I wrote a while back about Zonta Van De Goorbergh being reported as the first third generation GP rider.
However, it is possible that David Jefferies may have beaten him to it, having made 6 starts in 1993 in the 500c class. His dad Tony made 4 GP starts at the Ulster GP and TT in 1970 and 1971. Nick didn't win a TT until after it had been replaced as a world championship round, so to my knowledge didn't race in any GPs.
Did David's grandfather Alan Jefferies compete in any GPs? I have him down making 3 starts at the 1947, 8 and 9 TT in the Clubmans Senior TT races and the 1933 Manx GP. Would any of these races classify as World or European Championship GP races? It looks to me like the Clubmans TT wasn't a GP race, but was the Manx? If so was it part of the 1933 European championships?