Musical ability involves a lot of other abilities; fine motor skills, listening, memory, to name but three. Its possible to do well with a lot of hard work even if you're weak in one or more areas. Children may be taught that working around weaknesses is still of value whereas others may taught to give up early as they will never have IT and either style of teaching may be evident in a family where one or more parents are musicians. I figure genetics plays a part but only a part in the whole picture that determines whether a child is a musician or not.