Duchenne muscular dystrophy (md) is caused by a recessive allele located on the x chromosome (xd). affected people experience progressive weakening and loss of skeletal muscle. which of the sets of genotypes below would explain how two unaffected people (neither mom or dad have md) could have a child with duchenne md? a. option 1 b. mom is xdxd and dad is xdy c. mom is xdxd and dad is xdy d. mom is xdxd and dad is xdy