Health Promotion and Maintenance Q 79
A child has recently been diagnosed with Duchenne muscular dystrophy (DMD). The parents are receiving genetic counseling prior to planning another pregnancy. Which of the following statements includes the most accurate information?
A. Duchenne's is an X-linked recessive disorder, so daughters have a 50% chance of being carriers and sons a 50% chance of developing the disease.
B. Duchenne's is an X-linked recessive disorder, so both daughters and sons have a 50% chance of developing the disease.
C. Each child has a 1 in 4 (25%) chance of developing the disorder.
D. Sons only have a 1 in 4 (25%) chance of developing the disorder.
Correct Answer: A. Duchenne’s is an X-linked recessive disorder, so daughters have a 50% chance of being carriers and sons a 50% chance of developing the disease.
The recessive Duchenne gene is located on one of the two X chromosomes of a female carrier. If her son receives the X bearing the gene he will be affected. Thus, there is a 50% chance of a son being affected. Daughters are not affected, but 50% are carriers because they inherit one copy of the defective gene from the mother. The other X chromosome comes from the father, who cannot be a carrier.
Option B: DMD carriers are females who have a normal dystrophin gene on one X chromosome and an abnormal dystrophin gene on the other X chromosome. Most carriers of DMD do not themselves have signs and symptoms of the disease, but a minority do.
Option C: Advances in molecular biology techniques illuminate the genetic basis underlying all MD: defects in the genetic code for dystrophin, a 427-kd skeletal muscle protein (Dp427). These defects result in the various manifestations commonly associated with MD, such as weakness and pseudohypertrophy.
Option D: Minor variations notwithstanding, all types of MD have in common progressive muscle weakness that tends to occur in a proximal-to-distal direction, though there are some rare distal myopathies that cause predominantly distal weakness. The decreasing muscle strength in those who are affected may compromise the patient’s ambulation potential and, eventually, cardiopulmonary function.