Colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red-green colours. The gene for normal vision is dominant. The normal gene and its recessive alleles are carried by X-chromosome. In female, colour blindness appears only when both the sex chromosomes carry the recessive gene (X^cX^c). The females have normal vision but function as carrier if a single recessive gene for colour blindness is present (XX^c). However, in human males the defect appears in the presence of a single recessive gene (X^cY) because Y-chromosomes of males do not carry any gene for colour vision. Colour blindness, like other sex-linked trait, shows criss-cross inheritance (i.e., a male transmits his trait to his grandson through daughter, while a female transmits the traits to her grand-daughter through her son or it is transfer of trait from one sex to the offspring of the opposite sex).