Sickle Cell Anemia: Autosomal hereditary disorder in which the erythrocytes become sickle shaped under oxygen deficiency as during strenuous exercise and at high altitudes. The disorder is caused by the formation of an abnormal haemoglobin called haemoglobin-S. Haemoglobin-S differs from normal haemoglobin-A in only one amino acid- 6th amino acid of b-chain, glutamic acid is replaced by valine.
- During conditions of oxygen deficiency 6-valine forms hydrophobic bonds with complementary sites of other globin molecules.
- It distorts their configuration. As a result, erythrocytes having haemoglobin–S become sickle-shaped. The cells cannot pass through narrow capillaries. They have a tendency to clot and degenerate.
- Blood circulation and oxygen supply are disturbed. Spleen and brain gets damaged.
- The patient feels acute physical weakness.
- The homozygous having only haemoglobin –S usually die before reaching maturity because erythrocyte distortion and degeneration occur even under normal oxygen tension.
- Despite having harmful effect, the allele for sickle cell anemia continues to persist in human population because it has survival value in malaria infested areas like tropical Africa.
- Malarial parasite is unable to penetrate the erythrocyte membrane and cause any harm.
Autosomal Recessive Disorder: Diseases that have autosomal recessive inheritance affect only individuals who have two defective copies of the gene (one from each parent).