Explain sickle-cell anaemia.

Explanation

Sickle-cell anaemia. (i) It is caused by a recessive mutant allele on chromosome 11. :

(ii) The affected idividuals have sickle-shaped erythrocytes with defective baemoglobin. The gene controlling the disorder has two alleles, `Hb^A and Hb^B`. There are three possible genotypes: `Hb^A Hb^A, Hb^A Hb^s and Hb^s Hb^s`. Individuals homozygous for `Hb^s Hb^s` suffer from the disease and die very early. (iii) Heterozygous (HbA Hb) individuals appear apparently unaffected, but they are carriers of the disease, who can pass on the disorder to the next progeny. (iv) The defect is caused by substitution of glutamic acid by valine in the sixth position of the p-polypeptide of haemoglobin, the defective haemoglobin causes the sickling of RBCs. Symptoms (i) Due to aggregation of erythrocytes in the venous capillary system, several tissues suffer severe damage because of lack of oxygen, it is also known as sickle-cell crisis. (ii) Such erythrocytes are destroyed more rapidly than the normal red blood cells and that leads to anaemia. (iii) The mutant haemoglobin undergoes polymerisation under low oxygen concentration and causes sickling of erythrocytes. (iv) The person suffocates faster in crowded places and at high altitudes.