A sex-linked gene is a gene located on either the X or Y chromosome. Since males have one X and one Y chromosome, while females have two X chromosomes, sex-linked genes are typically expressed differently in males and females.
One example of a sex-linked gene is the gene responsible for hemophilia, a blood clotting disorder. Hemophilia is caused by a mutation in the gene that codes for a clotting factor located on the X chromosome. Since females have two X chromosomes, they can inherit one normal copy of the gene and one mutated copy, and therefore may not experience symptoms of hemophilia. However, males have only one X chromosome, so if they inherit the mutated gene from their mother, they will experience hemophilia.
Another example of a sex-linked gene is the gene responsible for red-green color blindness. This gene is also located on the X chromosome, so males are much more likely to be color blind than females. Females can be carriers of the gene and may have some color vision deficiency, but typically not as severe as in males.
A third example is the gene for androgen insensitivity syndrome (AIS), which is also located on the X chromosome. AIS is a condition where individuals with male chromosomes are unable to respond to male hormones (androgens), resulting in incomplete male sexual development.