Polymorphisms of red-green vision among populations of the tropics

Summary

INTRODUCTION

Though there is an apparent wealth of information on the occurrence of “colourblindness” among indigenous populations of the Tropics, there are serious shortcomings in much of it: many samples of well-defined populations are too small, large samples often combine subsamples of unspecified numbers of populations; in many studies, methods are used which underestimate the defect frequencies (Table 1) and others which cannot differentiate genuine red-green blindness from mild anomalous colour-vision, or indeed from underlying ophthalmological disorders.

This is particularly unfortunate, since such information is essential for any attempt to interpret this polymorphism in terms of evolutionary processes.

Needless to say, the Tropics and its multitude of peoples provide ideal opportunities for investigations of the outcome of genetic drift, migration, selection or relaxed selection, factors likely to be involved in the maintenance, or change, of this polymorphism. For the Tropics are the home of virtually all types of human societies, from small hunter-gatherer groups to some of the largest and oldest civilisations on earth, and the changes that are occurring there are particularly relevant to the hypothesis that the colour defect gene frequency variation seen today is the outcome of relaxation of selection pressures exerted in traditional life styles (Post, 1971).

BIOLOGY OF RED-GREEN COLOURBLINDNESS

Red-green vision is governed by two adjacent X-linked, multiple allelic loci, protan and deutan. Only two of the mutant alleles, one at each locus, cause red-green blindness. In the retinal cones of normal people (trichromats) the pigment is of 3 types - red-absorbing, green-absorbing and blue-absorbing.