The Ishihara test detects colour blindness — option 3.

How it works. Devised by Dr Shinobu Ishihara of Japan in 1917, the test uses plates covered with dots of many sizes and colours. Within each plate a numeral or a winding path is picked out in dots of one hue against a background of another, chosen so that the two are easily told apart by normal colour vision but appear alike to someone with a red-green defect. A person with normal vision reads one number; a person with the defect reads a different number, or none at all.

The condition itself. Colour vision depends on three kinds of cone cell in the retina, sensitive to long, medium and short wavelengths. If the pigment of one kind is faulty or absent, colours that differ only in that channel become indistinguishable. The commonest forms are protanopia and deuteranopia, both red-green defects.

Why it is far commoner in men. The genes for the red and green pigments lie on the X chromosome and the defect is recessive. A male has only one X, so a single faulty gene expresses itself; a female has two, and the normal allele on the second masks the defect. She is then a carrier. About 8 per cent of men and 0·5 per cent of women are affected — a standard illustration of X-linked recessive inheritance, alongside haemophilia.

Where it matters : the test is part of the medical examination for pilots, drivers, railway staff and the armed forces, where signals must be read by colour.

Hence, the answer is Colour blindness.