The Occurrence and Identification ofα-Thalassemia-2 among Hemoglobin S Heterozygotes

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Abstract
The in-vitro synthesis of hemoglobin (Hb) chains was studied among 60 Hb S heterozygotes (AS) having different quantities of Hb S, including five with an associated α-chain heterozygosity (ASAG). Hematologic values and hemoglobin composition were studied in these cases and in 15 other ASAG heterozygotes. The percentages of Hb S (which fell between 27% and 42%) and the mean corpuscular volume values correlated directly with the α/non-α values, confirming previous suggestions (Huisman, Hemoglobin 1:349, 1977) that the concomitant occurrence of an α-thalassemia-2 heterozygosity (αα0/αα; β/βs) or homozygosity (α0α/α0α; β/βs) resulted in intermediate or lower levels of Hb S compared with Hb S heterozygotes having four active α-chain genes (αα/αα; β/βs). Among ASAG heterozygotes, the occurrence of low (about 25%), intermediate (about 33%), or high (about 45%) proportions of an α-chain variant resulting from a variability in the number of active α-chain genes due to α-thal-2 coincided with high (39%), intermediate (34%), or low (28%) levels of Hb S, respectively. However, the overlap of biosynthetic data between Hb S heterozygotes with four, three, or two active α-chain genes prevents a reliable diagnosis in individual cases.