X Inactivation - (Aug/25/2018 )

Hello,

Could anyone please answer the following question on molecular biology?
Duchenne muscle dystrophy is X linked.That is the gene that codes for dystrophin is on the X chromosome.Boys have one copy of the X chromosome.If the dystrophin gene is mutated there is no fallback option as there is only one copy of the gene--- defective dystrophin is produced and dystrophy develops.
But , as is written in the text books, girls have two copies of the X chromosome.So if in one chromosome the dystrophin gene is mutated there is a fallback option and since some genuine dystrophin can be produced the symptoms are reduced-- that is they are " manifest carriers".
But this explanation goes against the logic that at the embroyonic stage one copy of the X chromosome is inactivated known as X inactivation.So the individual can only access gene products from one X chromosome.So how is it that the girls have reduced symptoms?
One explanation might be the " skewed X chromosome inheritance principle"
I await anyone's comments on the matter.
Regards,

Moved to the correct forum...

The wikipedia article on X inactivation explains it pretty well. I suggest you read the Mechanism section, especially the first two paragraphs and the section on selection of one X chromosome and the following one on expression of X-linked disorders in heterozygous females.