Thanks TT I'm convinced dh has it, he is 6'4 with a longer armspan and has an unusual shaped breastbone. Combine that with the strong family history (including super tall grandad and great-grandad who both died of sudden heart failure) and it seems likely. Apparently my niece who is 5 has been tested as not a carrier of the dodgy gene though so I'm wondering if it's as simple as a blood test.