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99% of beta-thalassaemia genes have been characterised at the DNA level in terms of DNA deletions or point mutations. These DNA changes result in defects in transcription control, RNA processing and translation.
The beta-thalassaemias can be classified as beta0 or beta+. In beta0 thalassaemia no normal beta-chains are produced whilst in beta+ thalassaemia beta-chain production is impaired.
Since alpha-chain production is normal there is an excess of alpha-chains. These alpha-chains combine with available delta- and gamma-chains resulting in increased levels of Hb-A2 and Hb-F respectively.
The nature and severity of symptoms is dependent upon the level of beta-globin production and the amount of Hb-A2 and Hb-F present. This is in part dependent upon whether the individual is homozygous or heterozygous for a beta0 or beta+ allele.
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