Chamber of Secrets - The Unexplained

[snow15145]

No: HPFGUIDX 108913

It depends which version you are reading. In my hard back, it says
ancestor. In the paperback, which naturally came out later, it was
descendent. But then, I think I read somewhere that JKR said that
she had intentionally used "ancestor". This one has always puzzled
me, and I still don't know. Perhaps I should go to the book store
and look for a newer version than either of mine, and see which way
it is written. I'll let you know.

Pat
Snow:
This is JKR's answer in a Scholastic interview October 16, 2000

Harry Potter for grownups again! Is Voldemort the last remaining 
ancestor of Slytherin, or the last remaining descendent of Slytherin?

"Ah, you spotted the deliberate error. Yes, it should 
read "descendent." That's been changed in subsequent editions. (Keep 
hold of the "ancestor" one, maybe it'll be valuable one day!)"

Hope this helps!