Q: Some 5 per cent of the male population is colour-blind. Why is the proportion so high? Are there any advantages in being colour-blind?
A: Any apparent disability occurring frequently in men and infrequently in women, such as colour blindness, should almost immediately give away the location of the gene responsible 鈥� it is likely to be on the X chromosome. Colour blindness is higher in males because if they are unlucky enough to inherit an X chromosome with the defective gene on it, they have no spare chromosome copy to correct the problem. Women, with their two X chromosomes, have to inherit two defective copies to be colour-blind. A boy whose mother carries the gene has a 50 per cent chance of being colour-blind, whereas a girl requires a mother who is a carrier plus a father who is colour-blind to stand the same chance of being colour-blind herself.
Female carriers are not affected, and male sufferers are not generally prevented from reaching breeding age by colour-blindness, so the evolutionary pressure against the gene is very slight indeed.
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A: The correct figure for men is about 8 per cent compared with about 0.4 per cent of women.
There are two photopigment genes on the X chromosome, one for a long wavelength-sensitive (red) photopigment and one for a medium wavelength-sensitive (green) photopigment. Colour deficiency arises when the amino acid sequence of these alters. Poor green vision is more common, just 2 per cent of men have poor red vision.
Colour deficiency is a disadvantage in some occupations and may cause difficulty with everyday colour tasks such as matching home decorating materials. The only reported advantage is that military camouflage may be ineffective and colour observers were employed on daylight bombing raids during the Second World War.
A: One of the possible advantages to human colour vision deficiencies, of which there are many types, is improved hunting skills at low levels of light.
The incidence of colour deficiencies ranges from 1 to 14 per cent among different indigenous human populations and is moderately correlated with the amount of twilight. The duration of twilight is short at the equator but progressively longer at higher latitudes. These trends may reflect periods in our past, such as the Palaeolithic, when colour deficients may have had a significant advantage. There is a great-deal of anecdotal data for enhanced visual powers for colour deficients (many other mammals which are active at twilight are colour-blind). However, the critical experiments on sensitivity to movement and contrast at low light levels have not yet been done.