Red-Green Colour Blindness

Red-green colour blindness is the most common sex-linked condition. People with it find it hard to tell apart reds, greens and colours containing them.Colour vision depends on cone cells in the retina. There are three types, containing pigments (opsins) most sensitive to red, green or blue light. The genes for the red- and green-sensitive pigments are on the X chromosome. A faulty allele gives an absent or altered pigment, so one type of cone does not work normally.The condition is X-linked recessive. Using X^B for normal vision and X^b for colour blindness:• males: X^BY normal, X^bY colour blind
• females: X^BX^B normal, X^BX^b carrier, X^bX^b colour blindAbout 8% of men of northern European descent are red-green colour blind, but only about 0.5% of women, because a woman needs two recessive alleles.Worked example 1: a colour-blind man (X^bY) and a homozygous normal woman (X^BX^B). All daughters are X^BX^b carriers; all sons are X^BY with normal vision.Worked example 2: a carrier woman (X^BX^b) and a colour-blind man (X^bY). Offspring: X^BX^b, X^bX^b, X^BY, X^bY. Each child has a ½ chance of being colour blind, whether a son or a daughter.Colour blindness is usually diagnosed with Ishihara test plates, patterns of coloured dots containing numbers.