Sickle-cell anaemia is caused due to the substitution of :
A. Valine by glutamic acid at sixth position of alpha chain of heamoglobin
B. Valine by glutamic acid at sicth position of beta chain of haemoglobin
C. glutamic acid by valine at sixth position of alpha chain of haemoglobin
D. glutamic acid by valine at sixth position of deta chain of haemoglobin
