Red-green colour blindness in the Isle of Man and Cumbria.

The frequencies of red-green colour-blind males in the indigenous population of two regions of Britain, the Isle of Man (5.6 per cent) and Cumbria (5.0 per cent), do not readily fit into the pattern of a SW-NE gradient in the frequency of the colour-blind gene in Britain suggested by earlier studies. Frequencies of red-green colour-blind females are also given for these two populations.