Sickle cell anaemia is a genetic disorder caused by a variant form of haemoglobin in red blood cells. Individual homozygous for this disorder die at an early stage. In heterozygotes also the redblood cells with variant form of haemoglobin become sickle-shaped and were unable to bind oxygen efficiently. Even then the natural selection could not eliminate this abnormal allele from the population. This variant is often found in tropical Africa where malaria is common. Since sickle-shaped RBC effectively kills the malaria parasite, the individuals heterozygous for this variant are able to cope with malaria infections much better than normal persons. The natural selection thus maintains this variant form of haemoglobin along with the normal haemoglobin in malaria prone areas.
Pernicous anaemia is caused due to deficiency of vitamin $\mathrm{B}_{12}$. Leukemia is cancer of the body's blood-forming tissues, including the bone marrow and the lymphatic system. Thalassemia is caused by mutations in the DNA of cells that make hemoglobin.