Are all females heterozygotes because of an inactive X

John Bohmfalk bohmfalk at
Wed Jan 29 23:16:29 EST 1997

In article <32E7AA70.117F at>, chiassa at wrote:


> What happens in the case of a sex linked gene for an illness such as
> hemophilia when the female is a carrier (heterozygote)? Would the
> production level of the protein responsable for coagulation be cut in
> half because half of all body cells would contain an inactive X.
> Illustration
> + dominant allele 
> - recessive
> X+X- (heterozygote female)
> If the X- is inactive there is no problem the dominant gene is on the
> other X.
> If the X+ is inactive the cell should produce either a defective
> proteine or none at all.
> Since the choice of the inactive X is random, how come female carriers
> for sex linked illnesses are not between being normal and showing
> symptoms? Are they somatic mosaics?
> Can anyone shed any light on the problem or perhaps my reasoning is
> flawed.
> Alyre Chiasson
> chiassa at

Any female is a somatic mosaic for all X-linked traits for which she is
heterozygous.  Apparently, being mosaic is generally enough to establish a
"normal" phenotype.  For some traits in humans, you can actually
demonstrate this mosaicism, for example, in x-linked colorblindness,
portions of the retina are colorblind, and parts see colors.


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