Possible changes in the frequency of the human ABO blood groups in Iceland due to smallpox epidemics selection.

The hypothesis is put forward that the low frequency of A and high frequency of O blood group genes in the Icelandic human population is the result of a selective disadvantage of A during severe smallpox epidemics. The hypothesis is supported by data from India in 1965-6, which show a marked selective effect of a smallpox epidemic against the phenotypes A and AB (Vogel & Chakravartti, 1971). The conclusion is drawn that the present-day ABO blood group gene frequencies of the Icelandic population should be used with reservation as markers in the study of the origin of the Icelanders.