Who is colour-blind?
• Red-green colour blindness is usually inherited, and occurs in about 8% of males and only about 0.4% of females. This is because of the way the genes for the different cone cells are carried on the chromosomes. The genes that lead to red-green colour blindness are on the X chromosome [males have only one of these and females have two]. • If a mother ‘carries’ the gene for red-green colour blindness (one normal and one altered gene) she will not have a colour vision problem. About 50% of the sons of women who are carriers will be colour blind (in any family it may not be 50% because the distribution of the genes is random). • A daughter will not normally be red-green colour blind unless her mother is a carrier and her father is colour-blind, but about 50% of daughters of women who are carriers will also carry the gene. • Only 5% of people who are colour-blind have blue colour blindness, and this is equal in males and females, because the genes for it are on a different chromosome (chr
