Two related but distinct neurological disorders are Prader-Willi syndrome (PWS) and Angelman syndrome (AS). Children with PWS are obese, have weak muscle tone, and delayed development. Children with AS have severe speech impairment, developmental delay, intellectual disability, and problems with movement and balance. However, both disorders are due to the deletion of the same part of chromosome 15. When the deletion is in the copy of chromosome 15 that came from the father, the child has PWS, but when the deletion is in the copy of chromosome 15 that came from the mother, the child has AS. What hypothesis can explain these observations?