Sickle cell anaemia is a biochemical disorder inherited as recessive trait. In normal haemoglobin a dominant gene codes for glutamic acid monomer at sixth position in β-chain. The recessive allele of this gene codes for the insersion of amino acid valine in place of glutamic acid at the sixth position of ( β -chain. In low oxygen tension valine monomers at first and sixth position become associated in such a way that the haemoglobin molecule become sickle shaped. Such defective haemoglobin is useless for oxygen.
Some genes regulate many phenotypic characters. The ability of a gene to have many effects is called pleiotropy. The trait of sickle cell anaemia forms a famous example of pleiotropy. Thus, the mutant allele Hbs causes the production of abnormal haemoglobin. As a consequence the shape of the RBC containing it becomes distorted and sickle shaped. Homozygous individuals normally die early in life due to severe anaemia caused by premature destruction of the sickled red blood cells. But heterozygotes who have both normal (haemoglobin A) and abnormal (haemoglobin S) haemoglobins and who also have mild anaemia, are naturally protected against contracting malaria as the parasite cannot live in these distorted cells. Heterozygotes may, therefore, survive better in region where malaria is endemic. Such population have both normal individuals and individuals heterozygous for the gene. The latter act as carriers of the gene from generation too.