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Ezra Smith (talk | contribs) →Sex differences in phenotype/genotypes and frequency: Added a citation |
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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 in a population 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 they 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.{{Citation needed|date=January 2020}}
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 causes the father to choose to only have children with women who aren't carriers (so as 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.<ref>{{
A few scholars have suggested discontinuing the use of the terms ''dominant'' and ''recessive'' when referring to X-linked inheritance, stating that the highly variable penetrance of X-linked traits in females as a result of mechanisms such as skewed X-inactivation or [[somatic mosaicism]] is difficult to reconcile with standard definitions of dominance and recessiveness.<ref>{{cite journal|last1=Dobyns|first1=William B.|last2=Filauro|first2=Allison|last3=Tomson|first3=Brett N.|last4=Chan|first4=April S.|last5=Ho|first5=Allen W.|last6=Ting|first6=Nicholas T.|last7=Oosterwijk|first7=Jan C.|last8=Ober|first8=Carole|year=2004|title=Inheritance of most X-linked traits is not dominant or recessive, just X-linked|journal=American Journal of Medical Genetics|volume=129A|issue=2|pages=136|doi=10.1002/ajmg.a.30123|pmid=15316978}}</ref>
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