A 29-year-old man presents with a 3-year history of progressive difficulty raising his arms above his head. He first noticed he could no longer reach high shelves; over the past year he cannot comb his hair without resting his elbow. His father had similar shoulder weakness beginning in his late 20s. On examination, the patient cannot fully close his eyes against resistance, and when asked to whistle, air leaks from the right corner of his mouth. When he pushes against a wall with both arms extended, the medial borders of both scapulae wing off the thorax, worse on the right. His deltoids appear well-preserved, but the periscapular musculature is visibly wasted. Grip strength is normal, and there is no sustained contraction after a handshake. No ptosis. Elbow extension is full bilaterally. Creatine kinase is 310 IU/L.
Which of the following genetic abnormalities is most likely responsible for this patient's condition?