Sickle-cell anaemia 

i. It is an autosome-linked recessive trait. 

ii. The disease is controlled by a single pair of allele Hb^A and HbS. 

iii. Only the homozygous individuals for Hb^S, i.e., Hb^SHb^S show the diseased phenotype. 

iv. The heterozygous individuals are carriers (Hb^AHb^S). 

v. Due to point mutation, glutamic acid (Glu) is replaced by valine (Val) at the sixth position of β-globin chain of haemoglobin molecule. 

vi. HbS behaves as normal haemoglobin except under oxygen stress where erythrocytes lose their circular shape and become sickle-shaped. As a result, the cells cannot pass through narrow capillaries. Blood capillaries are clogged and thus affect blood supply to different organs

Thalassemia 

i. It is an autosome-linked recessive disease. 

ii. It occurs due to either mutation or deletion resulting in reduced rate of synthesis of one of globin chains of haemoglobin. 

iii. Anaemia is the characteristic of this disease. 

iv. Thalassemia is classified into two types: 

▪ α-thalassemia—Production of α-globin chain is affected. It is controlled by the closely linked genes HbA1 and HbA2 on chromosome 16. It occurs due to mutation or deletion of one or more of the four genes. 

▪ β-thalassemia—Production of β-globin chain is affected. It occurs due to mutation of one or both HbB genes on chromosome 11.