A trait associated with a gene on the X sex chromosome. Any trait associated with a gene on the X chromosome will almost always be expressed in males, who have only one X chromosome. Women who inherit an X- linked disease gene are usually not affected by the disease because they have two X chromosomes and have a healthy copy of the gene on the other X chromosome (which overrides the effects of the disease gene). X-linked genetic disorders are often passed from mothers (who are unaffected) to sons.