i have a 3 year old border collie with a undershot jaw. i was recently asked if i would consider using him on a friends bitch as his father and brother are both doing well in the show ring. because of the jaw i refused but it has started me wondering if this is hereditary or not. can anyone tell me if it can be pased on

Yes. Sorry if you are disappointed but with mouth faults there will be a good chance that up to 50% of the puppies would be affected and possible more. Even if some did have good mouths, the undershot factor would still be inherited and those puppies that appeared to have "good mouths" would be very likely to produce the fault in their puppies too.