Sickle cell anemia is a genetic disorder caused by a codominant alleles that when present in the homozygous state causes a alter  in just one amino acid at two of the 574 amino acids sites of hemoglobin .this change so alters hemoglobin that the red blood cells carrying it take on a sickles shape rather than the normal disc shape. Sickle shaped red blood cells not only transport less oxygen resulting in anemia but also tend to clump together and clog small arteries . as less blood gets through these arteries the tissues and organs they nourish become impaired in their functions. May phenotypic variations occur as parts of the body fail to function in an optimal manner. Lethargy susceptibility to disease , edema and shortness of breath develop from insufficient  blood flow to the heart , kidney and spleen. Jaundice and weight loss occur from liver malfunction. Slow reflexes forgetfulness , speech defects ad personality changes become a consequence of insufficient blood flow to the brain. Thus a small change in the dna really just a single base substitution causes poor circulation and thereby significant changes in both the body and mind of the affected person. While the homozygous situation endangers the life of the individual the heterozygous state confers protection against the most dangerous form of malaria .heterozygous individuals rarely suffer sickling episodes under normal conditions. But if they contract malaria an red blood cells infected with the causative organism become sickle shaped and being defective are destroyed by the spleen.