The inherited disorders of hemoglobin, including the thalassemias, are by far the commonest monogenic diseases. Although several factors are responsible for their very high frequency, the major mechanism seems to be natural selection mediated by heterozygote protection against severe forms of malaria. Recent work has highlighted the complexity of the interplay among the different hemoglobin variants themselves and among different levels of malaria resistance, and is helping to explain the extraordinary heterogeneity in the distribution of the hemoglobin disorders even within short geographical distances. Some progress has also been made toward understanding the cellular and immune mechanisms that may underlie heterozygote protection against malaria in these conditions. In addition to providing valuable information about human evolutionary biology, work in this field has an increasingly important influence on the development of programs for the better management of the hemoglobin disorders, particularly in the poorer countries of the tropical world.
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