When it's not inherited, it's called sporadic. Either way, it's still a genetic disease, arising from a genetic mutation. Genes are arranged on structures in the cell known as chromosomes. Females have two X chromosomes, and males have an X and a Y chromosome. Duchenne and Becker muscular dystrophies can be inherited in the X-linked recessive pattern, or can occur when the dystrophin gene spontaneously mutates.
Girls are luckier than boys when it comes to disease-causing mutations in genes on the X chromosome.
The gene for dystrophin is on the X chromosome. In DMD and BMD, women can carry the mutation on one of their two X chromosomes, but they rarely show the disorder in any major way. A woman can, however, pass along to a son her X chromosome with the mutated dystrophin gene. The son, having no backup X, will show the disease. This is called an X-linked recessive inheritance pattern.