As female has XX chromosome so they can be carrier if one chromosome is recessive to the disease and have muscular distrpphy if female has homozygous condition for the disease !!
But in male ; which has only one X chromosome i.e XY , it can not be a carrier !! Male even if has the recessive gene but due to having only one X gene will get muscular distrophy.
I hope now you understand; if got any problem ; comment !!
Sexual reproduction offers genetic variation, as the offspring are a combination of 2 sets of genes opposed to one. This ultimately leads to evolution and the adaption to changing environments :)