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Blood clotting is a favourite source of one-mark questions because it is a sequence: get one step out of order and the answer is wrong. It also links to genetics through haemophilia. This post lays out the NCERT sequence, explains why biologists call it a cascade, and works through a haemophilia cross with probabilities.
This post is about biology for exams. For any bleeding or clotting concern, see a doctor.
What does the syllabus include?
The NEET (UG) 2026 syllabus lists "Composition of blood, blood groups, coagulation of blood" under Body fluids and circulation, and "Sex linked inheritance-Haemophilia" under genetics. The CBSE curriculum for 2026-27 has both. The text to learn is section 15.1.4 of the NCERT chapter on body fluids and circulation. Check the current year's syllabus before your exam.
What are the ingredients?
From the NCERT chapter:
Plasma is about 55 per cent of blood. Its major proteins are fibrinogen, globulins and albumins, and fibrinogen is needed for clotting.
Clotting factors are present in plasma in an inactive form. Plasma without them is serum.
Platelets, or thrombocytes, are cell fragments produced from megakaryocytes in bone marrow. Blood normally has 1,50,000 to 3,50,000 platelets per cubic millimetre. Too few can cause clotting disorders and excess blood loss.
Calcium ions play a very important role in clotting.
What is the sequence from injury to clot?
An injury stimulates platelets to release factors that start coagulation. Factors from the damaged tissue can also start it.
A series of linked enzyme reactions, a cascade involving many inactive plasma factors, forms an enzyme complex called thrombokinase.
Thrombokinase converts inactive prothrombin into the enzyme thrombin.
Thrombin converts soluble fibrinogen into insoluble fibrin.
Fibrin threads form a network that traps dead and damaged formed elements. That is the clot, the dark reddish-brown scum you see on a cut.
Why is it called a cascade?
In 1964 two papers proposed the idea. Macfarlane in Nature called it "an enzyme cascade in the blood clotting mechanism, and its function as a biochemical amplifier". Davie and Ratnoff in Science proposed a "waterfall sequence" in which each clotting factor except fibrinogen is converted into a form with enzyme activity, which then activates the next.
The point is amplification. Here is an illustration with invented numbers. If each active enzyme molecule activates 100 molecules of the next factor, then 1 molecule at step one gives 100 at step two, 100 x 100 = 10,000 at step three, and 100 x 10,000 = 1,000,000 at step four. A tiny signal at the wound becomes a large response.
Researchers now also stress the role of cell surfaces. Hoffman and Monroe (2001) proposed that coagulation happens in three overlapping stages: initiation on cells bearing tissue factor, amplification in which platelets are activated, and propagation in which large amounts of thrombin are made on the platelet surface. For your exam, the NCERT sequence is what counts; the newer model explains why platelets appear at every step.
How is haemophilia inherited?
According to MedlinePlus Genetics, haemophilia A is caused by variants in the F8 gene, which makes clotting factor VIII, and haemophilia B by variants in F9, which makes factor IX. Both are inherited in an X-linked recessive pattern. Haemophilia A affects about 1 in 4,000 to 1 in 5,000 males born worldwide, and haemophilia B about 1 in 20,000.
Worked example: a carrier mother and an unaffected father
Write the normal allele as X^H and the haemophilia allele as X^h.
Parents: mother X^H X^h (carrier), father X^H Y.
Gametes: mother X^H or X^h, each 1/2; father X^H or Y, each 1/2.
Children: X^H X^H (normal daughter), X^H X^h (carrier daughter), X^H Y (normal son), X^h Y (affected son), each 1/4.
Among sons: 1/2 are affected.
Among daughters: none affected, 1/2 carriers.
Probability that the next child is an affected son: 1/4.
Read the question carefully: "probability that a son is affected" (1/2) is not the same as "probability that a child is an affected son" (1/4).
Common mistakes
Confusing serum and plasma. Serum is plasma without the clotting factors.
Calling platelets cells. NCERT calls them cell fragments from megakaryocytes.
Putting thrombin before thrombokinase. Thrombokinase forms first and makes thrombin.
Forgetting calcium. It is needed for clotting.
Mixing up the two haemophilia probabilities in the worked example.
A practice routine
Write the five-step sequence from memory, then check it against NCERT.
Make a list of the plasma proteins and one function each.
Redo the haemophilia cross with an affected father and a non-carrier mother.
Revise sex-linked crosses in our post on linkage and crossing over.
If a cross keeps going wrong at the same step, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Tonight, draw the clotting sequence as a flow chart from memory and mark where calcium is needed.
