It occurs due to an autosomal recessive gene, that expresses in homozygous condition.
Two carrier or heterozygous parents can produce an affected child.
This genetic defect originated in Mediterranean region due to mutation or deletion in the genes controlling the synthesis of globin chains of Haemoglobin. It also occurs in the middle east, Indian subcontinent & South East Africa.
It is a Quantitative Inheritance.
Adult hemoglobin is a heterotetramer.
It consists of four polypeptide chains, 2 α & 2 β types.
Reduced synthesis of one type of chain causes an excess formation of the other type. Free globin chains that are left forms abnormal tetramers. They being insoluble accumulate in RBC forming precipitates in it, damaging RBC and causing its lysis. This results in anemia.
It is of 3 types:
α, β, δ – Thalassemia
α – Thalassemia:
- It occurs due to defect in the synthesis of α globin chain of Haemoglobin.
- Its synthesis is controlled by 2 genes: HBA1 & HBA2 present in the 16th chromosome i.e., 4 alleles of the two genes.
- α – thalassemia minor occurs due to defect in one allele.
- A defect in 3 alleles: leads to anaemia, jaundice, pale skin, bone changes & hepatosplenomegaly.
- A defect in 4 alleles: causes death of the foetus.
β – Thalassemia:
- It occurs due to defect in the synthesis of β globin chain of Haemoglobin.
- Its synthesis is controlled by HBB gene present in the 11th chromosome.
- A defect in one allele is less severe as some β globin chains are formed.
- A defect in both alleles is most severe form as it prevents the formation of β – globin chains, it is β – thalassemia major.
- It causes severe hemolytic anaemia, cardiac enlargement, skeletal deformities, pale skin & hepatosplenomegaly.
δ – Thalassemia:
- It occurs due to defect in HBD gene present of the 11th chromosome, that forms δ chain of haemoglobin.
- 3 % of adult haemoglobin has α & δ chains.