What profession do Francis and his father share in the novel, What’s Bred in the Bone?

Francis' father, and later Francis himself, joins the profession - espionage for MI5. It seems a likely choice given that secrecy is bred in the bone of the Cornishes. Francis' father is quite a respected member of the profession, and Francis does some important work for them himself. In Francis' case, he is to count trains headed to a concentration camp and encode those numbers into a report in his health. For much the book, Francis barely understands or appreciates the importance of what he does for the profession.