Class 12 Biology Inheritance Variation SICKLE-CELL ANAEMIA


  • Autosome linked recessive trait
  • It can be transmitted from parents to the offspring when both the parents are carrier for the gene.
  • Disease is controlled by a single pair of allele, HbA and HbS.
  • HbS HbS homozygous shows the diseased phenotype.


Pic shows the difference between normal and sickle cells.


  • Heterozygous individuals HbA HbS show normal phenotype but they are carrier of the disease.
  • The defect is caused due to substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. It results from single base substitution from GAG to GUG at sixth codon of the beta globin.
  • Due to this, mutant haemoglobin is formed. It undergoes polymerization under low oxygen tension causing the change in the shape of the RBCs from biconcave to elongated sickle-like.

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