In this hypothetical family, all three children in the second generation got the X chromosome with the flawed gene.

Duchenne muscular dystrophy (DMD) is an X-linked disorder in which females rarely show symptoms but can be carriers. The mother in this family is a carrier. The parents have two sons with DMD, each of whom received an X chromosome with a DMD mutation from their mother.

They also have a daughter who received an X chromosome with a DMD mutation; she, like her mother, is a carrier. The daughter has married a man with no DMD mutation, and they’ve had four daughters. Three of the daughters didn’t get the DMD mutation from their mother, but one did. She’s also a carrier.