What causes duchenne muscular dystrophy?

Hereditary. It is inherited because of a defective gene on the x chromosome (female sex chromosome). It is referred to as a "sex-linked recessive" inheritance pattern. Girls get two x chromosomes, and the healthy one overrules the unhealthy gene, preventing girls from contracting the disorder. Boys get an x and a y. If the x has the defective gene, there is no corresponding gene on the y to overrule it.