The primary defect which leads to sickle

The primary defect which leads to sickle cell anemia is:

1.An abnormality in porphyrin part of hemoglobin.
2.Replacement of glutamate by valine in B-chain of HbA.
3.A nonsense mutation in the B-chain of HbA.
4.Substitution of valine by glutamate in the a-chain of Hba.

Ans. 2