A six month old baby girl who was normal at bih, begins to show signs of motor retardation. While she could sit up at 5 months, she can no longer do so. As time goes on, the child continues to deteriorate, and eventually becomes unresponsive to visual or auditory stimuli. Funduscopic examination reveals cherry-red macular spots in both eyes. Which of the following genetic abnormalities is most often related to the development of this disease?