Final answer:
X-inactivation is the phenomenon that significantly affects the phenotype of female carriers of dystrophin mutations, potentially leading to milder symptoms or an asymptomatic state due to random inactivation of one X chromosome in each cell.
Step-by-step explanation:
The phenomenon that has a large effect on the phenotype of female carriers of dystrophin mutations is B) X-inactivation. Dystrophin mutations are associated with certain forms of muscular dystrophy, which are X-linked recessive disorders. In female carriers, one of the two X chromosomes is randomly inactivated in each cell early in development, a process that leads to the formation of a Barr body. This X-inactivation results in mosaicism, where some cells express the normal allele while others express the mutated allele. Due to this random inactivation of one X chromosome in each cell, female carriers may exhibit milder symptoms or may even be asymptomatic.