'''X-linked recessive inheritance''' is a mode of [[Mendelian inheritance|inheritance]] in which a [[mutation]] in a [[gene]] on the [[X chromosome]] causes the [[phenotype]] to be always expressed in males (who are necessarily [[homozygous]] for the gene mutation because they have one X and one [[Y chromosome]]) and in females who are homozygous for the gene mutation, see [[zygosity]]. Females with one copy of the mutated gene are carriers.
X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females[[Cisgender]] females have two X chromosomes, while cisgender 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|accessdate=3 May 2018|url-status=live|archiveurl=https://web.archive.org/web/20160307090758/http://www.ncbi.nlm.nih.gov/Omim/mimstats.html|archivedate=7 March 2016}}</ref>
==Sex differences in phenotype/genotypes and frequency==
