Human female is referred to as carrier of colour blindness because of the following reasons:
1. Females possess double Xchromosomes in her gametes.
2. If one X-chromosome is carrying recessive gene for colour blindness, her other dominant X hides the expression of colour blindness and hence she does not become a patient.
3. But such female can carry the defective gene to her progeny. Thus she is called carrier of colour-blindness.
4. A female having one recessive gene on X-chromosome is a carrier female, while a female possessing both recessive genes on both the Xchromosomes will be colour blind which is very rare.
