Updated by: Arabinda Naik
It is an autosomal linked recessive trade that can be transmitted from parent to offspring, when both the partners are carrier for the gene(heterozygous). This disease is controlled by a single pair of alleles HbA and HbS. Only homozygous individuals for HbS that is HbS HbS show the diseased phenotype. The heterozygous individuals our career HbA HbS. It is caused by the substitution of glutamic acid Glu by valine (Val) at the 6th position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to single base substitution at the 6th codon from GAG to GUG (which is also an example of point mutation). The mutant haemoglobin molecule undergoes polymerization under low oxygen tension causing the change in the shape of the RBC from biconcave disc to the elongated sickle like structure. Its gene is located on 11th chromosome. This defect is beneficial to people living in malaria prone area in Africa.
Sex linked dominant: Defective tooth enamel.
Sex linked recessive: Colour blindness, haemophilia.