It is an autosomal hereditary disease characterized by sickle-shaped (crescentic) red blood corpuscles under low oxygen content. It is caused by the substitution of valine for glutamine at the sixth position in the β chain of globin molecule of haemoglobin. The mutant haemoglobin molecule undergoes polymerization under low oxygen tension causing the change in the shape of the RBC from biconcave disc to an elongated sickle-like structure. The sickled red blood cells slowly creep in to and aggregate in the venous side of capillary systems. There the sickled erythrocytes are destroyed. This leads to two defects. First, damage to tissues and organs due to blocking of blood capillaries. Second, haemolytic anaemia– Besides, ulcerations appear on the lower extremities. There are attacks of abdominal pains and joint pains or arthralgia. Defects of various organs due to non-receipt of food and oxygen supply are in addition to these. The heterozygous individuals are the carriers of this disease. Marriage between two carriers gives rise to affected, carriers and normal children in the ratio 1:2:1. About half of the affected individuals die after attaining the age of 20. The affected women give birth to fewer individuals. Through genetic counselling, the incidence of metabolic disorders can be reduced.
Haemophilia (Bleeder’s Disease):
It is a sex-linked recessive disorder discovered by John Otto in 1803. The defect is known as bleeder’s disease because the exposed blood of haemophiliac patient fails to coagulate due to deficiency of blood clotting factor VIII (antihaemophilia globulin) or factor IX (plasma thromboplastin). The two types of haemophilia are respectively called haemophilia A and haemophilia B. This disease is transmitted from unaffected carrier female to male progeny. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female to become haemophilic is extremely rare because the mother of such a female has to be at least carrier and the father should be haemophilic. The family pedigree of Queen Victoria has a number of haemophilic descendants as she was a carrier of the disease.