Explanation

In sickle-cell anaemia only one nucleotide substitution takes place from T to A. Genetic or chromosomal symbol used for person who is having sickle-cell anaemia $Ps - Hb^{s}\ Hb^{s}$. Sickle-cell Anaemia (i) It is an autosome-linked recessive trait (ii) The disease is controlled by a single pair of allele $Hb^{s}$ and $Hb^{s}$ (iii) Only the homozygous individuals for $Hb^{s}$, i.e., $Hb^{s}Hb^{s}$ show the diseased phenotype (iv) The heterozygous individuals are carriers ($Hb^{A}Hb^{S}$) (v) Due to point mutation, glutamic acid (Glu) is replaced by valine (Val) at the sixth position of $\beta$-globin chain of haemoglobin molecule (vi) A single base substitution at sixth codon of the beta globulin gene from GAG to GUG. GAG code for glutamic acid and GUG code for valine. (vii) $Hb^{S}$ behaves as normal haemoglobin except under the 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