Colour blindness is a common hereditary (inherited) condition which means it is usually passed down from your parents.
Red/green colour blindness is passed from mother to son on the 23rd chromosome, which is known as the sex chromosome because it also determines sex. 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 different to those for people who have ‘normal’ colour vision meaning one cone cell type 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 in the usual way.
For the sake of simplicity we refer to a colour blind ‘gene’ but this is not strictly a true description.
The 23rd chromosome is made up of two parts – either two X chromosomes if you are female or an X and a Y chromosome if you are male. The ‘gene’ which causes (inherited, red and green types of) colour blindness is found only on the X chromosome. So, for a male to be colour blind the 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.
If a woman has only one colour blind ‘gene’ she is known as a ‘carrier’ but she won’t be colour blind. When she has a child she will give one of her X chromosomes to the child. If she gives the X chromosome with the colour blindness ‘gene’ to her son he will be colour blind, but if he receives the X chromosome which doesn’t carry the colour blindness ‘gene’, he won’t be colour blind.
A colour blind boy can’t receive a colour blind ‘gene’ from his father, even if his father is colour blind, because his father can only pass an X chromosome to his daughters.
A colour blind daughter therefore must have a father who is colour blind and a mother who is a carrier (who has also passed the colour blindness ‘gene’ to her daughter). If her father is not colour blind, a ‘carrier’ daughter won’t be colour blind. A daughter can become a carrier in one of two ways – she can acquire the ‘gene’ from a carrier mother or from a colour blind father.
This is why red/green colour blindness is far more common in men than women.
Blue/yellow 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