Explanation

Genetic or chromosomal symbol used for person who is having sickle-cell anaemia Ps − Hb S HbS . 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 SHbS show the diseased phenotype (iv) The heterozygous individuals are carriers (Hb ୅HbS ) (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) 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