One example of a sex-linked trait in humans is red-green color blindness, in which affected individuals are not able to distinguish certain colors. The genes for both red and green color perception are X-linked and the mutations are recessive; therefore, color blindness affects far more XY individuals than XX individuals. Color blindness is diagnosed by showing children images like the one on this page. The orange circle is visible to all, but individuals with one form of red-green color blindness are unable to distinguish the red star from the green background.
Another sex-linked trait in humans is hemophilia. People with hemophilia lack a protein involved in blood clotting, resulting in excessive bleeding from even a minor abrasion. This disease is of particular interest to geneticists, given its relatively high frequency in many of the royal houses of Europe. It is thought that the hemophilia gene spontaneously mutated in one of Queen Victoria's (a queen of England in the 1800s) parents, making her a carrier of the trait. She transmitted the hemophilia allele to one son and some of her daughters, and it eventually affected the imperial houses of Prussia, Russia, and Spain through a series of politically arranged marriages.