Sickle cell disease

Sickle cell disease is an autosomal recesive genetic disorder which results in misshaping of red blood cells. The cells won't have an oval shape, but will be shaped as a crescent.
This change is mediate by a mutation in a gene encoding hemoglobin. In this mutation glutamic acid, in the sixth position of the beta hemoglobin chain, is replaced by valin. This drastically changes the physical properties of hemoglobin, since glutamic acid is an hydrophil aminoacid, whereas valin is hydrophobic.
Homozygotes carry two recessive alleles: HbS/HbS, heterozygotes only one HbsA/HbsS and healthy people are HbA/HbA. This disease is very common mainly in tropic or subtropic areas, since heterozygotes for the HbS/HbA gene are resistant against malaria.