(-), Sickle-cell anaemia is an autosome linked recessive trait which is controlled by a single pair of allele $\mathrm{HbA}$ and Hbs only the homozygous individuals for Hbs, i.e., Hbs Hbs shows the diseased phenotype. The heterozygous individuals are carriers ( $\mathrm{HbA} \mathrm{Hbs}$ ). Glutamic acid (Glu) is replaced by valine (Val) at sixth position of $\beta$-chain of heamoglobin molecule due to point mutation. This substitution occurs due to the single base substitution of the beta globin gene from GAG (Glu) to GUG (Val). Whereas, the other codes GGG, AAG, GAA do not codes for valine.