depends on which fighter. A guy with an iron chin like Dan Henderson would never lose. A normal guy with no training can knock out a trained fighter with a clean hit unless the guy can't be knocked out. In grappling an untrained guy loses 100% of the time, look up Franklin submitting a random guy in like 6 seconds. The thing about grappling/subs/wrestling is if you have no clue what you are doing you can't do anything unlike striking where you can throw haymakers and pull off a miracle knockout
Tito Ortiz got in a street fight with some untrained guy and got knocked out. I'd say with mma gloves on against an average UFC fighter an untrained guy could win 1/1000 unless he is Hendo or Rampage then it's 0/1000.
That "untrained guy" was Lee Murray, who went the distance with Anderson Silva. He wasnt some untrained chump off the street, who was actually very well trained. Also, there is no real way to know how true that story is. Good try though.