Bibliographical noteFunding Information:
* Working with the aid of a grant from National Health and Medical Research Council of Australia.
Summary Two G.-6-P.D. genetic polymorphisms exist-one characterised by enzyme deficiency, the other by an electrophoretic variant. Both are determined by X-linked loci. The association between the deficiency syndrome and the electrophoretic variant in American and Nigerian Negroes suggests that these loci are closely linked. The observation that most G.-6-P.D.-deficient Negroes have a single type of electrophoretic pattern is most easily explained by assuming that X-linked alleles at the struc- tural locus and those at the deficiency locus are not in genetic equilibrium. Uncommon variants have been demonstrated in American and Nigerian Negroes, Sardinians, and Papuans; but none in American Caucasians, Iraqi Jews, Thailanders, Japanese, or Marshall Islanders. A combination of electrophoretic and enzymatic varia- tion at present distinguishes thirteen G.-6-P.D. phenotypes. Proof of X-linkage is available for only some of these. The observed proportion of G.-6-P.D. phenotypes among Maryland Negroes agrees with predictions based on simple admixture of Nigerian and European gene pools. This suggests that selective forces other than falciparum malaria may be involved in maintaining the G.-6-P.D. polymorphism, as does the observation that all common G.-6-P.D. phenotypes occur among Nigerian children with severe falciparum malaria. We are grateful for the cooperation of the Thai students at the University of Pennsylvania; and we would like to thank Dr. S. W. Wright for supplying us with blood-samples from Japanese subjects at the Pacific State Hospital, Pomona, California, Dr. B. P. K. Ryan, Port Moresby, for supplying us with blood-samples from Papuans, and Dr. R. A. Conard, Brookhaven National Laboratory, for supply- ing us with blood-samples from Marshall Islanders. This study was supported by United States Public Health Service grant RG-6642, I. H. P. was in receipt of a Wellcome Trust travelling grant, and E. J. W.-W. was in receipt of a World Health Organisation grant.