The increasing recognition that more than one gene locus can be involved in the genetic control of a polypeptide chain has forced the reevaluation of some accepted ideas. There are no available data suggesting the presence of nonallelic genes for human β- and δ-chains. Detailed evidence supports the existence of multiple nonallelic structural genes for the human γ-chain. Most mammals examined have at least two kinds of α-chains differing by one or more residues. The duplication of the human α-chain gene was postulated as the simplest explanation for the heterogeneity of α-thalassaemia and for the different proportions of α- and β-chain mutants. The study of a Hungarian family rendered the first data for a conclusion that nonallelic genes code for the human α-chains. The variability of the level of expression of multiple nonallelic haemoglobin loci, difficulties of detection of chains with identical sequence, equivocal data with regard to the universal existence of two α-chain loci, and the eventual advantage of chain duplication has been discussed.
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