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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 can lead to varying degrees of clinical expression in carrier females.
==Sex Differences In Phenotype==
In humans, ''generally'' "men are affected and women are carriers" for two reasons. The first is the simple statistical fact that if 1% (1 in 100) of all X-chromosomes carry an X-linked mutation and then 1 in 100 men (1% of them) are likely to express the mutation while 1 in 10,000 women (0.01% of them) are likely to. 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 (that's why he was male). 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 0.01%. (Note that the percentages are examples based on an assumption that 1% of X chromosomes carry the trait, if that number changes so do the other numbers).
==Examples==
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