ExplanationD. Duchenne’s muscular dystrophy is an X-linked recessive disorder in which affected boys develop progressive weakness of limb girdle muscles beginning at age 5 or earlier. By age 12 walking is impossible, and these patients usually succumb to respiratory failure by age 25. Most muscular tissues, including cardiac tissues, are involved. An abnormally high creatine kinase level is found in all these patients before disease onset and in many female carriers. The responsible gene has been identified. This 2000-kb gene codes for a product termed dystrophin, a 400-kDa protein localized to the muscle plasma membrane. Since about 60% of these patients have an exon deletion or duplication in the dystrophin gene, it is possible to test directly for these genetic abnormalities in utero, obviating the need for more cumbersome family studies to determine RFLPs for linkage.
