The context of this question is a spoiler.
In the latest Star Trek Discovery episodes we learn that Tyler is actually Voq, who has been surgically altered and had the personality of the real Tyler implanted over his own.
How is it possible that this wasn't detected by medical staff on board the Discovery? I would have assumed that Starfleet medical scanners would be capable of analysing DNA (I'm certain that it's been mentioned in other series') so how is it possible that:
the doctor didn't realise Ash's DNA was actually Klingon?