As part of a multidisciplinary study of the population of Bali, Indonesia, 2452 blood samples from people of both sexes were tested for treponemal infection and blood groups. Analysis of blood groups of the 81 patients reactive to the Treponema pallidum immobilisation (TPI) test, who were considered to have latent or inactive yaws, compared with a control group of 552 healthy Balinese, showed that the ratio of MM to MN and NN phenotypes was 2.25 times higher in the patients than in the controls (chi 2(1) = 10.2, p less than 0.005). A speculative hypothesis is that yews infection gives people with the MM phenotype a lower selective fitness. This hypothesis could explain the low prevalence of the M gene in the Australo-Melanesian populations.
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