A 6-year-old boy is brought to see you in clinic by his mother. She has noticed that he has developed difficulty running and getting up from sitting on the floor. He has a 12-year-old brother who presented at a similar age and is now wheelchair bound. On examination he has proximal muscle weakness with enlarged calf muscles. He is diagnosed with Duchenne’s muscular dystrophy. Which is the most likely mechanism of inheritance of the genetic abnormality?