Some inherited conditions are caused by a gene variant located on the X chromosome rather than on one of the other 22 pairs of chromosomes. Because the X chromosome is one of the two sex chromosomes, and because males and females carry a different number of copies of it, X-linked conditions are inherited in a pattern that looks different from the autosomal dominant and autosomal recessive patterns described elsewhere on this site. This page is part of our inheritance basics series and focuses on the two main forms of X-linked inheritance: recessive and dominant.
Why sex matters for X-linked conditions
Females typically have two X chromosomes, while males typically have one X and one Y chromosome. That single difference changes how a variant on the X chromosome behaves. A male who inherits an altered gene on his one X chromosome has no second, working copy to compensate. A female who inherits the same altered gene usually still has a second X chromosome carrying a working copy of the gene, which can offset the effect, at least to some degree.
X-linked recessive conditions
In X-linked recessive conditions, one altered copy of the gene is enough to cause the condition in a male, because he has only one X chromosome. A female with one altered copy and one working copy is usually described as a carrier: she generally does not show the full condition herself, though some carriers have mild or partial features. For a female to be affected in the same way as a male, she would typically need an altered copy on both of her X chromosomes, which is far less common. This is why X-linked recessive conditions are seen much more often in males than in females. A carrier mother can pass the altered gene to her sons and daughters, but a father cannot pass an X-linked gene to his sons, since sons receive their father's Y chromosome, not his X.
X-linked dominant conditions
X-linked dominant conditions behave differently. Here, a single altered copy of the gene is enough to cause signs of the condition whether that copy sits in a male's only X chromosome or in one of a female's two X chromosomes. Both sexes can be affected, though the severity and specific features sometimes differ between males and females, partly because of a process called X-inactivation, in which one X chromosome is randomly switched off in each of a female's cells. Fragile X syndrome is a condition discussed on this site that follows this less strictly dominant or recessive pattern, which is part of why some geneticists prefer the broader label "X-linked" rather than insisting every condition fits neatly into "dominant" or "recessive."
A real example: hemophilia A
Hemophilia A is a well-documented X-linked recessive bleeding disorder caused by variants in the F8 gene, which provides instructions for making coagulation factor VIII, a protein the body needs to form blood clots. According to MedlinePlus Genetics, hemophilia A affects approximately 1 in 4,000 to 1 in 5,000 males worldwide, and because it follows X-linked recessive inheritance, it is far less common in females. Some female carriers of an F8 variant do experience bleeding symptoms, which research links to skewed X-inactivation, where the X chromosome carrying the working copy of the gene happens to be switched off in more than half of a carrier's cells. In practice, this is a useful illustration of how X-linked recessive inheritance plays out in a real, well-studied condition, rather than a tidy textbook rule that always applies exactly the same way.
Other X-linked conditions on this site
This site covers several other conditions that are inherited in an X-linked pattern, each with its own page describing the gene involved and its clinical features. Hypohidrotic ectodermal dysplasia is most often X-linked and tends to affect males more noticeably. Fragile X syndrome involves a specific type of repeat expansion on the X chromosome. Rett syndrome, linked to variants in the MECP2 gene on the X chromosome, is an example where the pattern and presentation differ from the simpler recessive or dominant categories. Each of those pages goes into the specific gene and features in more depth than this overview page can.
X-linked inheritance is one of several patterns a family might encounter. Some conditions arise from a brand-new variant rather than one inherited from a parent, which our page on de novo variants explains, and others are shaped by which parent passed down the gene, a topic covered on our genomic imprinting page.
This page is educational, not diagnostic. Inheritance patterns described here are general patterns, not a substitute for an individual risk assessment. Anyone with questions about a specific family history or diagnosis should speak with a genetic counselor or clinician; our genetic testing and counseling page explains what that process can involve. A full list of the conditions covered on this site is on our conditions page.