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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|X chromosome 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>[http://www.ncbi.nlm.nih.gov/Omim/mimstats.html OMIM X-linked Genes]</ref>
==Sex Differences In Phenotype/Genotypes and Frequency==
In humans, ''generally'' "men are affected and women are carriers" for two reasons. The first is the simple statistical fact that if the X-chromosomes is a population that carry a particular X-linked mutation at a frequency of 'f' (for example, 1%) then that will be the frequency that men are likely to express the mutation (since thay have only one X), while women will express it at a frequency of f<sup>2</sup> (for example 1% * 1% = 0.01%) since they have two X's and hence two chances to get the normal allele. Thus, X-linked mutations tend to be rare in women. The second reason for female rarity is that women who ''express'' the mutation must have two X chromosomes that carry the trait and they necessarily got one from their father, who would have also expressed the trait because he only had one X chromosome in the first place. If the trait lowers the probability of fathering a child or induces the father to only have children with women who aren't carriers (so as not to create daughters who are carriers rather than expressers and then only if no [[genetic screening]] is used) then women become even ''less'' likely to express the trait than the statistical prediction of f<sup>2</sup> .
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