A form of inheritance where a gene lies on the X chromosome. X-linked genes can be dominant, but are usually recessive. Girls who carry an altered gene are usually not affected, since they have a second, normal copy of the gene. They are carriers, however, and can pass the affected gene on in 50% of their pregnancies. Boys who have only one X chromosome do not have a second normal copy of the gene, so will be affected by the disorder. They will pass on the altered gene to all of their daughters, who will be carriers, but to none of their sons.