Evidence for multiple origins of the beta E-globin gene in Southeast Asia.

To investigate whether recurrent mutation has contributed to the high frequency of the beta E-globin gene in Southeast Asia, we used the haplotypes at three polymorphic restriction sites within and to the 3' side of the beta-globin gene to predict the framework of 23 beta E-globin genes. These haplotypes suggested that beta E-globin genes are present in two different beta-globin gene frameworks. DNA sequence determination of one gene representing each framework demonstrated that the same mutation (GAG leads to AAG at codon 26) was present in both frameworks. Moreover, the frameworks differed at three nucleotide positions known to be polymorphic in Mediterraneans. These polymorphic sites are located 70 nucleotides to the 5' side of the beta E mutation and 382 and 1032 nucleotides to the 3' side of it. The existence of the beta E mutation in these two beta-globin gene frameworks can be explained by (i) recurrent mutation giving rise to beta E-globin, (ii) a double crossing-over event, or (iii) two single crossing-over events. Mathematical analysis suggests that the first alternative, recurrent mutation of G leads to A at the first nucleotide of codon 26, is most likely.