Drepanocytic anaemia --> sickle cell anaemia
The cause is a point mutation in the allele that codes for the beta chain of haemoglobin with a substitution of (valine for glutamic acid at position 6. The defective haemoglobin (HbS) crystallizes readily at low oxygen tension.
in consequence, erythrocytes from homozygotes change from the normal discoid shape to a sickled shape when the oxygen tension is low and these sickled cells become trapped in capillaries or damaged in transit, leading to severe anaemia.
in heterozygotes, the disadvantages of the abnormal haemoglobin are apparently outweighed by increased resistance to plasmodium falciparum malaria, probably because parasitised cells tend to sickle and are then removed from circulation.