I read a study a few years ago (
afraid that there is near zero chance I can find it again, so you'll have to take this as an undocumented source
- or seek to research it yourself) that stated that it is equally plausible from the DNA evidence that Randolph Jefferson (Thomas' younger brother) was the Jefferson ancestor who contributed DNA to the Hemmings line. I seem to recall that there was (naturally) some controversy around this, and some sources even claimed that it was likely.
Regardless, it seems that the Randolph Jefferson theory is plausible, and therefore, interjects "reasonable doubt" should there be any inquiry held to a legal standard.