Red/green colour blindness is a common hereditary condition which means it is usually passed down from your parents. Colour blindness is usually passed from mother to son on the 23rd chromosome, which is the sex chromosome. Chromosomes are structures which contain genes – these contain the instructions for the development of cells, tissues and organs. If you are colour blind it means the instructions for the development of your cone cells are wrong and the cone cells might be missing, or less sensitive to light or it may be that the pathway from your cone cells to your brain has not developed correctly.
The X chromosome is the sex chromosome: males have an X chromosome and a Y chromosome and females have two X chromosomes. For a male to be colour blind the faulty colour blindness gene only has to appear on his X chromosome. For a female to be colour blind it must be present on both of her X chromosomes. This is why red/green colour blindness is far more common in men than women. Blue colour blindness affects both men and women equally, because it is carried on a non-sex chromosome.
For the sake of the following explanation a normal X chromosome is shown as (X) whilst a colour blind carrying X chromosome is shown in bold (X).
The colour blind gene is carried on one of the X chromosomes. Since men have only one X chromosome, if his X chromosome carries the colour blind gene (X) he will be colour blind (XY). A woman can have either:-
(i) two normal X chromosomes, so that she will not be colour blind or be a carrier (XX),
(ii) or, one normal X and one colour blind carrying X chromosome, in which case she will be a carrier (XX), or rarely
(iii) she will inherit a colour blind X from her father and a colour blind X from her mother and be colour blind herself (XX). She will pass on colour blindness to all of her sons if this is the case.
See the tables below to understand how people can become colour blind and how colour blindness is passed on to future generations.
A colour blind man and a non-colour blind woman
A non colour blind man and a colour blind carrier woman
A colour blind man and a colour blind carrier woman
A non colour blind man and a colour blind woman