Question

what is the difference between haemoglobin a and haemoglobin S

Answer 1

Well, hemoglobin A is a normal human hemoglobin.
It does what it's supposed to:
it helps our red blood cells carry oxygen.

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On the other hand,
hemoglobin S is an ABNORMAL hemoglobin. It's not normal.
Hemoglobin S causes our red blood cells to sort of curl up into an S shape.
And when this happens, the red blood cells have trouble carrying oxygen.

We call this condition "sickle cell anemia."

That's why it's called Hemoglobin S.
S is for Sickle Cell.