested early on, the Nobel biography notes, that because blood groups are inherited they could help decide cases of doubtful paternity.
Common mistakes:
Writing group A as always I^A I^A. A group A person can be I^A I^A or I^A i; read the question for clues such as an O child.
Calling ABO "incomplete dominance". AB shows both antigens fully, so it is co-dominance.
Mixing up antigens and antibodies in the donor table. Antigens are on cells; antibodies are in plasma.
What else did Landsteiner discover?
He was far from a one-discovery scientist. The Nobel biography records that he showed poliomyelitis could be transmitted to monkeys, work that helped lay the foundations for understanding polio, which later made the polio vaccines possible. He also identified and named the immunological factors called haptens.
At the Rockefeller Institute in New York he worked with Philip Levine and Alexander Wiener on further blood groups. With Wiener he published a 1940 paper on an agglutinable factor recognised by immune sera for rhesus blood, the Rh factor. NCERT notes that nearly 80 per cent of humans are Rh positive and explains why Rh incompatibility matters in pregnancy. The Nobel biography records that he died two days after a heart attack in his laboratory in June 1943, "pipette in hand".
What can a student learn from Landsteiner?
Start with the simplest experiment that can answer the question. Mixing serum and cells from a few people needed no special equipment, only a clear question.
Look for the pattern, then the rule. He did not stop at "some samples clump". He found the rule that explains every result, and the rule is what you should learn too.
Connect chapters. ABO links circulation (Class 11) with genetics (Class 12). Questions often sit on that bridge.
For practice on crosses like the one above, work through our guide to genetics problems for NEET. If you get a cross wrong and cannot see why, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Try it now: work out the possible blood groups of children of an AB parent and an O parent, then check that none of them can be AB or O.
