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If one of the two X chromosomes in female mammals is randomly inactivated (i.e. becomes a Barr body) in each cell, shouldn't half of all cells in a female who is a carrier for an X-linked recessive disorder (say, Duchenne muscular dystrophy) express the disorder?

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Yes and No... You are correct that X-Chromosome Inactivation will randomly inactivate the good X-chromosome (i.e. the one NOT carrying the mutation/disease) half of the time. However, this doesn't always seem to be associated with a disease phenotype. It likely depends on the gene and other factors, like the gene allele's function, to determine whether there is a disease phenotype that manifests with inactivation. Check out this paper that looked at Duchenne muscular dystrophy in females that is available freely on PubMed.

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