I think one of your letters might be wrong, but I don't know which one!
I can find: "heterosis" and "heterotic" which require you to change either the s or c.
Thank you for your help Norberto - you are right and the answer is heterosis. One of the letters was wrong - I had taken Don Quixote's horse to be Rocinante and it should have been the alternative spelling of Rosinante, giving me the last letter as 'c' when it should have been 's'. Many thanks for helping me out with this one.