Black and white: not shades of grey

When referring to abnormal colour vision eye care professionals are trained to use the term ‘colour deficiency’, in preference to the traditional phrase ‘colour blindness’ which implies seeing only in black and white. Whereas colour deficiency is relatively common (8% of males, 0.5% of females), true colour blindness, or achromatopsia, is much rarer. Congenital achromatopsia is estimated to affect one in 30,000 people. How remarkable, then, to find a tiny island in the western Pacific where between one in 25 and one in 10 people are affected by the condition.

Achromatopsia is a congenital hereditary condition that has two forms. The more common variety is rod monochromatism, an autosomal recessive condition, requiring both parents to pass on a defective gene, and is seen more or less equally in males and females. Blue cone monochromatism, actually a form of incomplete achromatopsia, is an x-linked recessive condition that is much rarer, affecting around one in 50-100,000 males and possibly only one in 10 billion females.