My connection is through the South Carolina branch of the Witherspoon family. The marriage of John Knox's third daughter, Elizabeth, to the rev. John Welsh of Ayr in 1592 is well documented. Their youngest daughter Louise Welsh (b. c. 1613), married James Alexander Witherspoon in Scotland c. 1635. There is some dispute among genealogists about the connection regarding Louise, but it has been part of the family history of the Witherspoon family going back to the early 18th century, and is even mentioned in Boddie's History of Williamsburg, South Carolina.
My grandmother was a Knox, and the family always used to say we were descended from John Knox the Reformer.
But as near as I could tell from my own brief research, his son(s) didn't pass down the family surname and the Reformer's Y-DNA to future generations.
On the other hand, because you are thought to descend from one of the Reformer's daughters, it's obviously a different story. Sorry for my initial misunderstanding.