Sex-linked genes are located on the X-chromosome or Y-chromosome. In humans, females have two X chromosomes while males have one X and one Y chromosome. Since sex chromosomes determine the sex of an individual, genes located on the X or Y chromosome are called sex-linked genes. Genes on the X chromosome are considered X-linked, while genes on the Y chromosome are Y-linked. Because males only have one X chromosome, they express any X-linked gene, whether it is dominant or recessive, whereas females must inherit two copies of a recessive X-linked gene to express the trait. Therefore, sex-linked genes play an important role in the inheritance of many genetic disorders and traits.