X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females have two X chromosomes while males have one X and one [[Y chromosome]]. Carrier females who have only one copy of the mutation do not usually express the phenotype, although differences in [[X-chromosome inactivation]] (known as [[skewed X-inactivation]]) can lead to varying degrees of clinical expression in carrier females, since some cells will express one X [[allele]] and some will express the other. The current estimate of sequenced X-linked genes is 499, and the total, including vaguely defined traits, is 983.<ref>{{cite web|url=https://www.ncbi.nlm.nih.gov/Omim/mimstats.html|title=OMIM X-linked Genes|website=nih.gov|access-date=3 May 2018|url-status=live|archive-url=https://web.archive.org/web/20160307090758/http://www.ncbi.nlm.nih.gov/Omim/mimstats.html|archive-date=7 March 2016}}</ref>
== Patterns of Inheritanceinheritance ==
[[File:Haemophilia family tree.GIF|thumb|Patterns of X-Linkedlinked Recessiverecessive Inheritanceinheritance in thea Royalroyal Familyfamily]]
In humans, inheritance of X-linked recessive traits follows a unique pattern made up of three points.
-* The first is that affected fathers cannot pass xX-linked recessive traits to their sons because fathers give Y chromosomes to their sons. This means that males affected by an xX-linked recessive disorder inherited the responsible X chromosome from their mothers.
* Second, xX-linked recessive traits are more commonly expressed in males than females.<ref>{{Cite web|date=8 July 2009|title=Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals|url=https://www.ncbi.nlm.nih.gov/books/NBK115561/|access-date=9 June 2020|website=National Center for Biotechnology Information}}</ref> This is due to the fact that males possess only a single X chromosome, and therefore require only one mutated X in order to be affected. Women possess two X chromosomes, and thus must receive two of the mutated recessive X chromosomes (one from each parent). A popular example showing this pattern of inheritance is that of the descendants of Queen Victoria and the blood disease [[Haemophilia|hemophilia]].<ref>{{Cite web|date=2014-03-04|title=History of Bleeding Disorders|url=https://www.hemophilia.org/Bleeding-Disorders/History-of-Bleeding-Disorders|access-date=2020-06-09|website=National Hemophilia Foundation|language=en}}</ref>
-* The last pattern seen is that xX-linked recessive traits tend to skip generations, meaning that an affected grandfather will not have an affected son, but could have an affected grandson through his daughter.<ref>{{Cite book|last=Pierce|first=Benjamin A.|title=Genetics: A Conceptual Approach|publisher=Macmillan Learning|year=2020|isbn=978-1-319-29714-5|pages=154–155}}</ref> Explained further, all daughters of an affected man will obtain his mutated X, and will then be either carriers or affected themselves depending on the mother. The resulting sons will either have a 50% chance of being affected (mother is carrier), or 100% chance (mother is affected). It is because of these percentages that we see males more commonly affected than females.
