An X-linked condition can occur when a person has a change, or disease-causing variant, on their X chromosome. Males have an X and Y chromosome while females have two X chromosomes. In most cases, females with a disease-causing variant in an X-linked gene are not affected because they have a second functioning copy of the gene. When X-linked conditions occur in females, symptoms are typically milder. Since males have only one X chromosome, they have no such ‘backup’ copy and are therefore more likely to have an X-linked condition.

Because females can have a disease-causing variant in an X-linked gene without displaying symptoms, they are more likely to be carriers of X-linked conditions than males. As a result, X-linked conditions are usually passed down from carrier mothers to sons.