Color-blindness is the inability to distinguish the differences between certain colors. This condition results from an absence of color-sensitive pigment in the cone cells of the retina, the nerve layer at the back of the eye.
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.
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 faulty ‘gene’ for colour blindness is found only on the X chromosome. So, 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.
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 faulty ‘gene’ to her son he will be colour blind, but if he receives the ‘good’ chromosome 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 faulty ‘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.
HoPe It HeLpS :)
