The recessive genes located on X-chromosome of humans are always expressed in males eg., colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green colours. The gene for the normal vision is dominant. The normal gene and its recessive allele are carried by X-chromosomes. In females colour blindness appears only when both the sex chromosomes carry the recessive gene (X∘X∘). The females have normal vision but function as carrier if a single recessive gene for colour blindness is present (XX∘). However, in human males the defect appears in the presence of a single recessive gene (XcY) because Y− chromosome of male does not carry any gene for colour vision.