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General principles: The pathophysiology of beta thalassemia depends absolutely on the amount of excess unpaired alpha globin chains. Therefore any genetic variant that increases or decreases the amount of these unpaired alpha chains (eg, by altering the rate of alpha or beta chain production, or by substituting for the missing beta chains) will modify the phenotype. Disorders that present as β-thalassemia intermedia, rather than as the major or minor variants, are listed below: Homozygosity for mild forms of β+ thalassemia Compound heterozygosity for β+/βº thalassemia Compound heterozygosity for β thalassemia andanother beta chain variant (eg, β-thal/Hgb E) Coinheritance of homozygous β thalassemia withgenes for increased gamma chain synthesis (ie, HPFH) Coinheritance of homozygous β+ thalassemiawith alpha thalassemia (eg, β+/β+ with -a/-a, --/aa,-a/aa, or --/-a) Coinheritance of heterozygous β thalassemiaand triplicated or quadruplicated alpha genes (eg, aa/aaa or aa/aaaa) Dominant forms of beta thalassemia β º thalassemia: no production of beta chains; β + thalassemia: reduced production of beta chains (may be mild, moderate, or severely reduced); HPFH: Hereditary persistence of fetal hemoglobin. Table provided by Stanley L. Schrier, MD.