A patient with thalassemia major is described in which two types of thalassemia genes are present, one that reduces β-chain synthesis and the other that suppresses it completely. The identification of these two types of β-thalassemia genes is based on the presence or absence of HbA in other members of the same family who are also HbC carriers. The genetic implications of the interaction of these two thalassemia genes are discussed in view of recent theories on the nature of the molecular defect characteristic of each type of thalassemia.

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